Curious to know if anyone has an opinion on this. I've debated with a colleague that I've seen vertical error introduced when over 500' using a tiny 360?ø prism, and the response was "if it reflects back it's good". I didn't agree. Any thoughts?
Is it autolock (i.e. robotic)?
If not, the vertical error depends on where you point and how accurate the HI is. It may be that at a longer distance you cannot "see" the center of the prism.
Another issue (but not restricted to small prisms)is that if you are close to the ground, then vertical refraction is a guess. Data collectors usually apply a value of around 0.13, which may not be even close for grazing lines, or even lines that aren't grazing but are parallel to the ground.
John Hamilton, post: 449282, member: 640 wrote: Is it autolock (i.e. robotic)?
Thanks-
Correct- I'm referring to robotic/ autolock
Is there a way to align the ccd camera with the cross hairs.
It's not only a distance measurement. The angle is critical in trigonometric leveling. If the vertical collimation is off in the gun it will not give good vertical results no matter how good the distance measurement is. Same goes for rod height and instrument height measurements. If those are wrong the result will be wrong.
Run a loop with 500'+ long shots.
Run another one with 250'-ish shots.
Report results here.
Probably be fine, if you're a flat earther... The rest of us have to deal with curvature and refraction at that distance.
D+R will take care of vertical discrepancies due to cross hairs, vertical circle, etc. But not refraction. Reciprocal observations (near-simultaneous) can help with that issue.
Survey Chad, post: 449278, member: 12063 wrote: Curious to know if anyone has an opinion on this. I've debated with a colleague that I've seen vertical error introduced when over 500' using a tiny 360?ø prism, and the response was "if it reflects back it's good". I didn't agree. Any thoughts?
If you get different answers with the two prisms, after allowing for the prism height, then maybe there is something wrong. Have you isolated the problem to the small 360 though?
I missed the original post that it was a 360?ø prism. There can definitely be some error introduced by not having a prism pointed directly back at the instrument. Seems like it would be more H than V, but it could be V as well.
squowse, post: 449351, member: 7109 wrote: If you get different answers with the two prisms, after allowing for the prism height, then maybe there is something wrong. Have you isolated the problem to the small 360 though?
Good point. At least do some experimenting with using the two different types of prisms. You can at least see if you're getting the same measured vertical difference between monuments. That doesn't cover all the other sources for error, but you could at least answer the question about whether a different type of prism will affect your results.
There is no question in my mind - and the data backs me up - that I get better "splits" of angles when using a standard 30mm glass over using a 360 prism.
Thanks for all the responses. I think Mark may have answered my question above. I was confirming my own findings with a 30 mm over an older spectra focus 10 instrument with a mini 360?ø prism, which is much smaller than the MT1000 . Only difference is that we had to run with the 30 mm in standard mode running it like your non-robotic total station since this one wont work as a robot with a passive target. My biggest issue with the 30 mm taking a 1000' shot, it's very difficult to get perfectly centered on it. I see people do this all the time, but I suppose the best way would be another trav.
Per the "flat-earther" comment (thanks for the laugh) and beyond the apparent sarcasm, revisiting curvature and refraction was very helpful. I prefer shorter shots in general (under 300').
[USER=12063]@Survey Chad[/USER] you're definitely on a better option at max 300' if you're interested in decent vertical,
Pushing to 1000' is asking for trouble due to uncertainty in refraction. If people say they can do it then good luck to them.
I'm a firm believer in the value of EDM levelling, BUT only with reciprocal readings and short lines. Length is governed by atmospherics which one can get a feel for.
A quick double ended reciprocal observation soon gives the expected refraction differences.
I always calibrate my instrument at the start of any job where verticals are a vital part of the survey, along with ensuring legs are tight and screwed together properly.
It can be very rewarding to finish a closed EDM traverse and find elevation misclose minimal.
