"Slope distance" as measure of distance between target and camera pinhole. Tips on how to accurately measure it
The slope distance is the distance between the target (prism heart) and the sensor (the pinhole aperture). This is used to convert angles on the image to displacements in millimetres, and to accurately compensate for sensor motions when using reference points. Often sub-mm accurate measurement of slope distance is not required, as it concerns a percentage error of the total distance. Note that any error in your slope distance measurement will result in a similar error in the computed displacement. For example, an error of 1 cm in slope distance estimation for a monitoring point at 20 m, is equal to 0.05% which is equivalent to an error of 0.0025 mm error for a 5 mm displacement. Hence, with slope distance typically within the range of 5 to 70 metres, an accuracy of a few centimetres in slope distance will suffice. Below we will give some ways to measure the slope distance:
- The best way to measure the slope distance is using a total station and determine coordinates for the prisms heart and the pinhole aperture. The pinhole aperture is located 1 cm inwards the centre of the front glass and ca. 1.5 cm inwards of the housing. When measuring on the TotaLite housing front on both sides of the pinhole, you can define the centre position (in 2D). Applying a depth offset of 1.5 cm and you can define 3D coordinates of the pinhole aperture.
- A more quick and dirty method is to use a laser distometer. The accuracy of most devices is within limits but pointing them accurately is a challenge, in particular for networks with targets at slope distances larger than 40m. Put the distometer on top of the TotaLite sensor and aim it at the prism (or a colleague holding something in front of the prism that is easier to aim on).
NOTE: As for the accuracy, the slope distance acts as a scale factor to go from measured angles to displacement. The rule of thumb is as follows, if you’re 1% off with your slope distance, your displacement will be 1% off as well.