A level is an optical instrument used to establish or verify points in the same horizontal plane. It is used in surveying and building with a leveling staff to measure height differences and to transfer, measure, and set heights.
In 1832, English civil engineer William Gravatt, who had worked with Marc Isambard Brunel and his son Isambard on the Thames Tunnel, was commissioned by Mr. H.R. Palmer to examine a scheme for the South Eastern Railway's route from London to Dover. Forced to use the then conventional Y level during the work, Gravatt devised the more transportable and easier to use dumpy level.
Both the Y-level, and the dumpy level, consist of a precision telescope with crosshairs.
The level is held in two brass arms, which are part of a mount, supported by a tripod. The Y-level can be removed from its two Y shaped brass arms, in which it rests without being secured. The dumpy level is permanently secured to its two support arms.
The mount allows the level to freely rotate 360° in a plane. It is the responsibility of the surveyor to adjust the tripod such that the plane is horizontal. The tripod has a screw length-adjustment in each leg. These allow the surveyor to adjust the position of the mount such that the rotational plane is horizontal. The surveyor does this with the use of a bull's eye level built into the mount.
The surveyor looks through the eyepiece of the telescope while an assistant holds a level staff which is a graduated, rigid, measuring stick marked off in inches or centimeters. The level staff is placed with one end on the ground at the point where the mesurement is to be made. The telescope is swiveled until the level staff is in the crosshairs. The graduation under the crosshairs is recorded, and a stake is placed where the level staff touched the ground marked with this value.
Measurement generally starts from a surveying benchmark, which is a permanent marker giving a known height determined by a previous survey. If such a marker is not available then an arbitrary point is chosen, with an assumed height.
After careful setup of the level, the height of the crosshairs is determined. Then the assistant surveyor holds a graduated staff vertical at the point under measurement. The surveyor looks through the eyepiece of the level, as they swivel the level until the graduated stick is in the crosshairs. The surveyor reecords the graduation seen in the crosshairs. As the level is constrained to rotate in the horizontal plane the recorded value is the height above, or below, that of the level. Measurement starts from a benchmark with known height determined by a previous survey. If that is not possible, then an arbitrary point with an assumed height is used.
A site survey is conducted to produce a level map of the land prior to the initial design phase of a construction project. As construction begins, the level surveyor sets posts in the ground marked with their exact height, and placed where the arcitech, or civil engineer requests, to interface with the blueprints of the project.
The term dumpy level (also builder's level) endures despite the evolution in design.
A dumpy level is an older-style instrument that requires skill to set accurately. The instrument requires to be set level (see spirit level) in each quadrant to ensure it is accurate through a full 360° traverse. Some dumpy levels will have a bubble level intrinsic to their design which ensures an accurate level.
An alternative to the dumpy level and one that was often used by surveyors, where greater accuracy and error checking was required, is a tilting level. This instrument allows the telescope to be effectively flipped through 180°, without rotating the head. The telescope is hinged to one side of the instrument's axis; flipping it involves lifting to the other side of the central axis (thereby inverting the telescope). This action effectively cancels out any errors introduced by poor setup procedure or errors in the instrument's adjustment. As an example, the identical effect can be had with a standard builder's level by rotating it through 180° and comparing the difference between spirit level bubble positions.
An automatic level, self-levelling level, or builder's auto level includes an internal compensator mechanism (a swinging prism) that, when set close to level, automatically removes any remaining variation. This reduces the need to set the instrument truly level, as with a dumpy or tilting level. Self-levelling instruments are the preferred instrument on building sites, construction, and during surveying due to ease of use and rapid setup time.
A digital electronic level is also set level on a tripod and reads a bar-coded staff using electronic laser methods. The height of the staff where the level beam crosses the staff is shown on a digital display. This type of level removes interpolation of graduation by a person, thus removing a source of error and increasing accuracy. During night time, the dumpy level is used in conjunction with an auto cross laser for accurate scale readings.
In popular culture
In the first chapter of Thomas Hardy's 1887 novel The Woodlanders, the narrator states, "He knew every subtle incline of the ten miles of ground between Abbot's Cernel and Sherton—the market town to which he journeyed—as accurately as any surveyor could have learnt it by a Dumpy level."
In the online game World of Warcraft, there is a quest in Wetlands given by Surveyor Thurdan to retrieve his lost dumpy level. He even comments on the name, saying, "I didn't name the bloody thing, alright? Go look it up!"