Figure 1. Illustration of EDM sweep test.


When I used optical theodolites, I checked and adjusted the horizontal and vertical collimation myself. I noticed in my new total station's instruction manual that I only have 30 arc seconds or so of adjustment I can use. The manufacturer requires that I return it to an authorized service center for larger collimation adjustments. Why? Is this a way for my dealer or the manufacturer to get additional repair shop revenue?

Collimation refers to the alignment of the horizontal and vertical cross hairs with the optical axis of the telescope of the total station. The old-fashioned way to check for the error in the vertical cross hair position (the cross hair used for horizontal angle measurement) is to perform the double centering procedure. Modern total stations that have a 1" or 2" least count can be checked by taking several sets of F1 and F2 readings to a well-defined target. The difference between the averaged F1 and F2 horizontal circle readings should be 180°. Any observed difference represents twice the collimation error. The sum of the vertical circle readings should be 360°. The observed difference from this number represents twice the vertical collimation error. Ordinarily, this error can be fixed by simply moving the position of the cross hairs in the telescope. However, because modern total stations are also used to measure distances, proper adjustment of the instrument requires a further collimation adjustment--to also align the axis of the EDM's measuring laser with the optical axis of the telescope. Adjusting the cross hairs without adjusting the EDM collimation causes the cross hairs and EDM lines (of sight) to the target to diverge. This introduces additional errors in the EDM measurement, and thereby also introduces errors in the three dimensional coordinates of the point being observed. Adjusting the axis of the EDM requires sophisticated tools and highly trained technicians. The collimation limit stated in the manufacturer's instruction manual for your total station should be followed strictly.

Many users are unaware of the importance of pointing the center of the EDM's measuring beam at the center of the prism. You can understand this error by performing a sweep test (see Figure 1). Set up your instrument and prism on a tripod approximately 100 to 150 ft apart, then point the cross hairs carefully at the target, take a distance reading (slope distance) and record it. Now, with the instrument clamped, use the tangent screw to move the vertical cross hair laterally 30"; take and record another distance reading. Repeat this at 30" intervals. You will observe a variation in the distance to the prism. While you don't know how well the EDM is collimated with the cross hairs of your instrument, by repeating this test right, left, up and down, you can get some idea of the collimation.

I've only been using GPS at my company for about three years. Recently, we surveyed a large network that required us to use addtional receivers from the same manufacturer that we hadn't used in a very long time. I noticed that there were north arrows on the ground planes, but nobody was able to give me a good reason why they were there. When we did the surveys, I did align those arrows with north, but I don't know why. Can you help?

The measuring point of a GPS antenna is what is referred to as the phase center. This phase center is a theoretical point that is not located on the antenna element--usually it is above it. With all antennae, this phase center location changes as the azimuth and elevation of the angle change. Furthermore, especially with early GPS antennae, the phase center movement varied across the area of the antenna. Since precise GPS surveying measurements require the differential analysis of measurements across two or more antennae, aligning all antennae with north increases the accuracy of the vectors determined between each pair of GPS receivers. This is because the phase center movement for each antenna for a given constellation of satellites will be the same. The process of differencing measurements, because the phase center error at each antenna is the same, results in cancellation of many of the phase center errors. Why don't GPS antennae today have the north arrows? Improvements in GPS antenna design have significantly decreased the magnitude of phase center shifts so that orienting all antennae in the same direction for a session has marginal improvement. While there might be some small improvements possible, they are so small that they are likely to be overshadowed by other sources of error.

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