|Developer(s)||Martin Budaj and Stacho Mudrák|
5.3.16 / December 30, 2014
|Written in||Metapost, TeX, C++Tcl/Tk|
|Operating system||Microsoft Windows, Mac OS X and Unix-like|
|Available in||13 languages|
|License||GNU General Public License|
Therion is an open source cave surveying software package which is designed to: process survey data; generate maps and 3D models of caves; and archive  the data describing the cave and the history of exploration.
Therion was developed by Slovakian cavers Martin Budaj and Stacho Mudrak but is available in English. It runs on a wide variety of platforms including Linux, Windows and Mac OS X. Therion is available as part of standard distribution of Debian and Ubuntu (operating system) Linux distributions.
It is free software, released under the terms of GNU GPL, with source code available. It does not require any other commercial software to run. The format of all files is human readable plain text (excluding 3D models), which semantically describe the cave, and are compiled by the program into various output forms such as 2D PDF or SVG maps, or 3D models. Other files like map overlays, terrain models and pictures can be incorporated into the output. A graphical editor is provided to help with the drawing process, and a 3D viewer ('loch') for viewing the models. Survex is used for the centreline error distribution if installed.
The separation of drawing/semantic data entry and output rendering makes the program both complex to learn to use, but also capable of dealing with cave systems still being explored, where new finds and surveys need the drawing to morph to fit. It also allows a survey to be rendered with different national cave-symbol sets.
The (UK-based) Cave Surveying Group has been using Therion along with PocketTopo and DistoXs to train cavers and improve the quality of surveying by the use of real time measurement that is available in Paperless surveying. 
Therion is used in several large projects for the documentation of cave systems.   It is listed among 10 of the Best Free Linux Earth Science Software. It was used in a number of scientific projects.
Not everyone finds the results satisfying: "Beginning in 2003, the Mulu Caves Project attempted to use Therion on several cave surveys. After many attempts over several years by many different cave surveyors, one of whom was closely affiliated with the software itself, no aesthetically pleasing results were produced."
The unmatched feature of Therion software among other software tools for cave surveying is straightforward creating of 3D presentation for WEB pages. One may export 3D model from Therion to 3D format .lox, open it in Therion's 3D viewer Loch and export data as VTK. ParaView software is able to open such data and export them in WebGL format.
To draw the maps the Speleo-Vulcain group from France is using Visual Topo for simple systems. However, because of the difficulties to build a rigorous synthesis and to update the survey of the complex Jean-Bernard System, they passed to the open source software Therion.
Therion was analysed and used in thesis of Eliška Rákocy
Two articles in the Annual Report of Cave Administration of the Czech Republic 2010 describe how Therion was used to document the Javoříčko Caves show cave.
Described as "State of the Art Cave-Drawing software" Therion was reviewed in Compass, the Cave Surveying Journal of The British Cave Research Association. The review covered installation, use and development of the software. Since this article, written in 2004, Therion has continued to be developed and is now considered to be the most capable cave drawing software available.
- "New Version of Therion Released: 5.3.13". Caving News. 2014.
- UBSS and MCG. "Cave Registry Data Archive". British Caving Association. Retrieved December 16, 2011.
- UBSS. "UBSS Survey Archive". UBSS. Retrieved April 23, 2013.
- Budaj, Martin; Mudrák, Stacho (2008). "Therion – Digital Cave Maps" (PDF). Spelunca. Spelunca Mémoires. French Federation of Speleology. 33: 138–141. ISSN 0249-0544. Retrieved December 30, 2011.
- "Surveying Training". CSG Paperless Surveying Training. Wildplaces. Retrieved April 25, 2013.
- "Therion Survey". Mulu Caves Project. The Mulu Caves Project. Retrieved January 3, 2012.
- Dave Clucas. "Mulu Caves 2007". Retrieved December 31, 2011.
- "Groupe Speleo Vulcain (France) - Surveys made by the club". Retrieved December 1, 2011.
- "Groupe Speleo Vulcain (France) - Expeditions in Borneo, Indonesia, 2010". Retrieved December 31, 2011.
- "Cheddar Catchment". Cheddar Catchment. MCG and UBSS. Retrieved April 25, 2013.
- Atkinson, A; Moody, A.A.D (2012). "Palmer's Cave: History and exploration.". UBSS Proceedings. 25 (3): 319–326.
- "10 of the Best Free Linux Earth Science Software (2011)". LinuxLinks.com. Retrieved December 31, 2011.
- P Schuchardt, DA Bowman (2007). "The benefits of immersion for spatial understanding of complex underground cave systems". Proceedings of the 2007 ACM symposium on Virtual reality software and technology.
- M Boggus, R Crawfis (2010). "Prismfields: a framework for interactive modeling of three dimensional caves". Advances in Visual Computing: 213–221. doi:10.1007/978-3-642-17274-8_21.
- Rákocy, Eliška. "Mapovací práce v amatérské speleologii - diplomová práce" [Mapping in non-professional speleology - thesis] (PDF). Prague, Czech Republic: Czech Technical University in Prague, Faculty of Civil Engineering. Retrieved April 28, 2013.
- Vybíral, Stanislav (2010). "Javoříčské jeskyně" [The Javoříčko Caves]: 16. and Bohanes, Radek; Svojanovský, Tomáš (2010). "Speleologický rok 2010 v Javoříčských jeskyních" [Speleological year in the Javoříčko Caves]: 30, 31. "Zpřístupněné jeskyně 2010" (PDF). ročenka Správy jeskyní české republiky (in Czech). Průhonice, Czech Republic: Správa jeskyní České republiky, Květnové nám. 3, 252 43 Průhonice. 2010. ISBN 978-80-87309-10-0. Retrieved April 28, 2013.
- BCRA Cave Surveying Group, Compass Points 33, March 2004, pp 5-12
- Bernard, Joey (2014). "Spelunking with Linux" (digital). Linux Journal. Belltown Media, Inc. of Houston, Texas (March 2014): 23–28. ISSN 1075-3583. Retrieved March 2014. Check date values in:
- Petr, Radek (2006). "Speleologické mapování" [Speleological mapping]. Zeměměřič (in Czech). Prague, Czech Republic: Zeměměřič (Klaudian Praha, s.r.o.) (06+07/2006). Retrieved December 31, 2011.
- Homepage of Therion (software)
- French wiki page about Therion (software)
- A review from Linuxlinks.com
- Paperless Surveying Disto X tool and PocketTopo software for in-cave drawing maps. Data could be exported to Therion - check User Guide.
- TopoDroid free software for Android platform for in-cave surveying. Surveying data and maps are in Therion format.
- Auriga free software for Palm OS for in-cave surveying with bidirectional data exchange provided with Compass, Survex, Therion, GH Topo/Toporobot, VisualTopo and Walls.
- BCRA Cave Surveying Group, Compass Points 33.
- Short review of Therion and other survey software packages (German).