Per my other posts, we purchased an older but practically new Geodimeter robot. We do not need anything super accurate but decided to run some tests.
What I did was setup up 3 control points in a field with rebar and center punched them for an alignment point. I set all points roughly level by setting my gun on one point, then setting my RMT at the same height as the gun, then set the other two points to about HA 90.00 +/- .02.
From one point, I shot in the other two, collected their HD and the angle between them and calculated the 3rd leg of the triangle. Then I moved to another point and shot that leg directly. Because we did not use a bipod on the RMT/pole, I was expecting a little variance, which would be fine.
However, I calculated the 3rd leg at 114.405ft, and the direct shot was 114.238ft. That seems to be a LOT off.
I tried to offset this a bit by entering and SD offset but all I get is legs that are all longer but the variation seems to be consistent within reason.
So at this point I led to believe that either I errored in my field layout somewhere and missed something, or my gun HA could be off? Now, I did not shoot these in two faces! Is that a big issue? I did not do two face testing but did some trials at another site and did not detect much issue there. maybe I not shoot far enough to see any error?
So, did I miss a critical step here? I am certainly not a mile off but 3" is no where near accurate enough to be acceptable IMO.
bob james, post: 422714, member: 12050 wrote: Per my other posts, we purchased an older but practically new Geodimeter robot. We do not need anything super accurate but decided to run some tests.
What I did was setup up 3 control points in a field with rebar and center punched them for an alignment point. I set all points roughly level by setting my gun on one point, then setting my RMT at the same height as the gun, then set the other two points to about HA 90.00 +/- .02.
From one point, I shot in the other two, collected their HD and the angle between them and calculated the 3rd leg of the triangle. Then I moved to another point and shot that leg directly. Because we did not use a bipod on the RMT/pole, I was expecting a little variance, which would be fine.
However, I calculated the 3rd leg at 114.405ft, and the direct shot was 114.238ft. That seems to be a LOT off.
I tried to offset this a bit by entering and SD offset but all I get is legs that are all longer but the variation seems to be consistent within reason.
So at this point I led to believe that either I errored in my field layout somewhere and missed something, or my gun HA could be off? Now, I did not shoot these in two faces! Is that a big issue? I did not do two face testing but did some trials at another site and did not detect much issue there. maybe I not shoot far enough to see any error?
So, did I miss a critical step here? I am certainly not a mile off but 3" is no where near accurate enough to be acceptable IMO.
Sounds like a wrong prism offset.
Now, I am green here so excuse my ignorance but the prism offset would affect the SD, correct? This is basically the difference of where the center in the prism is vs where the center of the RMT mounted on the pole is? I seem to remember this being set at 0, with the factory and correct RMT for the gun?
Even if the SD was off, the shot distance would only be affected with the offset? And since I leveled everything to always ensure my RMT center line was set to my gun, there was effectively vertical angle.
EDIT to my first post, I mean that I set my "VA" to 90.00*. IE, I setup the gun over a point, then measured the distance from top of point to center axis of gun, and then put that same measurement on the pole at the engraved hash on the side of the RMT. This ensured that I was not dealing with any significant VA.
First thing to do is re do the test and use tripods or at the very least bipods on the shots.
I wouldn't change any other variable before doing that first. If you left the rebar sticking up, perhaps you could turn the angles directly to the marks?? Just a thought.
If I was attempting to determine the length of the third leg using the method you described with any significant accuracy, I would certainly be measuring direct/reverse angles to compensate for angular errors present in the total station.
I'd be running some simple field checks to verify that the instrument is in adjustment. First thing would be to measure some direct/reverse angles and check the splits.
Geodimeter had a course and fine dist. measurement mode. Make sure you are using the fine measurement mode.
Andy, can you better explain "turn the angles directly to the marks"?
I did leave the field setup as I figured I would have to return for more testing.
Do you think a 2 faced measurement should be required here?
I back calculated a couple things to see how I could get my measurements to work out and it looks like maybe .100* or so would do it. I am hoping this is just poor operator procedure or I did not have something set right on the gun. However, shooting back to my first station point from the second point showed an identical HD, which is expected.
I further verified my weather conditions and was getting changes of 4ppm to 0ppm depending on what I entered, but I looked up how this would affect my shots and looked like at these distances, it would take more than 100ppm to really see much change. Furthermore, after changing the SD offset to lie to it, it seems that if all the distances change by the same amount, it really will not change much.
imaudigger, post: 422728, member: 7286 wrote: If I was attempting to determine the length of the third leg using the method you described with any significant accuracy, I would certainly be measuring direct/reverse angles to compensate for angular errors present in the total station.
I'd be running some simple field checks to verify that the instrument is in adjustment. First thing would be to measure some direct/reverse angles and check the splits.
Geodimeter had a course and fine dist. measurement mode. Make sure you are using the fine measurement mode.
I believe you are referring the "D bar mode"? I was not using that.
I was only using the "autotrack" function of the gun too as that is how I would typically use it and seems they are very accurate anyway. The way this RMT is setup, the crosshairs in the gun are not coincident with the laser so the prism sits about 2" above the crosshairs. I guess it is entirely possible that I missing a setting in the gun and there is a discrepancy in this area.
Yes, chasing 4 PPM on a pressure adjustment seems unlikely to help at this point, as you are holding the range pole by hand, if I'm reading this right.... You are using an unknown instrument so eliminate the largest error sources first. Then worry about 4ppm tweaks. The Geodimeter is an incredible instrument, but if your level bubbles on the range pole are out, or perhaps, your field skills at holding it level aren't super sharp... (no offense intended) that would be where I'd start working on it.
Turning doubles would help you determine if your Geo needs adjusting, for sure.
I would have the instrument calibrated by a trained technician...
First thing to do with any instrument/prism combination is to check the prism offset. It's easy to do, I do it at the conference table. You can do it with tripods if you want, all you have to do is measure between the prism and instrument with a tape then with the EDM. Those should match. The two don't need to be very far apart, I like doing it at the conference table so I can have the documents handy and all the prisms available so you can see just what settings need to be imputed and how that all works.
Mike Moe has a very good answer.
Andy J, post: 422733, member: 44 wrote: Yes, chasing 4 PPM on a pressure adjustment seems unlikely to help at this point, as you are holding the range pole by hand, if I'm reading this right.... You are using an unknown instrument so eliminate the largest error sources first. Then worry about 4ppm tweaks. The Geodimeter is an incredible instrument, but if your level bubbles on the range pole are out, or perhaps, your field skills at holding it level aren't super sharp... (no offense intended) that would be where I'd start working on it.
Turning doubles would help you determine if your Geo needs adjusting, for sure.
I just realized that I had a note to myself that I needed to do a proper collimation calibration at a proper distance. I think I previously calibrated at only 50ft! That could be a massive error once I shoot any real distance.
I think I will get my pole on a tripod and so more testing.
Basically as I recall, when I was testing at short distance, I would shoot a point, then flip the gun and my horizontal alignment was off.
I will get on this and report back.
thanks for the help!
bob james, post: 422743, member: 12050 wrote: I just realized that I had a note to myself that I needed to do a proper collimation calibration at a proper distance. I think I previously calibrated at only 50ft! That could be a massive error once I shoot any real distance.
I think I will get my pole on a tripod and so more testing.
Basically as I recall, when I was testing at short distance, I would shoot a point, then flip the gun and my horizontal alignment was off.
I will get on this and report back.
thanks for the help!
Check it on an NGS Baseline.
I wouldn't worry about the NGS Baseline at this time. I'd save that until he has a better grip on what's going on. IMHO.
Man, you guys are going to have to throw some ideas or better testing procedures my way. I cannot make any sense of my data.
I setup again today. The first thing I did was shoot out to 660ft for a collimation test. Everything was done with a tripod this time. My previous settings for HA was .0012, after the collimation at that distance, it was .0005. I switched face and was nearly dead on.
So I moved back to testing my 3 triangulated pins. I realized that my pole needed the plumb bubble tuned by seeing the obvious angled pole so we used the bubble on the RMT top. That got things straightened right up. I felt very good that we were getting this nailed down.
I swapped faces several times to double check how things looked and realized you have to wait maybe 5sec for the gun to calm down and dial itself it. You can watch it work in the telescope.
However, after all the data was in, I was still off on my calculated leg lengths. I am almost convinced I am just doing it all wrong. When I start to back calculate things, I realize that either my gun is WAY off, I am measuring things WAY wrong, or I am just needing a better way to do this. When I run back calculations, in order to make things technically right, my angles might need to be as much as .2-.4* higher. Now, I might buy that except the gun will do a 180 flip, read back 180* and be dead on its target, plus when I calculate how far .2-.4* of HA might be in horizontal position, we are talking inches off the target. There is no way I am off on my sightings more than .125". I am using autolock, then confirming the gun is sighted properly to the RMT, it is. I even turned it off and manually shot, same result.
So then I end up back looking at the EDM. However, if I consider that the EDM is giving me a false distance, I can enter any shot distance I want with my triangulation and it hardly changes anything because I have to add whatever the error might be to any shot I might have taken.
I am completely puzzled here because it seems that by the math, I would have to have a huge blaring error somewhere that I am missing. I just don't think if a gun has a major HA calibration issue, it would return a collimation value of .0005.
On the plus side, I did find out that I have an NGS baseline about 20 miles from me with shots out to 1400m. It sounds like a crapshoot though because at least one mark was damaged and no one can tell me which, lol.
I did do some basic EDM testing months ago by measuring a few points on the ground and it seemed to return reasonable values.
My ignorance and apology! I would delete above posts if I could.
It just hit me..... That gun confuses me because there is no colons to separate the minutes and seconds and I am used to working in degrees only. So................I forgot to convert my units back to degrees. Once I saw those values, I knew where I screwed up and were about the .2-.4* I was looking for....
Now I have triangulated one line to be within .108" of calculated vs directly measured. The other was within .300". This is using a 6ft pole and a kinda cheesy tripod.
Does this gel with everyone in terms of reasonable accuracy? Playing with the numbers I realized that I probably could have goofed slightly on my numbers or setup just a little better and got even better results but I think the point of the exercise was to ensure the gun was accurate.
Maybe a couple minor questions though.
1. The way I setup, I roughly set tripod over pin, then set gun but loose, then play a dancing act to get tripod half level while over the pin. Make adjustments, and do it all 10 more times before I finally get there. Is there an easier way????? OMG!
2. I adjust the tribrach bubble to level while ensuring I am centered on my mark. Once I engage the compensator, I see the digital floats move. out of center a bit. Should I turn the tribrach screws after the gun calibrates and make those fine adjustments or is it just fine being only slightly out? Typically once I dial in the tribrach bubble, the digital level shows perfect until the fine digital adjust kicks in.
bob james, post: 422862, member: 12050 wrote: 1. The way I setup, I roughly set tripod over pin, then set gun but loose, then play a dancing act to get tripod half level while over the pin. Make adjustments, and do it all 10 more times before I finally get there. Is there an easier way????? OMG!
I'm sorry I cant imagine what you are trying to do to level the tripod. You can try going youtube, there are many basic steps on how to set and level instruments on a tripod over a point.
Thanks. I did look up some vids and appears I do everything I should, just probably don't do it every day like the pros.
Well, the problems you seem to be having are solvable.
"If it was built by a man, it can be fixed by a man".
For starters:
Set it up, and shoot a prism, 10 feet away. NOW check that with a box tape. Get a good mechanism to MEASURE it. I'm serious. Do this.
Prisms, have offsets. Total Stations have offsets.
These offsets are CHANGEABLE. They are settings.
AFTER you confirm that EDM reads 10.000' and box tape reads 10.000' then go to step B.
Step B:
Do all the above, THROUGH the data collector. DATA COLLECTORS can have offsets too. And, scale factors, for State Plane work etc.
Until you do that box tape vs edm check, this stuff will tend to obfuscate the problem.
Differing prisms have differing offsets. Do all this with ONE prism. (later, compare them)
N