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INCREASING THE PRECISION OF THEODOLITE TRACKING: MODIFIED TECHNIQUE TO CALCULATE THE ALTITUDE OF LAND‐BASED OBSERVATION SITES
Author(s) -
Bailey H.,
Lusseau D.
Publication year - 2004
Publication title -
marine mammal science
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.723
H-Index - 78
eISSN - 1748-7692
pISSN - 0824-0469
DOI - 10.1111/j.1748-7692.2004.tb01199.x
Subject(s) - shire , theodolite , geography , citation , library science , archaeology , cartography , computer science
Knowledge of the location and speed of cetaceans is necessary to address a wide range of ecological and behavioral questions. Estimating the locations of cetaceans at sea can be extremely challenging, particularly where there are few notable landmarks or buoys to use as reference points. This has led to the increasing use of a surveyor’s theodolite to track animals from land in a non-invasive manner (e.g., Acevedo 1991, Harzen 1998, Bejder et al. 1999, Williams et al. 2002). A theodolite provides both the horizontal and vertical angle to a target. The vertical angle is given relative to gravity and the horizontal angle relative to a selected reference point of known position. These angles, together with the altitude of the theodolite and the position of the horizontal reference point, can be used to calculate the position of the target in (x,y) coordinates. The accuracy of these coordinates will depend on the accuracy inherent to the theodolite and that to which the altitude of the theodolite is known. In some situations the altitude of the theodolite can be calculated using a surveying landmark of known altitude and location. However in remote field sites, such landmarks are often unavailable. Würsig et al. (1991) overcame this by providing a technique to calculate the altitude of the land-based observation site in the absence of such surveying landmarks (Fig. 1). This technique involves locating two points near the waterline with the same vertical angles (and thus equal distance from the theodolite). The distance between the theodolite and the mid-point between the two waterline sites can then be determined and consequently the altitude of the theodolite using trigonometry. However, practicalities in the field often make it difficult to position the points at equal distance from the theodolite and to place them near the waterline. This may be because of the rugged nature of the shoreline terrain or the orientation of the beach. It is also possible, especially on rocky shores, that two points of different height have the same vertical angle, but be unequal distances from the theodolite. Errors in these measurements will consequently reduce the precision with which the height of the theodolite is known. Lack of precision in the estimates of theodolite altitude will inherently lead to increased errors in the estimated location and hence distance travelled and speed of the target.