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EDM Baseline results – Help
Posted by subman on September 19, 2010 at 2:30 amWent to the City of Los Angeles baseline along the LA River this morning. I set the total station on 4+50 (metric) and set up tripods and prisms at 0+00 and 6+00.
I shot from 4+50 to 0+00 and got the following results (in feet): 1476.755, 1476.750, 1476.755, and 1476.755. It was 86 degrees and the atmospheric pressure was 29.46.; set ppm accordingly before the shots. Needless to say, I am feeling pretty good at that point. The established horizontal distance between those stations is 1476.754.
Next, I turn and shoot from 4+50 to 6+00. The established horizontal distance between these stations is 492.049. I got the following results: 492.090, 492.090, 492.090, and 492.090.
How can I be off by 0.041 ft? Is it possible that a tribrach and tribrach adapter (with plate vial) set up can be out of adjustment and produce that much error?
Unfortunately, I didn’t have a spare tribrach/adapter to try a different set up. In hindsight, I guess I could have brought the tripod/prism setup from 0+00, but it didn’t occur to me at the time.
Any suggestions/thoughts/input?
The total station is a Topcon ET-1 and it has been into the shop within the last 6 months for service/adjustment.
Paul Plutae replied 13 years, 8 months ago 9 Members · 18 Replies- 18 Replies
When was the last time the baseline was checked or calibrated?
Don’t know Beerlegs, the notes I have are dated 9/16/2002
A tribrach could be out of adjustment that much, especially if the tripod was set up tall, but you did check its alignment before going to the range, didn’t you?
You should be doing multiple combinations of distances. The redundancy helps a lot in diagnosing any problem.
One check you can do is AB+BC=AC+error on any 3 stations in a straight line. If AB is long and BC is short by the same amount, then you have reason to suspect the mark at B. Scott Z [msg=18420]posted about a week ago[/msg] about a baseline where the data had that pattern, so it appears one of the marks has shifted 5 mm=0.016 ft in his case.
.> Went to the City of Los Angeles baseline along the LA River this morning. I set the total station on 4+50 (metric) and set up tripods and prisms at 0+00 and 6+00.
>
> I shot from 4+50 to 0+00 and got the following results (in feet): 1476.755, 1476.750, 1476.755, and 1476.755. It was 86 degrees and the atmospheric pressure was 29.46.; set ppm accordingly before the shots. Needless to say, I am feeling pretty good at that point. The established horizontal distance between those stations is 1476.754.
>
> Next, I turn and shoot from 4+50 to 6+00. The established horizontal distance between these stations is 492.049. I got the following results: 492.090, 492.090, 492.090, and 492.090.
>
> How can I be off by 0.041 ft? Is it possible that a tribrach and tribrach adapter (with plate vial) set up can be out of adjustment and produce that much error?> Any suggestions/thoughts/input?
Dennis, in Los Angeles don’t all the surveyors just report the distance that you measured as 492.090 ft. as the “record” value of 492.049 ft.? :>
Kent, only for points next to fence post corners… 😉
> Kent, only for points next to fence post corners… 😉
To give you a straight answer to your calibration problem, though, I’ll ask why you don’t calibrate your EDM/total station on a line of unknown length with perhaps three or four intermediate stations between endpoints.
You can use static GPS vectors to check the overall scale (the distance between the endpoints of the whole line), or have a repair facility just check the frequency of the quartz oscillator in the EDM. The rest of the calibration is done by measuring every possible combination of distances between the points on the line, both endpoints and intermediate stations, and calculating the instrument addition constant correction from a least squares adjustment.
You either use a function of the adjustment software to solve the addition constant or by manually adding or substracting a constant value to or from the measured ranges until the weighted sum of squares of the residuals generated by the adjustment is at a minimum value.
The large advantage to that method is that you measure lots of distances and can chose lengths in the range that you will ordinarily be measuring.
What is you prism offset (for the prism), and what is the setting in the instrument? Are they the same? Are all your prisms from the same manufacturer?
You can check by setting 3 points (pk) in a line A-B-C about 300 feet long. B should probably be somewhat off center, maybe 1/3 Set the prism offset in the instrument to 0. Set on A, measure to C. Set on B, measure to A and C.
Of course, you should get AB + BC = AC, except for the prism error (e) in each shot, which results in (AB+e) + (BC+e) = (AC+e). Plug your distances in and solve for e. It should be very close to what you think the prism offset should be. Set this value in the instrument and try again. You should get AB + BC = AC now.
We’ve had some baseline monuments hit by DOT mowers which isn’t good.
Good luck.
Dennis
I would say that you have no data to check anything at this point. The middle monument (OCC) could have moved or either of the end ones or it could be entirely something else.You just have no independent data to verify anything at this point.
Deral
You definitely should check tribrachs before going out to the calibration baseline. Maybe setting on a point and doing a quick set of measurements to all of the other points is a good quick check especially if everything checks within reason. The best evaluation would be to go out and set up a tripod already on all the points, and adjusted tribrachs on each tripod you aren’t set up on. Then measureing to every other monument from each monument. Then you can see if you are different whenever you shoot to and from a particular point, and evaluate the whole baseline better. It is always possible that a monument out there moved, but having a measurement from every point out there to it would tell you so much more.
Why go to all the trouble of going to a CBL and measuring only two segments? While many folks seem happy when their measurement is “close to” the published/well-determined value, this is hardly a good use of your time.
While some might assert that your observations show one of the monuments has moved, you have merely brought the baseline into question rather than identifying the problem.
With two segment measurements you can solve for the scale and constant with zero degrees of freedom. BTW, there is a small program on the NGS site: CALIBRAT that will do the computations for you. You will need to create two small text files one with published values the other with measured values. These are inputs for CALIBRAT.
The NGS hoped that folks would measure all segments from each point. When they were established all points were observed from each point on separate days. Measuring more segments provides redundancy allowing us to estimate how confident we can be in our results.
Excuse the rant. I set and remeasured a few of these (not in CA) and find it annoying that my hard work was for naught.
> The NGS hoped that folks would measure all segments from each point. When they were established all points were observed from each point on separate days. Measuring more segments provides redundancy allowing us to estimate how confident we can be in our results.
>
> Excuse the rant. I set and remeasured a few of these (not in CA) and find it annoying that my hard work was for naught.Geod(esist) Mike (great pseudonym, BTW) do you by any chance have an opinion about the merits of a calibration baseline with all intervals at nominal 10m multiples? Didn’t the original NGS design assume that a CBL user would have previously determined the cyclic error of hie or her EDM? In the case where a CBL user has no detailed information about the cyclic error, is a baseline with all intervals being multiples of 10m really that useful?
In my view, the original design and implementation of the CBL program was intended to provide a low-cost, easy-to-use tool for practicing surveyors. Little attention has been given in the last decade or more (as best I know) to any reevaluation of the layout. As a means to validate instrument specifications it works.
Having watched the great work on issues related to EDM calibration in Australia (see: http://www.land.vic.gov.au/CA256F310024B628/0/2AC14DC8753E005DCA257339000516E3/$File/EDM+Handbook+V9.0.pdf ) I have been disappointed that, given its limited resources, NGS chose to fund priorities other than the enhancing the current activity.
I understand there were moves by some state societies (not Texas) to establish EDM baselines using the Hobart design. I have not followed up on the early discussions having since retired.
As highlighted by this thread, no matter how rigorously established, they must be maintained. The Australian model in this regard is one I wish had been adopted. If users make their measurements on baselines and submit them for processing and adjustment to a web-based tool they, in fact, maintain the baselines.
The current model is for the agency/entity interested in a CBL to find the property, obtain clearances to set monuments according to the NGS design, set the monuments, and make observations according to NGS guidelines. Not much room for innovation that I can see.
Hope this is responsive,
Mike
GeeOddMike
No need to apologize. Why bother going you ask? I had a few hours of “me time” and I enjoy spending it this way. I was essentially by myself. My son was with me for company (and a free lunch after wards), but more for securing the equipment than anything else. There is a lot of foot traffic on the path the base line is located. His first time around surveying equipment so I took time explaining what I was doing. The site is only 30 minutes from my home and I only had a few hours available (other family errands). I would have taken more shots if I had more time. In addition, there was almost 1/3 mile of fence line overgrowth that needed trimmed back for line of sight. I did what I could given the time I had.
I just ordered a Seco tribrach adjuster (#2002-00)to check my tribrachs’ optical plummets and bubbles before any more visits to the baseline.
Thanks for everyone’s input. Your knowledge and experience are appreciated!
I hate being the fly in the soup Dennis…Since you need two tribrachs, how do you know the base one is in adjustment?
> Having watched the great work on issues related to EDM calibration in Australia (see: http://www.land.vic.gov.au/CA256F310024B628/0/2AC14DC8753E005DCA257339000516E3/$File/EDM+Handbook+V9.0.pdf ) I have been disappointed that, given its limited resources, NGS chose to fund priorities other than the enhancing the current activity.
>
> I understand there were moves by some state societies (not Texas) to establish EDM baselines using the Hobart design. I have not followed up on the early discussions having since retired.
>
> As highlighted by this thread, no matter how rigorously established, they must be maintained. The Australian model in this regard is one I wish had been adopted. If users make their measurements on baselines and submit them for processing and adjustment to a web-based tool they, in fact, maintain the baselines.Yes, that would be a very good model for the US, with NGS mainly involved in maintaining the online calibration processing software and archiving the results, also in an online database. I’d be very surprised if repeated static GPS vector solutions wouldn’t be sufficient to fix the overall scale of a CBL (or at least one with stations suitable for GPS occupations) for all practical land surveying purposes.
Any adjustment software would probably need to include the periodic GPS vectors. A baseline design that involved measuring all possible intervals between baseline stations (and submitting them for processing) would (at least in theory) be an adequate method of maintaining the works and getting usable lengths of the intervals.
At least we all know..
Where the 0.04′ is hiding out.
Paul
From what I have read, I don’t think it matters for the bottom tribrach. The plate level in the top of the “puck” is used to establish a level horizontal plane. The tribrach to be tested is placed upside down on top of the “puck” so the optical plummet is focused on a target on the ceiling.
The “puck” plate level can also be used to check and adjust the bubble on the bottom tribrach. Someone feel free to correct me if I am interpreting anything I have read incorrectly. The following video provides a brief overview.
>The plate level in the top of the “puck” is used to establish a level horizontal plane.
Gotcha!!
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