I have an old prism that I think is a topcon. I use it for a survey we do 6 times a year that has a long sight (6000 feet) where the seco mini will not return a signal. I am wondering what the model number is and what is the published prism offset? I will shoot it on my baseline to compare against the seco mini prisms (-29), but it is cold outside right now! Actually the problem is I don't have the seco mini prisms here to compare, they are out in the field.?ÿ
Comparing it against GPS from yesterday (long static session, 1 mm centering, GPS on top of prism, 1700 meter line) I get -35 mm. The EDM is 1 mm ?ñ 1 ppm, so that has an uncertainty of a few mm's.?ÿ
I have a couple of those single prism and several triple prism holders in orange stamped Sokkia.
When the bolt pattern is centered on top and bottom, it should be -30mm offset.
I also have one that style in orange that is stamped Sokkia where the bolt patterns are offset near the back of the holder (the end with the setscrew to hole prism) making it 0.035ft closer to the instrument.
Some people refer to that style as Retro prism holders.
I like them to stack on top of each other to view from different directions and the prism is like 65?ñmm and the regular prism are like 55mm.
I concur.?ÿ We have some of those and although I have never checked them, the boss always said they were a 30mm o/s.?ÿ Every not and then I use one for a quick backsight on the robot, and the distance is always 0.1' off, which jives with the 30mm
You could check it indoors. Set up a total station with the bottom of the tribrach resting on a table top (no tripod needed). Level it and power it up. Shoot the prism 8 or 10 feet away, and then tape from the instrument center to the center of the prism mount. Taping that way might not be accurate enough to distinguish between 29mm and 30mm, but it would distinguish between 30mm and zero.
The prism looks like the Topcon/Seco prisms I use, which are -30mm, i.e. the 30 mm needs to be added to the measured distance.
you could set up a base-line consisting of three co-linear points.?ÿ set up your prism on one of the outer most point, and your EDM on the other outer most point.?ÿ Measure and record your distance.?ÿ Set the EDM on the center point and the prism on one of the outer points, and measure and record the distance.?ÿ Repeat the process with the prism moved to the other outer point.?ÿ If the two shorter measurements match the overall, your offset is "0".?ÿ If it is not your offset is half the difference.
