When I set a backsight, shoot a few rounds in F1/F2 at say, 300 feet. Things are looking normal.
Now when I try to shoot rounds on a point, at say 100' or less, I get a message saying Zenith and/or horizontal angle difference between F1/F2 is 30" or more.
Sometimes it's a minute or more difference.
Does anyone know why this happens? I usually shoot a fixed single prism with a 30mm offset in passive mode. I'll use auto lock to lock on the prism, and then turn it to passive for the shots.
But even if I shoot it all in autolock, I get the same thing.
The shot ends up being good, even if the error is quite a bit. As in, I can switch set ups, backsight and stake points out, and they're within tolerance. But I just don't understand how a shot of 100' or less with F1/F2 could produce these errors, while one farther away doesn't.
You are seeing large differences at short distances in angular measurements in passive mode. Do the math to see what that distance error would be for 30 seconds in 100 ft first. Then compare that to a manual sighting at same distance. Then compare that to your instrument specs. sighting and angular accuracy of the instrument itself. 30 seconds in 100 ft is nothing to worry about once you do the math. Heck with a 5 second S5 when turning rounds to a property corner the other day it was only 75 ft +/- away I had the 360 on a pole set up with bi pod had my BS about 350 and FS at 350 ish and a cross check to another control point at 200 ft ish as we were going around the property I was doing all rounds F1/F2 at same time in one set up and had almost a minute in ZA and around 45 to 50 seconds in HZ to the short distance. You can use sine and or cos trig to do a little rough check on what you are seeing. I had a young crew chief that is learning these principles as well.
Some food for thought. The human eye is good to about 2 seconds on average, A 3 second gun in calibration (this depends on ISO vs DIN specs) is not equivalent to 3 seconds until two rounds of F1/F2 have been met that's when you get the 3 seconds if the measured angles fall within the mean tolerances. The F1/F2 takes error from the instrument the extra sightings help reduce the pointing error. In older instruments that read a plate vernier manually we could unlock and rotate the plate to re measure direct and reverse so we could mitigate the error in the plates. Even fully digital Total stations that read the plates for us we could still unlock those and do more rounds trying to measure in each quadrant of that plate. Now many instruments today do not have plates but we are still doing some of the same historical principles to account for instrument error and sighting error Now it might be the gun sighting or locking itself vs our human eyeballs. Which mine are not as good as they once were.
A good rule to have in your head is 1 second in a mile. broken down to half a mile 1/4 mile 1000 ft 500 ft 100 ft Then for average everyday distances look at the 5 second 10 seconds 30 and 1 minute at distances of say 50 ft through 500 ft . Now keep in mind 30 seconds on your 100 ft shot if you continue on traversing this little math game can help you in understanding angular mis closures for longer loops with those errors on short sites. For especially typical classical traverse loops. Keep a good site distance averaged out BS and FS good geometry even if it means a little extra cutting on line. If you get this down you will be able to almost predict your closures on jobs. And yes sometimes some jobs are just tough to make everything work perfectly. Sometimes its better to have a main traverse balanced and smaller interior fly traverse to get into those tight spots. I hope this helps. Also keep the instrument in calibration the different routines. Don't just set it up from a nice cold room and start turning angles first thing on a hot day let it acclimate a little to the temperature parts move etc..
I’ll use auto lock to lock on the prism, and then turn it to passive for the shots.
Why would you turn off autolock??? That instrument can point a lot better than you or I. I used to be pretty darn good with a manual TS, but once I switched to a robotic with autolock, there was no way I could turn better sets than a calibrated TS using autolock.
But even if I shoot it all in autolock, I get the same thing.
Now that is definitely not typical. While short sights will definitely return a warning more often, if you're seeing similar split results to manual readings, and all the time, that's a red flag that the instrument needs to be collimated. I would run Check & Adjust and go through all the steps and see if the corrections change...
I found that calibrating your instrument with the prism you are going to use at the distance you want yields best results, at least with Leica robots.
Not sure if the OP is using Leica, but with Leica both 'lock' and 'ATR' passively site the prism. ATR, auto target recognition, sites a prism in its field of view only when a distance is measured while the 'lock' mode constantly tracks the prism. ATR is a little more precise than lock and is preferred to lock of turning angles. If the OP is using Leica then his statement makes sense.
As others have noted, there is no way you can site as precisely as the instrument.
