Determining the position of animals at sea can be particularly difficult and yet, accurate range and position of animals at sea are essential to answer a wide range of biological questions. Shore-based theodolite techniques have been used in a number of studies to examine marine mammal movement patterns and habitat use, offering reliable position measurements. In this study we explored the accuracy of theodolite measurements by comparing positional information of the same objects using two independent techniques: a shore-based theodolite station and an onboard GPS over a range of 25？km from the shore-based station. The technique was developed to study the habitat use of sperm whales (Physeter macrocephalus) off Kaikoura, New Zealand. We observed that the position accuracy fell rapidly with an increase in range from the shore-based station. Results showed that the horizontal angle was accurately determined, but this was not the case for the vertical angle. We calibrated the position of objects at sea with a regression-based correction to fit the difference in distance between simultaneously recorded theodolite fixes and GPS positions. This approach revealed the necessity to calibrate theodolite measurements with objects at sea of known position. 1. Introduction Knowing the accurate geographical position is essential for studying the spatial behaviour of animals at sea. Accurate positional data can answer a wide range of biological questions related to their movement patterns, habitat use, and the effects of human activities [1, 2]. A number of tracking methods can be used in order to obtain the position of animals at sea including recoverable data loggers, satellite tags [3–8], acoustic monitoring [9–12], and boat surveys [13–16]. All of these methods require expensive equipment, and time to collect data and the observer can be a source of potential disturbance [17]. As a result, the geographical coordinates of animals at sea are ideally determined from shore using a surveyor’s theodolite, first introduced by Roger Paine in 1972 (Describe in Würsig et al. 1991) [18]. Shore-based theodolite tracking is a technique offering an inexpensive and nondisturbing alternative to other tracking techniques. By tracking animals at sea from land, a small amount of equipment is required and a larger area can be monitored in a shorter amount of time compared to boat-based station. The theodolite readings (horizontal and vertical angle) can be converted into longitude and latitude when exact theodolite position and height above sea level are known [18, 19]. However,
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