S6 versus SX10

Today I was using a high precision S6 (1", 1mm + 1 ppm) doing a deformation survey. Also on the same project at the same time was an SX10 (1", 1 mm + 1.5 ppm). The S6 had just come back from a major repair, and had not been used since it came back.?ÿ

So I was seeing high misclosures using the S6 versus old coordinates, we usually see mm, I was seeing up to 8 cm on a long line. So of course I suspected that something happened during the repair, this particular S6 is 14 years old but had always given very consistent high accuracy results. It so happened that we were both setup at the same time at the end of a line about 375 meters long, we each shot at about the same time to the other, the distance agreed exactly (to the mm).?ÿ

When I got home I setup a test, using the two pedestals I have (140 meters) and a third point using a tripod (115 meters). Turned the angle and measured the distances in 16 rounds with each instrument. Forced centering, same tribrachs, prisms, etc.?ÿ

I was surprised at the results. The horizontal angle differed by 0.5", which is 0.25 mm over the 115 m foresight distance. The distances agreed 1.0 mm to the FS (116 m) and 0.9 mm to the BS (140 m). Delta V was 0.6 mm to the BS and 0.9 mm to the FS. So everything is well within spec.?ÿ

Now I gotta wonder what has moved at the dam being monitored. I suspect that one of the points (once as a setup point and once as a backsight point) I was using with the S6, at the?ÿ end of a long wall, has been hit by a barge. Still have another day of surveys to complete. We surveyed it the same way last fall, but using only one instrument (SX10). At least it is good to know that we aren't introducing any significant error by using the two instruments rather than just one. We do 4 rounds to each point, there is a big difference in time, as the SX10 EDM is noticeably quicker, hence the reason we are now mostly using the SX10.?ÿ

An interesting note...When I do the adjustments in Star*Net, the error factor for GPS, horizontal angles (0.15 mgons), and vertical angles (0.15 mgons) always come up near 1. The EDM error factor (1 mm + 1 ppm) always comes in around 0.5 to 0.6. My client wants me to change the distance constant (1 mm) and distance PPM (1 ppm) settings in the adjustment to get the error factor closer to 1.0. However, I am reluctant to do that because we are not able to measure temperature and pressure well enough to get the uncertainty in the PPM correction under 1 ppm.?ÿ?ÿ