# Talk:Geographic coordinate system

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## Geocentric latitude vs true latitude

Is there an explanation of the difference between geocentric latitude and true latitude somewhere in Wikipedia? --Begemotv2718 23:52, 24 August 2005 (UTC)

See: Types of latitude. ~Kaimbridge~ 20:07, 4 November 2005 (UTC)

## Only lat/long is mentioned

This article needs a lot of work. Why is the title "Geographic Coordinate System" when only lat/long is mentioned? What is the relevance of the diagram on the right? (Obviously that's rhetorical - I mean: what does it show?) Also, the whole section on lat/long is pretty incomprehensible. It should also probably mention that different meridians were in common use until the 20th century... Finally, the first sentence is wrong: a geographic co-ordinate system need not express *every* location on the earth - or indeed *any* location. Rolypole 21:23, 16 January 2007 (UTC)

## Hexadecimal earthgrid

Match 213 added a long section which proposed a hexadecimal earthgrid. I am reverting this section because it violates official Widipedia policy which prohibits new ideas. It can be included only if those ideas have been published in some peer-reviewed journal, which must be cited. — Joe Kress 06:00, 14 November 2005 (UTC)

Hi Joe Kress, it was me as SP Match 213 (user contribuations) who introduced the paragraphe "The proposed hexadecimal geographic coordinate system".
I always think that this (not so new) scientific work is encyclopedia worthy. But at the moment I don't have the time to discuss and to argue on this topo.

I know you very well as a competent and sincere editor in Wikipedia. Therefore I don't resent you, that you have – for the moment – deleted this chapter.
As soon as I'll have more time, I'll come back to this topo. Interim, I invite you to follow the related discussions at the Talk:Fairway Rock Island page.

Have a good day, -- Paul Martin 11:37, 23 November 2005 (UTC)

## Unsourced?

It looks pretty sourced (and common-sensical) to me. Why the 'unsourced' tag?Guinnog 21:44, 18 February 2006 (UTC)

## Decimal div

"Decimal division is now the most common and standard.": division?

"By combining these two angles, the horizontal position of any location on Earth can be specified.": at this point please hammer home that in some countries, if one doesn't add a third, datum, even more important than elevation, rescue crews could easily be sent somewhere a kilometer away.

Of course there is just as good a chance that rescue crews get their datums mixed up too anyways, or never thought about them.

Hammer it home here even though datums are mentioned below it.

--jidanni 2/06

## In wikipedia

It would help if the article talked about how the cooridinate system in wikipedia was used, since every page with cooridinates points to this page...
~ender 2007-09-09 13:26:PM MST
That is not possible given the Wikipedia guideline to avoid self-references to Wikipedia or to any Wikipedia project such as geographical coordinates. — Joe Kress 06:17, 10 September 2007 (UTC)
But I believe it does allow for disambiguation links at the beginning of the article. I added such a link to this article. Dbiel (Talk) 02:37, 6 November 2007 (UTC)
I've seen a template something like {{selfref}} for wrapping around such selfreferences. Maybe it's mentioned in WP:ASR. (SEWilco 02:48, 6 November 2007 (UTC))

## Baltimore

Why is the example town Baltimore? Seems completely arbitrary to me. +Hexagon1 (t) 00:16, 11 November 2007 (UTC)

I don't know why Baltimore was chosen. Seems like a good choice to me; it's far from the origin (Greenwich), easily identifiable on many maps, and the nearby coastline helps during confirmation between maps of various styles. (SEWilco 15:53, 12 November 2007 (UTC))
Still arbitrary, is there no other widely used geographical beacon in the world but Greenwich? +Hexagon1 (t) 04:37, 16 November 2007 (UTC)
Removed in the 20:20, 15 July 2010 edit. The example was misleading. It used the value for geodetic latitude but the vector for geocentric latitude. -Ac44ck (talk) 15:51, 28 August 2010 (UTC)

## Spring Cleaning

For such an prominent article, this one seems tired and in need of a good cleanup and revision. Is anyone going to object if- I remove the purple prose, remove the irrelevant links zB Greeks. Correct the facts in line with "A guide to coordinate systems in Great Britain v1.7 Oct 2007", explain datums and Helmert transformations, make Baltimore grid reference consistent with Baltimore article, explain relevance of time, and accuracy constraints? ClemRutter (talk) 16:18, 13 April 2008 (UTC)

Be my guest, although I have no idea what "irrelevant links zB Greeks" means. I don't think that Helmert transformations would be appropriate here because it has its own article, unless you can reduce its complexity substantially. — Joe Kress (talk) 04:28, 14 April 2008 (UTC)

Here is the first draft- Greeks are in a footnote, Baltimore has gone and I have structured it so that the casual user has a few paragraphs which discuss all GCS, then the heavier geography and maths follows. I want to replace the diagram that says little, and look at why the map uses Eckert VI which is difficult to explain. What do you think.ClemRutter (talk) 16:43, 15 April 2008 (UTC)

I started to copyedit your first draft but immediately ran into significant problems, so I thought I'd give you a chance to correct them first. One signifcant point is that latitude is not measured via the center of the Earth, but is the angle between a line perpendicular to the surface of the ellipsoid (either a geoid or a datum) and the equatorial plane—the line usually does not pass through the center of the Earth, except for rarely used geocentric coordinates. The other problem relates to your introduction of the three basic methods of specifying coordinates, DMS, DM, and DD. Arabic numerals never replaced Roman numerals in geographic coordinate systems—they replace Greek numerals, which, in the system used by Ptolemy, were a place value system regularly using six sexagesimal places (a precision of 2×10−11), at least in astronomy. In geography, if he only used degrees, minutes and seconds (he may have only used DM), he did so simply because of the roughness of his data, not because he couldn't use more precision.
Within each sexagesimal place Arabic numerals have been used ever since they were introduced into Western Europe about 1200, from 0 to 59 for each place. These places were named minutes, seconds, thirds, fourths, fifths, sixths, etc. Astronomers were still regularly using six sexagesimal places as recently as 1740 when Jacques Cassini did so. He was a noted geographer as well, using triangulation to measure an arc of the French meridian, so it would be interesting to see what notation he used geographically. Thus the change to our present mixed sexagesimal/decimal system must be relatively recent. Astronomy now uses a mixed system (at least for right ascension) just like other time and angles do, including geographic coordinates: minutes and seconds plus decimals, or minutes plus decimals, or only decimals. — Joe Kress (talk) 21:05, 15 April 2008 (UTC)
Cassini used d ' " (degrees, minutes, seconds) throughout his book wherein he presented his triangulation, De la grandeur et de la figure de terre (1720), for example page 135. But on page 236 he used thirds, e.g. 2d 12' 9" 30'", as well as a simple fraction after seconds, e.g. 51d 2' 25" ½. On page 286 are several thirds. I still find at least one 'third' in 1829 in Nouvelles tables astronomiques et hydrographiques by V. Bagay on page vi. — Joe Kress (talk) 06:20, 16 April 2008 (UTC)

Fantastic response. I know I read all this once- but didn't buy the books, at the libraries closing down sale! All this needs to be recorded- to me the question is where. I'll tell you my thinking.

• This article is accessed by a link from every geotag on Wiki, so it is incredibly important that the first two or so pages are kept focussed on what this reader needs to know.
• From the point of view of 'Dr. Joe Public' or a doctoral student accessing the article, they must understand the 4 myths (see OSGB ref) so they don't waste time assuming their primary source map from say (1910) will have the same lat/long as their GPS/Wikipage.
• After that, the article flies in two directions, the nature of geodesic transformations, and the relation between this and the History of mathematical thought and numerical representation.
• I am torn between the desire to include everything and the desire to keep on subject

So I suggest that we need a section on the History of GCS, which gives a synopsis of the fields development, and a full article called something like the History of GCS. In the same way the Helmert is obliquely described, then a link given. In the History Case I think it should be For more information see.... tagged. I readily concede, that my latitude definition is an approximation, but so early on in the article nothing was defined (geoid, planes of equal gravitation potential) and I wanted to keep it concise and not take the reader down a mental rabbit hole, but to keep focussed. I see the problem and would be inclined to use a footnote to help clarify. I haven't wanted to introduce Greek numbers as for outside the world of Maths, Greek numbers are not a current concept so they obfuscate rather than illuminate. I am acutely aware of the number of new concepts one can hold in one's head at one time. There are a few thought's- sorry it's only brief- life get's in the way of Wiki! ClemRutter (talk) 09:45, 16 April 2008 (UTC)

There are other history articles distinct from a modern treatment of the same subject, so your suggested split is not new. I'll let you decide how you want to lead the reader into the subject, but I do feel that the fact that latitude is not via the center of Earth is more important than a footnote. Indeed, upon reading A guide to coordinate systems in Great Britain PDF (718KB), I find that this is included within Myth 1, that their is no single coordinate system. See latitude for its many types. A compromise might be to say near or about the center of Earth with a statement to see a later section where the distinction is explained in more detail. Before the prolate vs oblate spheroid arguments of the 18th century, Earth was assumed to be spherical so latitude was via its center at that time. Jacques Cassini was on the losing side, believing that Earth was a prolate spheroid. It is also a myth that Earth was ever believed to be flat, at least in Europe. See Flat Earth. I see no real need to discuss the history of geographical coordinates here—my explanation was to point out that the distinction between DMS, DM, and DD was not due to ancient history, but is quite modern. I do not know of any reason for that distinction other than the personal style of each writer and his relative importance in the last few centuries. — Joe Kress (talk) 22:37, 16 April 2008 (UTC)
Geodesy for the Layman PDF (3.42MB) or Geodesy for the Layman (html) by the United States Defense Mapping Agency (now the National Geospatial-Intelligence Agency) is also a good source of info, especially for GPS. — Joe Kress (talk) 23:14, 16 April 2008 (UTC)dvvbbbvbcvbdcvbvbdcvbvcvbcvdcvbvbcvbdcvbbcbccvbvcbcvbvbcvbvbdc
I have made a couple of holding changes and added the pdf link. Doing a bit more thinking. ClemRutter (talk) 09:22, 17 April 2008 (UTC)

## Graticule

Redirected here from Graticule, trying to find out what that was and where it came from. Graticules are not even mentioned on the page. It seems odd to search for a topic, then be referred to a page that makes no mention of it. (Maybe I missed the section on graticules? Apologies if that's the case.) --Thomas Btalk 04:41, 21 May 2008 (UTC)

I assume you came here after reading the latest XKCD blag post? I did. So, I think a "graticule" is a grid square on the coordinate system, at some level of precision. -- Phyzome is Tim McCormack 13:54, 21 May 2008 (UTC)
Note quite. A graticule looks like a grid, but there are technical differences between the two. A square grid is a regular set of straight parallel and perpendicular lines, usually aligned approximately north, and used to give positions a grid reference (for example, the British national grid reference system). A graticule is the "grid" formed by the lines of latitude and longitude drawn on a map, which may or may not be parallel, perpendicular and regular, depending on the projection used. Wardog (talk) 17:11, 15 December 2008 (UTC)
Yes it is——it got lost in an HTML remark, during an edit last month. It has now been restored and augmented. P=) ~Kaimbridge~17:49, 21 May 2008 (UTC)

## US or British spelling

As I understand it, "centre" is British (as against "center" in American), but neighboring is American (as against neighbouring in British). I really don't mind whether the article uses American or British English, but it should be consistent. I am changing it to British spelling, I can't think of a good reason to choose one over the other though but I'm British myself so I claim first mover advantage. Also "centre" pre-dates "neighboring" in the edit history. — PhilHibbs | talk 14:44, 29 May 2008 (UTC)

I think we should go with the American spelling. Wikipedia started, and still is, in the United States. It only makes sense. WikiFanD 21:40, 25 February 2010 (UTC)

Unlike, copyright, where the location of server is legally significant, there will never be agreement on reached on style or spelling. On could say it is a matter of preference. There is however a solution- a simple template {{ortho}} needs to be written that checks the language preference stored in the readers profile: #ifeq:UserLanguagePreference|en-us|{{{2}}}|{{{1}}} - so if the user has opted for us-english, they will always get the second spelling- otherwise they will get European English. Editors need only write {{ortho|Centre|center}} to ensure everyone is happy. --ClemRutter (talk) 01:00, 26 February 2010 (UTC)
The question of American versus British spelling has already been resolved by policy: WP:ENGVAR. The short version of the rules: if there is a strong national tie to the subject, that variety of English is used (e.g. List of English monarchs has "centre", while NASA has "center"). Failing that, whichever edit breaks the tie first, remains in effect after that time. There is no strong national tie for this article and it looks like the British spelling has been in effect for quite a while, so it should remain. The thing about user preferences is interesting, but if I understand correctly, it only works for the 1% of readers who log in; 99% of Wikipedia users browse anonymously, so there still need to be rules to avoid needless conflicts. CosineKitty (talk) 01:35, 26 February 2010 (UTC)

## Map projection

The caption under the world map says "Eckert VI projection", but this does not seem to be the case. Eckert VI is an equal area projection, with sinusoidal meridians; the one in the image is not equal area - polar regions are stretched. It looks like the Robinson projection, but I'm not sure. The caption under the larger version says 'creu projection'; googling for that phrase produces only 2 backreferential hits, so I'm not sure if that's correct either. 217.173.186.154 (talk) 11:07, 2 December 2008 (UTC)

## Positive/Negative for North/South and East/West

For someone (such as myself) who comes to this article looking for an explanation of notations used, there seems to be a very basic omission in explaining, when North/South and East/West are not used, which direction positive angles are and which direction negative angles are.

I am making a guess from..

   * DMS Degrees:Minutes:Seconds (49°30'00"N, 123°30'00"W)
* DM Degrees:Decimal Minutes (49°30.0', -123°30.0'), (49d30.0m,-123d30.0')
* DD Decimal Degrees (49.5000°,-123.5000°), generally with 4-6 decimal numbers.


that positive angles for latitude are "north", negative are "south", and for longitude positive is "east" and negative is "west".

Spxl (talk) 07:40, 20 May 2009 (UTC)

I second that a short accurate discussion of negative coordinates should appear early. Many GPS users not very familiar with degree measurements outside the first quadrant will end up here looking for clarification on that specific question. I hope someone more familiar with typical GPS usages than I am will add that.

NStahl (talk) 15:42, 13 July 2009 (UTC)

As a non-native English speaker I would like to know how do you actually say or pronounce all this, i.e. can someone read this out loud for me: 49°30'00"N? —Preceding unsigned comment added by 85.27.49.139 (talkcontribs) 17:57, 6 September 2010
Forty nine degrees, thity minutes, zero seconds north. — Joe Kress (talk) 22:12, 7 September 2010 (UTC)

## Broken Link

https://www.cia.gov/library/publications/the-world-factbook/reference_maps/pdf/political_world.pdf is broken. However the closest I got was: https://www.cia.gov/library/publications/the-world-factbook/graphics/ref_maps/pdf/political_world.pdf Unfortunately there are no any lat/lon values attached. I suggest removing that CIA link alltogether. (Feel free to delete this post when you feel the issue is settled.) Curt Contributions/147.172.172.29 (talk) 13:50, 15 June 2009 (UTC)

## Graphic Could Be Improved

The second graphic (Latitude phi and Longitude lambda) could be improved by moving the label "phi = 0deg" away from the point where phi and lambda are both zero. Just move it right a little ways. The "lambda = 0" label is well placed, for instance. —Preceding unsigned comment added by NStahl (talkcontribs) 15:54, 13 July 2009 (UTC)

## Intro sentence - spherical coordinates

The first sentence is a little confusing about the relationship to spherical coordinate systems:

A geographic coordinate system is a coordinate system that enables every location on Earth to be specified in three coordinates, using mainly a spherical coordinate system.

What does "mainly" mean here? The article on Spherical coordinate system says that a geographic coordinate system is different than a spherical one. -Pgan002 (talk)

## In some sense, a cube is an approximation of a sphere

It says in the article that, "The Earth is not a sphere, but an irregular shape approximating a biaxial ellipsoid." That may be true, but if the earth approximates a biaxial ellipsoid, then it is also true that it approximates a sphere - even if it isn't one. After all, the earth is not a biaxial ellipsoid either, it merely approximates one.

Surely the whole point of associating the earth with a known shape is to simplify understanding for the reader. If that is the case, then wouldn't it be better to point out that the earth approximates a sphere, a shape which most readers will be familiar with, rather than with a biaxial ellipsoid, which doesn't help with understanding at all. —Preceding unsigned comment added by 81.187.233.172 (talk) 22:54, 18 December 2009 (UTC)

## A better approximation of a longitudinal degree

Whence 0.9966472 in the formula?

If it is a function of the flattening of the ellipsoid, please state that function. - Ac44ck (talk) 03:21, 28 August 2010 (UTC)

Replaced with b/a. Thanks. -Ac44ck (talk) 15:31, 28 August 2010 (UTC)
All the distances in this section assume the GRS80/WGS84 spheroid-- right? So might as well give people the actual value of b/a for those spheroids.
(Truth to tell I haven't checked the numbers in the table to confirm they are GRS80/WGS84-- I'm assuming if they were something else the author would have said so.)
The terminology (reduced/parametric latitude) is of course secondary to the formula itself-- you need the formula, you don't need to know the terminology-- so the formula ought to come first, with the terminology mentioned as an afterthought, for those who are interested. Tim Zukas (talk) 20:54, 29 August 2010 (UTC)

It is good to have the value of b/a spelled out, to the number of decimal places one editor deems to be adequate. I don't know who may need more decimal places, but telling them that it is calculated from b/a lets them calculate as many as they want. Some may think it should be obvious that the value is b/a or that it should be discernible without too much trouble. It may not be obvious, and why make it any trouble if we don't need to? If one is writing a computer program, there is one less opportunity for a typo if the constant can be derived in the program instead of hard coded. -Ac44ck (talk) 02:30, 30 August 2010 (UTC)

On the WGS84 and GRS80 spheroids, b/a is itself a calculated result-- in other words, it's never going to be the ultimate in accuracy. For WGS84, the exact value of b/a is

$\frac {297.257223563}{298.257223563}$

If you think that fact belongs in this section, feel free, but it looks incongruous to me.
(On GRS80 it isn't possible to give an exact value of b/a, so the programmer is in trouble.) Tim Zukas (talk) 18:49, 30 August 2010 (UTC)
While this is as precise as it can be, it isn't general and leaves an avoidable opportunity for a typo.
In writing a program, I would prefer to define a constant
inverse_flat = 298.257223563
and compute
b_over_a = (inverse_flat - 1) / inverse_flat.
The inverse flattening for GRS 80 is given here:
World_Geodetic_System#A_new_World_Geodetic_System:_WGS_84.
Could not the same program fragment be used, changing only the one constant, for GRS 80 or even WGS 66 (with flattening = 1/298.25)?
My point was that, in the "information age," we shouldn't needlessly obscure the origin of the only constant in the formula. One might surmise that 0.9966472 is b/a, but they may have a nagging doubt that it came from elsewhere. So let's tell them where it came from. If need be, one can derive 'b' from 'a' and 'f'. A string of digits like 0.9966472 may not have such an accessible pedigree. I'm not saying we shouldn't spell out the value; just that it seems fitting in an encyclopedia to also give its origin if it isn't too convoluted. Neither "b/a" nor "(inverse_flat - 1) / inverse_flat" seem so convoluted to me that we should shy away from mentioning one of them. I think "b/a" would be more useful to most interested readers. -Ac44ck (talk) 04:03, 31 August 2010 (UTC)

## Definition of "the" latitude

The body of the article starts with:

The Latitude (abbreviation: Lat., φ, or phi) of a point is the angle between the equatorial plane and a line joining that point with the earth center.

But that isn't the definition of "geographic latitude." -Ac44ck (talk) 18:17, 20 November 2010 (UTC)

That common misconception was added within the last few hours. I have reverted and clarified it. — Joe Kress (talk) 18:57, 20 November 2010 (UTC)

## References

At least three items in the "References" section seem to be parenthetical thoughts. Isn't this section to be used for links or other forms of cites? -Ac44ck (talk) 18:38, 21 November 2010 (UTC)

I believe we do not have another method to insert footnotes. But you are right, the title of that section is misleading. I am going to change it to "Footnotes and references". Paolo.dL (talk) 18:42, 21 November 2010 (UTC)
I've separated notes from references using the group parameter. See WP:REFGROUP. — Joe Kress (talk) 19:58, 21 November 2010 (UTC)

## Needs more clarity, examples, practical explanation

I came here trying to understand where on a map I might find a particular coordinate reference (because Google Maps didn't understand them) and I have left none the wiser. I'm not a geographer or cartographer obviously but surely this is basic stuff. I've been given coordinates N 00 02.632 WO 53.229 in Ecuador - what does it mean? You should give a list of examples of notations and coordinates, and their corresponding locations. Then us ignoramuses can work out the rest. Just a wordy description of a particular notation system is really not helpful in deciphering coordinates at all. —Preceding unsigned comment added by 88.209.79.178 (talk) 19:06, 9 January 2011 (UTC)

"I've been given coordinates N 00 02.632 WO 53.229 in Ecuador" -- probably nobody can make sense of the numbers as you gave them. Ordinarily if you see "N 00 47.2 W 67 57.6" you would interpret it as latitude 0 degrees 47.2 minutes North, longitude 67 degrees 57.6 minutes West. So "WO 53.229" doesn't make sense for Ecuador.
But you're right, of course; articles like this do tend to attract writers that like pointless complexity. Some of the complexity isn't pointless, but it still conceals the essentials that people like you are after. Tim Zukas (talk) 20:24, 13 January 2011 (UTC)
Thanks Tim. I've worked out that WO means W (fairly obviously) - it seems to appear on several sites and may be a mistranslation of West/Oueste, as Spanish doesn't have a 'W' normally, or perhaps this is just the notation in some hispanic countries.
Complicating matters, the crucial longitude degree part is missing in the example I gave (it should be 78), for some reason, but I have found other examples, which are correctly formed, where simply substituting WO for W gives the correct location in Google Maps. —Preceding unsigned comment added by 80.94.110.73 (talk) 09:53, 4 February 2011 (UTC)
P.S. FWIW, I've found a reasonable example of a practical explanation on the geotagging page http://en.wikipedia.org/wiki/Geotagging - the examples boxes under GPS formats and JPEG photos. This shows fairly simply (in fact it could be a lot simpler) how to correctly form and read a GPS coordinate, with examples in decimal and minutes, and NSWE. Surely similar boxes have a place in this article? All the other stuff is of course very welcome - depth of information is great if that's what you're looking for - but don't forget us idiots! —Preceding unsigned comment added by 80.94.110.73 (talk) 10:42, 4 February 2011 (UTC)

I don't understand this statement describing coordinate systems: “With the origin at the center of the ellipsoid, the conventional setup is the expected right-hand: Z-axis along the axis of the ellipsoid, positive northward X- and Y-axis in the plane of the equator, X-axis positive toward 0 degrees longitude and Y-axis positive toward 90 degrees east longitude” The right hand, positive northward, and positive toward 0 degrees longitude are particularly confusing. Bcantwell77 (talk) 00:24, 19 September 2011 (UTC)Brian Cantwell

See right-hand rule for a picture of a right hand and these coordinates, although I prefer to align the thumb along the axis connecting the north and south poles, pointing north; the index finger along the axis connecting the center of the Earth with the intersection of the equator and the prime meridian, pointing from center to surface; and the second finger along the axis connecting the center to the intersection of the equator and 90°E longitude, pointing from center to surface. Thus the X and Y axes are in the plane of the equator, and the Z axis is perpendicular to the plane of the equator. — Joe Kress (talk) 21:53, 19 September 2011 (UTC)

## The Annoying Degree-Minute-Second System

In entries of places, Wikipedia has used the Degree-Minute-Second (DMS) system to provide geographic coordinates. I am curious if there are lots of people out there who find this preferable to the decimal-degree system. It is very annoying to make a note of the latitude and longitude of a place in the DMS system or to copy/paste them for other purposes, and it is especially so when you want to use the coordinates as inputs for some mathematical computation, because you have to convert them to decimal degrees. So could Wikipedia use decimal degrees for the default units of geographic coordinates in the future? --Roland 06:42, 12 January 2011 (UTC)

I agree the archaic second system should be used as little as possible. Decimals are so much clearer (but you still need degrees and minutes, don't you?). Maybe this is a US preference? I know they feel very attached to imperial units - one of the reasons one of their space shuttles crashed, I seem to remember. —Preceding unsigned comment added by 80.94.110.73 (talk) 10:49, 4 February 2011 (UTC)
I can hardly think of the possibility that I would need any angle in DMS. If the need does arise, which I doubt, I can convert the decimal degree to DMS.
Additionally, how can we pass the suggestion to Wikipedia that the decimal degree be used in entries about places in the future? --Roland 11:16, 18 February 2011 (UTC)
I don't know what the background of previous posters on this topic is... but I have a background in remote sensing and defense; DMS are the preferred notation for Lat/Long with few exceptions in my experience. It's also the easiest way to plot on an aeronautical chart, or any map for that matter. It's likely preferred because it's simple and useful for practical applications. So while it's not the easiest to use in a trilateration calculation, for example, it is for many more immediate purposes. Additionally, it is not a "US preference" to my knowledge. I would strongly advise against changing the current coordinate format on Wikipedia. --75.111.228.197 (talk) 03:03, 25 May 2011 (UTC)
"It's also the easiest way to plot on an aeronautical chart, or any map for that matter."
Any paper map that shows lat-lon in deg-min-sec, you mean. It is true that paper maps rarely show decimal degrees, which is the only possible disadvantage to decimal degrees. When Wikipedia gives a position it's likely to be a link to an online map that likes decimal degrees fine.
For any given precision decimal degrees are of course more concise than deg-min-sec; "10.123456 deg" is more precise and more concise than "10 deg 7 min 24.44 sec". Even the few people that want to plot the point on a paper map will not find the longer form much help; if you did want to plot on a 1:24000 map, how would you go about it? You'd probably need to convert it to a linear distance you could measure with dividers or ruler-- and for that decimal degrees are at least as good as deg-min-sec. Tim Zukas (talk) 21:14, 5 July 2011 (UTC)
DMS is very useful for navigational purposes, because it's linked to time. Eg, if you know your position, course, and speed it is quite easy to plot your destination using DMS with very little math. Likewise it's easy to take a position, speed, and destination and plot your ETA. 12.117.176.190 (talk) 20:48, 3 January 2012 (UTC)
Let's see an example of how much easier such a calculation is with deg-min-sec. Tim Zukas (talk) 03:18, 4 January 2012 (UTC)

It looks that there is not a single reason to justify the use of the inconvenient DMS system. Or is it simply the inertia from the past? --Roland 00:56, 5 September 2011 (UTC)

Just learn to use both. I plot in degrees.deci-degrees but read in ddd:mm:ss. There are good historic reasons why the base 60 system is preferred, in the same way that time is told using a Hour:minute:second system. You would go as far as to suggest we have a 10 hour day, with deci-hours would you? Someone referred to a 1:24000 map- which I believe is what they use in the US. My reference maps IGN or OS are 1:25000- and are marked in ddd:mm and in ddd:mm:ss.
At wikipedia we report from secondary sources- a GPS reading is a primary or even WP:OR- all the secondary source printed material uses ddd:mm::ss so to use anything else is not an option. Carpe diem.--ClemRutter (talk) 09:19, 5 September 2011 (UTC)
"Just learn to use both."
We know how to use both. No need to bother Wikipedia readers with that, tho. We know how to use grads, and some maps use them-- but Wikipedia readers can live without them too.
"There are good historic reasons why the base 60 system is preferred"
Like what? Tim Zukas (talk) 22:16, 27 September 2011 (UTC)
I fully agree with Tim Zukas and Roland. Over the last 25 years or so we have seen a clear trend that DMS is gradually being replaced by decimal degrees (with very good reason), and I think it is just a matter of time when the old DMS system has been abandoned. The question is probably not IF wikipedia should switch to decimal degrees, but when. Isaac Euler (talk) 12:05, 29 September 2011 (UTC)
The GPS devices I am familiar with, use neither degree-minute-second nor degrees and decimal fractions of degrees. They use degree, minutes and decimal fractions of minutes.

## Degree-length formula is wrong

The recently added formula for the length of a degree of longitude

$\frac{\pi}{180}\cos(\phi)\sqrt{\frac{a^4\cos^2(\phi)+b^4\sin^2(\phi)}{a^2\cos^2(\phi)+b^2\sin^2(\phi)}}\,\!$

seems to be wrong. Anybody know what distance it's intended to give us? If you try it at 45 deg latitude it doesn't give geodetic distance, or distance along the parallel, or great-ellipse distance. (Didn't check chord distance, but probably not that either.) Tim Zukas (talk) 16:25, 14 October 2011 (UTC)

I don't know about the outside factors, but $\scriptstyle{\sqrt{\frac{a^4\cos^2(\phi)+b^4\sin^2(\phi)}{a^2\cos^2(\phi)+b^2\sin^2(\phi)}}}\,\!$, itself, is the actual “radius”.  ~Kaimbridge~ (talk) 14:04, 15 October 2011 (UTC)
Good point-- so he must have been trying to get the distance along the parallel and forgot he needed geocentric latitude for that. So no need for that variation. Tim Zukas (talk) 20:05, 16 October 2011 (UTC)

## Second-person textbook prose

The section Latitude and Longitude In Practice of this article seems to have been lifted straight out of some other work. It reads like a textbook, written in the second person, and describing things that are not addressed elsewhere in the article (what's a "Wild T4"?). The whole thing has good information but requires cleanup for style and tone. If anyone can find where this was lifted from (a quick Google search only turned up Wikipedia copycat sites), it may need to be referenced or removed entirely for copyright reasons. --- AeroIllini (talk) | 01:04, 1 December 2011 (UTC)

No one can find where it's lifted from, and no need to worry about copyright.
All the data (the lat-lons and the deflections of the vertical) are on the NGS website, as is the calculator to compute distance between two lat-lons if you don't have the formula on your own calculator. FWIW the C&GS paper that explains the Hawaiian triangulation is also online, but in principle that triangulation wasn't much different than any other one. And that's the point-- that in the real world a point's "latitude and longitude" is defined by its distance and direction from other already-defined points, and not by its relationship to the equator and the Greenwich meridian. (I guess that's less true now, with GPS, but I think a GPS lat-lon that's supposed to be correct to a centimeter is to some extent still based on other already-determined points. But I don't know much about GPS.)
I figured no need to tell folks what a T4 is; they can infer it's a device for getting lat-lons from the stars. It's the sort of theodolite you need if you want astro lat-lon accurate to 0.0001 degree. If somebody can find a pic to link to that would be nice.
Got an example of "cleanup for style and tone"? Tim Zukas (talk) 17:39, 1 December 2011 (UTC)
(I see I messed up the NGS links-- they're fixed now, if you want to try them again.) Tim Zukas (talk) 17:55, 1 December 2011 (UTC)
"No one can find where it's lifted from"? That is not the way copyright laws (and Wikipedia) work. Please tell us where you lifted it from, and then let the community decide if that is acceptable or not. But actually, don't bother. It really was so far from Wikipedia's encyclopedic style that it should not be in article-space at all. The best bet, I think, is to thoroughly re-work the point(s) you feel are missing from the article here on the discussion page, or in your user space, and bring them back into the article, with citations to secondary sources making the same point(s), when they are ready. Some years ago, when Wikipedia was very young, those kinds of lapses of quality were sometimes overlooked temporarily, as being better than nothing, but we have got a long way past that now. --Nigelj (talk) 10:47, 25 December 2011 (UTC)
It wasn't lifted, of course. Why do you imagine it was?
"It really was so far from Wikipedia's encyclopedic style..." Show an example.
I think you said you didn't understand it-- where do you think it needs elaboration?
The example is in the US, but the principles apply everywhere. If a site exists that gives the same data for points outside the US, tell us about it. Tim Zukas (talk) 23:19, 27 December 2011 (UTC)
I do agree on this matter, style is far from enclyclopedic. As an example first sentance "Say you set up your Wild T4 next to the water tank north of the airport at Hilo, Hawaii, intending to determine its latitude and longitude by the stars." Well I am not setting up anything, I am not on Hawaii nor do i have a Wild T4. Also sentance "What went wrong?" is something not found on other pages of wikipedia. Also "but the T4's level vials don't know where that is" ofcourse they dont know they are vials aren't they? Instead of mentioning a specific device by brand and model one should use a generic name for such a device. Would it not be stupid if for example the page on photography used "Nikon D1000" instead of camera? Also phrases such as "So we need a different plan" are unscientific, better alternative would be something like "A different method should be used". Also mentioning specific coordinates are rather unrelevant and a generic theory should precede the example. First paragraph should tell what the section is about. The point is now obscured by coordinates meaningless to average person, poor writing style and rather confusing presentation of facts. Currently it looks like a 6th grader wrote that part of article, no offence. I suggest this part to be removed or to be completely rewritten with sources 109.204.158.244 (talk) 20:42, 7 March 2012 (UTC)
I agree that the section Latitude and Longitude In Practice is poorly written. I'm not an expert on the matter, but it seems the section discusses two distinct issues: I would suggest a section about the continental drift to be added (or appended to?) the section on Shape of the Earth. In addition, I would suggest a section that discuss the methods to measure the geo coordinates (from this talk page, I understand that there is a device that uses the gravity -- the Wild T4, while I only imagined devices that uses the GPS satellite system, or devices that uses the position of the sun and stars). I would say that this section would go after the sections on Shape of the Earth and Continental Drift. MacFreek (talk) 11:09, 1 June 2012 (UTC)
"it seems the section discusses two distinct issues: I would suggest a section about the continental drift to be added..."
This section doesn't discuss continental drift at all.
"I would suggest a section that discuss the methods to measure the geo coordinates..."
The point of this section is that geo coordinates are almost never measured at a point-- rather, the surveyor tries to figure how far away it is from an already-known point, and in what direction. The Wild T4 is used to measure astro latitude and astro longitude-- which most people don't need to know. But that's the best we can do, if we don't have a known point anywhere around. Tim Zukas (talk) 21:00, 2 June 2012 (UTC)
Well since no one has done anything about the "Latitude and longitude in practice" section I am going to remove it. Motivations can be found above. I leave a copy of the text here below if anybody is interested. Petter Ljungqvist (talk) 22:11, 10 September 2012 (UTC)
Well, that didn't work. But I am not up for a editing war.
Tim Zukas, you have it your way, can't be bothered any more. — Preceding unsigned comment added by Pljungqv (talkcontribs) 00:22, 1 October 2012 (UTC)

## Transverse mercator

The article says that TM maps are spaced at 6 degree intervals. I've worked on surveys which have TM maps with the central meridian at 3 degree intervals. Is there some special significance to the 6 degree statement that I am missing?--Robert EA Harvey (talk) 09:35, 28 December 2012 (UTC)

UTM zones are 6 degrees apart. If you need better accuracy (more constant scale) you could always set up narrower TM zones-- e.g. some US state-coordinate zones. Tim Zukas (talk) 23:40, 14 January 2013 (UTC)

## Sentence needs clarification/grammar check

I don't quite understand the sentence "As the longitudinal rings — geographically defined, all great circles — converge at the poles, it is the poles that the conjugate graticule is defined." It looks like one or more words are missing in the last part. Should that be "at the poles"? Thanks! Zip00 (talk) 19:46, 24 April 2013 (UTC) zip

I've removed this paragraph since it is confusing and uses non-standard terms. "Conjugate diameter" and "transverse diameter" were lifted from a Victorian handbook on mechanics and I'm not aware of their use for the dimensions of an ellipsoid. If you google the term "conjugate graticule", you will see that it originated in this article. It's not clear why the "transverse" and "oblique" graticules are being introduced here. These are only useful in constructing some forms of spherical map projections and they are not printed on any maps. No support is given for the sweeping assertion that all of spherical trigonometry is based on the transverse graticule. All the major results in spherical trigonometry were obtained without using any graticule. cffk (talk) 09:40, 4 July 2013 (UTC)

## Gravity doesn't point to the center of the earth

The paragraph discussing theodolite measurements in Hawaii is misleading. "We would like the T4's axis to point to the center of the Earth" repeats a common misconception. Presumably if we are worried about the perturbing influences of a nearby mountain we should already be accounting for the fact that the gravity of the ellipsoid is normal to its surface (and thus its gravity vector doesn't intersect the center of the earth). cffk (talk) 10:08, 8 August 2013 (UTC)

## Latitude and longitude in practice

I see that there's been a flurry of word-smithing on this section. So before this goes too much further, perhaps it's worth addressing its content! This section is confusing because it doesn't start by distinguishing geographic coordinates (introduced in the previous section) and astronomical coordinates (introduced in this section by the code-phrase "by the stars"). Once this distinction is made the rest of the discussion become less mysterious. The typical amounts that astronomical coordinates differ from geographic coordinates (up to about a 1km?) can be given. Some cases where the difference changes dramatically over a short distance leading to discrepancies in the estimate of the distance (the Hawaii example?) can be discussed. cffk (talk) 13:57, 8 August 2013 (UTC)

I pondered whether to edit this section to correct its deficiencies. In the end I decided just to remove it. The issues that need to be dealt with (astronomical vs geographic coordinates and the relative positioning of difference reference ellipsoids) are out of place in an introductory article like this. cffk (talk) 22:10, 28 August 2013 (UTC)

The article says
"The latitude (abbreviation: Lat., φ, or phi) of a point on the Earth's surface is the angle between the equatorial plane and a line that passes through that point and is normal to the surface of a reference ellipsoid which approximates the shape of the Earth."
And similarly for longitude. How do we measure that angle? We don't-- we can't. In practice, in the real world, the lat-lon of a point on a map is defined by its distance and direction from some other point whose lat-lon we already know, or have semi-arbitrarily assumed. (Or, nowadays, by its distances from several satellites whose X-Y-Z we know.)
No other Wikipedia article gives any inkling of how lat-lon is actually defined and measured. The articles talk about angles near the center of the Earth. If the reader is halfway alert he will wonder: since we don't dig tunnels to the center of the Earth, how do we measure lat-lon? By the stars, might be his first guess. Our first priority is explaining why that's not at all true (except when we're out in the middle of the ocean and have nothing to go by but the stars). Tim Zukas (talk) 23:19, 28 August 2013 (UTC)
Ideally the article would explain the related question that alert readers might wonder about: how the WGS84 Greenwich meridian ended up 5? seconds away from the traditional Greenwich meridian. I'm too lazy/ignorant to do that, tho. Tim Zukas (talk) 23:34, 28 August 2013 (UTC)

• I have once again reverted the changes, which do not belong to a simple theoretical article like this one. Why not create a new article about the various methods used today to geo-reference geographic information? -- Alvesgaspar (talk) 10:18, 29 August 2013 (UTC)

Let me relay my concerns with this section in more detail.

It you asked someone nowadays how to determine her latitude and longitude I expect she would either pull out a GPS unit or else bring out a map and locate the position on that. Determining her position by the stars would be far down on the list of possibilities.

You are right that a connection needs to be made between position on the ellipsoid and latitude and longitude; but that's straightforward. This leaves open (in the case of the map) of how the map was made. The answer to that, in the pre-satellite era, is given (in part) by the article on triangulation. Accurate triangulations are made on a network of points. These are supplemented by astronomical readings (paying due attention to the article on vertical deflection). And the whole system is subject to a least squares adjustment. Again, in the pre-satellite (and maybe pre-radio) era, there's the question of how to join up the triangulation networks between Europe and N. America and between N. America and Hawaii and, indeed, how to maintain accuracy over thousands of kilometers of N. America. These problems account for the 100m or so discrepancies between different reference ellipsoids (or within a single reference ellipsoid). With satellite positioning those problems are history.

Now to the specific issues with the text in this section.

• The whole business about the vertical deflection is a bit of a straw man. The effect has been well appreciated for at least 200 years. Even boy scouts learn how to position themselves using more reliable means.
• If the goal is to measure short distances (which is what this section seems to focus on), then the subtracting two geographic locations found by astronomical means will lead to huge relative errors (even neglecting the effect of vertical deflection).
• Why the product placement for the Wild T4. Is this an instrument that the average reader will have on hand?
• Won't the user also have to have an accurate chronometer and astronomical tables? These aren't mentioned. To get a longitude reading to 0.0001 deg required that you need to be able to read the chronometer accurate to a 0.01 secs. That sounds like additional expensive equipment our imaginary user will need.
• Will the user easily be able to take accurately synchronized astronomical readings without some sort of means to cancel the effect of the earth's rotation?
• You talk about a "different definition" of latitude and longitude being needed. However, the definition in the first section is just fine. What you mean is a different way of measuring these quantities.
• You way "We would like the T4's axis to point to a point on the Earth's axis near its center". Well, no, I wouldn't. I prefer my instruments to obey the laws of physics. Otherwise I don't know how to interpret their readings.
• You bury the reader in a lot of fussy numbers which obscure the points you are trying to make.
• To compound this problem, you have 4 ways to denotes angles
   19.7323 deg North
19.7244 N
21 deg 18 min 13.889 sec North
21-18-02.54891 N

• The language: "NGS now says ..." and "Eventually they deemed ..." implies a degree of arbitrariness about the process which is misplaced.

I hope this helps explain my concerns. cffk (talk) 14:09, 29 August 2013 (UTC)

"I expect she would either pull out a GPS unit or else bring out a map and locate the position on that"
Does she care how far off the GPS unit's lat-lon is? If she does care, she should wonder how the "correct" lat-lon is defined and determined. An article titled "Geographic Coordinate System" should try to tell her.
She may have noticed that some maps are NAD27 (or Old Hawaiian) and some are NAD83, and the two don't fit together. Which one is "correct", and how did the difference arise? An article titled "Geographic Coordinate System" should try to tell her.
"Even boy scouts learn how to position themselves using more reliable means."
What "reliable means"? They use a GPS, or a map, you mean? And they don't care how correct the GPS/map is, so those means are reliable enough for them?
"subtracting two geographic locations found by astronomical means will lead to huge relative errors (even neglecting the effect of vertical deflection)"
And the article should say that, and the best way to say that is to give an example.
Why the product placement for the Wild T4"
It seems C&GS used the T4, and you need something like that if you're trying to measure astro lat-lon to 0.0001 degree. What's the objection?
"Won't the user also have to have an accurate chronometer...[etc]"
Yes, and they'll need a truck, and fuel for the truck, and flashlights, and batteries... why waste the reader's time telling him all that?
To get a longitude reading to 0.0001 deg required..."
One way or another, 0.0001 deg is about the accuracy C&GS claimed.
Will the user easily be able to take accurately synchronized astronomical readings without..."
Huh?
"You talk about a "different definition" of latitude and longitude being needed. However, the definition in the first section is just fine."
The angle at the center of the earth, you mean-- an angle that we can't measure. If we can't measure it, the definition is "just fine"?
"I prefer my instruments to obey the laws of physics."
Gravity, you mean. That's what we're stuck with, with the T4, and that's not good enough for determining lat-lon for a map.
"you have 4 ways to denotes angles
   19.7323 deg North
19.7244 N
21 deg 18 min 13.889 sec North
21-18-02.54891 N"

I quote NGS lat-lons as NGS gives them; elsewhere might as well use decimal. In each case, it's spelled out in the first instance and shortened in the second instance. Tim Zukas (talk) 17:15, 29 August 2013 (UTC)

"Will the user easily be able to take accurately synchronized astronomical readings without..." Every second you spend getting the cross hairs aligned on a star, the apparent position of the star changes by 15 arcsec. So astronomical instruments typically need a motor to keep the telescope pointing in a constant absolute direction. It's not clear therefore that the T4 would be an appropriate instrument for making such observations. A typical surveyor probably isn't taking star sightings during the course of his field work. cffk (talk) 17:47, 29 August 2013 (UTC)

"It's not clear therefore that the T4 would be an appropriate instrument for making such observations"
I assumed you knew about astro lat-lon. I'm guessing the T4 is appropriate-- in any case, it's what C&GS/NGS used; what do you think would be appropriater? Tim Zukas (talk) 18:56, 29 August 2013 (UTC)
Have you looked at any of the NGS datasheets, like http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=tu1252 or http://www.ngs.noaa.gov/cgi-bin/ds_mark.prl?PidBox=ku3602 ? Tim Zukas (talk) 19:16, 29 August 2013 (UTC)

I suscept that USCGS/NGS would not routinely have used portable instruments for accurate astronomical observations because of their difficulty (compared to observing a "fixed" trig point), the need to conduct the observations as inconvenient times of the day (i.e., at night), the need only for sparse coverage of astronomical fixes. Instead they would relied on observatories (their own or those belonging to a university). Incidentally, the need to compensate for the earth's rotation is most acute for the determination of longitude. Latitude readings would be much easier.

One of the problems with your reference to the T4 and making an astronomical observations at some random spots in Hawaii is that you it sound as though this a plausible scenario. I don't have any specific knowledge one way or another, although it seems unlikely to me. Unless you know that such measurements were routine (preferable with a citation), I think it better that you leave out such imagined details. The point you want to make doesn't depend in this. You could refer to the vertical deflection article which quotes ranges for this quantity and merely state that mistaking the astronomical latitude and longitude for the geographical ones can introduce an error of nearly 1'. cffk (talk) 20:29, 29 August 2013 (UTC)

As you see at

http://docs.lib.noaa.gov/rescue/cgs_tech_memorandum/NOAA%20Technical%20Memorandum%20NOS%20NGS%2018.pdf

NGS did use T4s, and apparently they were portable, and they did claim accuracy to about 0.0001 degree (not that that matters to us).
Take a look at the NGS datasheets. You see how they give lat-lon to 0.00001 second-- do you think they're claiming to know the angle at the center of the earth to 0.00001 second? Tim Zukas (talk) 21:40, 29 August 2013 (UTC)

Thanks for the link to Lake Tahoe observations. Yes I see that T4 was indeed used for astronomical observations, so I stand corrected on that score. "Portable" is probably a rather generous term for the T4 and the associated equipment, however. Referring to Hoskinson and Duerksen, SP 237, on longitude measurements it appears that the party making the observation typically consisted of 4 men and the equipment needed 2 x 1.5 ton trucks to transport and that a reading typically required observations over two nights. So "mobile" might be a more appropriate term. This is in contrast with the equipment needed for an ordinary triangulation which (in my modest experience) involved 2 people, an observer and a recorder, carrying a theodolite and tripod which were light enough that access to the triangulation station could involve modest rock climbing.

Also I will observe that the Lake Tahoe observations specifically called for astronomical coordinates to resolve a border dispute presumably because when the border between California and Nevada was defined that was all that could be determined.

Bottom line is that the T4 and the supposed readings in Hawaii are still a red herring. This just wasn't what was used to determine locations over small areas.

The accuracy of NGS readings... I'm not sure why you need to refer the angles to the center of the earth. The plain meaning of the coordinates is that this is the direction of the normal to the reference ellipsoid or equivalently the position on the ellipsoid. So are the coordinates accurate to 0.00001 arcsec (i.e., less than 1mm)? I believe that this is possible with modern GPS devices (with supplementary ground stations and accumulating the data over several hours). Everything depends on knowing your reference ellipsoid and I suppose this is defined in terms of the main ground stations for GPS. With conventional triangulation, it would not be possible to maintain this accuracy over large distances. So in practice the "reference ellipsoid" would have been a slightly warped ellipsoid. However, in many cases that didn't matter because everything was referred to the triangulation network and the maps made from it; the error over small distances (100km, let's say) would have been pretty small. It would still have made sense to quote the positions of triangulation stations to an accuracy of 1cm-10cm because the error relative to neighboring stations was of that order. cffk (talk) 14:03, 30 August 2013 (UTC)