My data collector told me not to listen to you.
Stephen
[sarcasm]I have heard that tin foil wrapped around one's head can help relieve those external voices....[/sarcasm]
I either localize, or use state plane coordiantes, usually SPC. I was never a big fan of setting one point and saying "here" and taking off and working. I can't tell you why I believe this, probably some old Trimble training I had years ago.
As far as trusting GPS, I've done enough checking over the years to believe in it. Do any of us really understand what all the equipment we use works? Think about, EDM's, total stations, data collectors, CAD, computers etc. How can we trust any of it? The answer is through checks. I will stand up in court and swear my RTK GPS is the perfect answer, my total station is always right, my data collector is reliable. I will do this, because by the time I get to court, I will have checked my work enough to know this is a fact.
Dual base rtk would likely provide you with the defensible data you speak of. I am thinking of property corners here, not topo.
Are you sure your total station is correct?
this is why i raised the question about trusting everything that comes out the "black box" if surveyors are willing to use gps technology then they need to become semi experts in geodesy. i have only been to court a couple times and never lost because i always had more data than the other side. its about whose got the most evidence and sounds the most convincing. good acting works too!
> ... I was never a big fan of setting one point and saying "here" and taking off and working. I can't tell you why I believe this, probably some old Trimble training I had years ago.
Back in the days of SA a HERE position would likely differ enough from the true to introduce errors into your vectors.
Nowadays I find that HERE positions usually differ from true by no more than a few feet. I commonly collect static data for OPUS at the base while working off a HERE position, then update everything to a real base position back at the office. Slicker than slug snot.
That's why they have baselines.
That's why I take my GPS to the local baseline, and the matches are excellent.
Are your court objections based upon real experience in court, or conjecture about what might happen. I've testified in 106 trials and have yet to have anyone successfully cross examine me. I fondly recall the attorney who got so frustrated with me that he actually squatted down on the floor, then complained to the judge. I thought the attorney was going to scream, which would have been fine with me.
i only use the OPUS solution if geodetic positions have to be accurate and transformed into SPC or UTM which is not the norm for me. Otherwise it doesnt matter if geodetic positiions are acccurate or not its all in the same localization. if another surveyor needs my positons he/she would have to localize on my control. i work mostly in rural areas of southern iowa that doesnt have any passive control points to calibrate to. if i am doing a large site like a township then i set at least 4 base points near the twp corners
> i only use the OPUS solution if geodetic positions have to be accurate ..
Still, it might be a good idea to confirm that your HERE base positions are close to reality by collecting and sending data into OPUS or OPUS-RS. Just to eliminate that as a potential source of the errors you are experiencing.
> I am curious how well GPS would stand up to traditional surveying in a court of law were the minutia of measurement to become a factor in a dispute.
People don't go to court over 0.3' feet of Iowa farmland. Sometimes they shoot each other on general principles exacerbated by a dispute over 0.3' of Iowa farmland, but they don't go to court.
:good:
Are you sure your total station is correct?
> Are your procedures solid enough to explain to a jury how you know your GPS data is and was accurate?
> Yes
> Do you truly understand enough to sit and be cross examined by an attorney who questions your understanding of the sheer amount of math involved in calculating a GPS signal return?
> Just as much so as with any other technical aspect of surveying
>
> How easily could a hired-gun expert touting a PhD tear your data to shreds?
> Not very
:good: :good:
yes i agree that this would be a check on the dc software that it is producing a reasonable geodetic position. afterall this affects the scale of the project. i have heard from other surveyors that they prefer using SPC projection because it cuts down the distortion from ellipse to ground by about half. do you agree with this?
"i have heard from other surveyors that they prefer using SPC projection because it cuts down the distortion from ellipse to ground by about half. do you agree with this?"
In a word...NO!
The relationship of the SPC "grid" (developed surface) and the Ellipsoid (OR GROUND) varies a LOT depending on where you are.
GPS (Static or RTK) "measures" ON the SURFACE of the Earth (ground), NOT on the ellipsoid, or the SPC Grid.
Loyal
> Most of my projects start from an autonmous base position using assumed ground coordinates (10K, 10K, 100).
> i have heard from other surveyors that they prefer using SPC projection because it cuts down the distortion ...
I am beginning to wonder if you are somehow convincing your dc that you are at 10000,10000 on the state plane grid and thus causing it to produce wild scale factors. Grid /Ground scale factor distortion should be on the order of 1:10,0000 when working in the appropriate state plane system.
Why not just use the coordinates generated by the state plane projection? It would be less work. So yes, I think I agree with those other surveyors you have been talking to.
I beieve my data collector. Its the information that the GPS unit is sending to the DC that I have to watch. Here are 4 shots, 4 initializations, on the same nail set yesterday. This is in a lightly wooded area with 5 minutes between shots, GPS and GLONASS. See the problem with 594... about 2.5 feet from the other 3.
593,385258.1591,641206.8653,1584.0308,NAIL SET
594,385257.2077,641209.2533,1597.8276,NAIL SET
595,385258.1871,641206.9320,1583.9655,NAIL SET
596,385258.1509,641206.9165,1584.0168,NAIL SET
Anyone who takes only one RTK shot on any point is asking for trouble. You need to know what your equipment is capable of and learn not to accept the first solution it gives you. For wide open topo, most likely you should have no problems. But if you're seeing greater distance differences as your distance between RTK points increases, there is a problem for sure, one that you should be able to track down and take care of. When I was using a Leica 530, I would always see less than 0.03' difference between RTK points when checked with a total station - regardless of the overall distance between them. With the equipment I'm using now the results aren't always within 0.03' but are OK for what I am doing with these control points in this project.
"Grid /Ground scale factor distortion should be on the order of 1:10,0000 when working in the appropriate state plane system."
ONLY IF (or when) your project is AT (on, or very near) the surface of the NAD83 Ellipsoid).
At my house in Wyoming, the number is more like 1:2600.
The infamous "1:10,000" associated with State Plane Coordinate systems, is the relationship of the SPC “developed surface” and the surface of the Ellipsoid. Your height above the Ellipsoid (or below in some cases), CHANGES this number about 1ppm for every 21 vertical feet.
Loyal
> At my house in Wyoming, the number is more like 1:2600.
True. But the highest peak in Iowa is 5000 feet below the average height of Wyoming.
“True. But the highest peak in Iowa is 5000 feet below the average height of Wyoming.”
Agreed!
However, Iowa elevations range from ~480 to ~1,670, with an “average” of ~1,100 feet (USGS Professional Paper 909).
1,100 vertical feet, would impart as much as 52ppm into the grid-ground equation (depending on where you are at in the SPC Zone). This "could" mitigate the combined effect, OR make it worse. It depends on where you ARE (in Iowa)!
I'm sure there are a many places in Iowa where the “grid-Ground” distortion is 1:10,000 (or even less), but that doesn't mean that one should ASSUME 1:10,000 as a upper limit.
🙂
Loyal
Who believes their: tools, methods, judgement
:good: :good:
Good Post Gavin
Ralph
