I am trying to post process some gps observations that I observed a few weeks ago. I am using Topcon Tools V8.2.
I have two points onsite that I set a Topcon Hiper on, and the second one had a Promark 3 unit on it. The files were several hours long. All observations were occupied using 2 meter rods and bipods.
I have a pair of points to the northwest that I occupied with another Hiper and Promark 3. This pair of points were set to side shoot in azimuth marks for an NGS horizontal control point. (This work was done when the government was shut down). I broke both units down, and moved to my next ocupation point.
My last point was on an offset nail that I set for vertical control. I set the nail about 50 feet southeast of an NGS vertical control point. I ran levels to the point, and then occupied the nail with my Hiper on a 2 meter rod and bipod.
The three Hiper observations were submitted to OPUS, as a check. I decided to use the OPUS observation on-site for my horizontal control coordinates. The file was about 3 or 4 hours long.
My question is this: I set my vertical point as control, and the horizontal point as control. When I process using Topcon Tools, the elevation is changing by 1.417 feet. The horizontal coordinates are not changing, only the elevation of the vertical control point.
Any ideas? The state plane zone is Missouri East.
Thanks in advance.
Edit: I generally process through GNSS Solutions as well, but I haven't processed the data in that program yet to compare. I just upgraded to the latest version on Friday, and haven't updated the NGS antennas or anything yet.
Is the NGS vertical control 29 or 88?
Can you choose whether you hold the vertical or horizontal value of control points in TT?
Kevin,
Thanks for the reply.
The vertical control is NAVD88. In TT you can choose to hold either the horizontal, vertical, or both values for a point.
I set the one point as vertical control, and the other as horizontal control.
I just found and downloaded a reference manual (quick start guide). I am going to do some more reading.
Thanks
I assume you ran a level loop that closed within reason?
Any chance that you ran your level off of a RM and not the actual NGS mark?
Kevin,
I was able to shoot the job from the two points.
I did not run a closed level loop between the vertical control point and the points on-site. It was about two miles or so as the crow flies. My goal was to establish the vertical control with the static GPS observations.
I have used this method before, and had good results, and the results generally check well within reason with the OPUS checks.
Not sure what is going on.
Thanks,
Jimmy
I was wondering about your level loop between the NGS vertical and your nail 50 feet away. Did you do more than one turn to check that? If not, I might suspect a misreading of the level maybe the upper or lower wire instead of a true reading?
Kevin,
Sorry, I misunderstood the question. Yes, closed loop between the monument and the offset nail (always a closed loop).
Thanks,
Jimmy
Jimmy,
Last year, when I started using Topcon gear (Hipers & GR-3 rentals from a local dealer) and TT v8.2, I recall seeing a similar problem, and it seems that was close to the vertical difference that I was seeing. At the time, I was using an FC-200 DC and TopSurv. The numbers in TopSurv on the DC seemed to jive, but once I downloaded the data and looked at it in TT, that vertical error crept in, but only on the points measured with the rover. It didn't seem to affect the base station. I initally thought it was a busted HI or improper antenna offset, but, if I recall correctly, it crept into all the GPS project's which I downloaded data to TT.
The projects I was working on with this gear were smallish boundary surveys, so I wasn't to concerned with it. The dealer hadn't heard of anyone else experiencing this issue, but it was always in the back of my mind. If I did any staking, elevations were based on leveled marks and I used a GPT-9003 total station, so it never went through TT.
I'm still using Topcon GPS (Hiper V's), but have ditched TopSurv and the FC-200 and switched to Microsurvey FieldGenius running on a Mesa, and don't use TT very much now.
I will have to go back and look, but I did have an OPUS observation on the offset nail as well, so I also used the OPUS horizontal coordinates and fixed that point as both horizontal and vertical. The elevation did not change when I reprocessed after fixing both points as horizontal control, and then holding the offset nail as vertical control.
Thanks for the replies. I have not downloaded and run the data thru GNSS Solutions yet.
this surveyor connect post of yore has something about NAD83 NO TRANS in TTools, might be a clue.
Did you input the antenna types for each of your observations into Topcon Tools? You would need to do this since you are using 2 different receivers.
