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Project Localization, Least Squares, & Grid to Ground
Hello all, I have been sitting on the sidelines for quite some time and have enjoyed all the extensive, useful information this site has to offer. I cannot seem to find the answers I need so I guess it’s time to come out from the shadows and add to the community. Hopefully I can give back some knowledge in the future…
I have recently moved from sea level inland to a bit more of a hilly terrain and started mapping with a new company. With that said, I am a little hazy on the whole step by step process of what they do concerning least square adjustments, project localization, and placing the project in/on ground coordinates. Everything was so flat previously that I guess there wasn’t much use for the methods of my new company.
When mapping a survey, their standard is to locate everything with the total station (keeping the data collector set at scale factor of 1.0000 – or no projection/no datum), come back and GPS control points, then localize the entire project from one of these GPS points using a combined grid factor. This I get; holding this point, everything gets scaled out using the factor.
My question is when exactly do you do this? Here are a couple scenarios that I could see making sense, but I am pretty confused on which way to go about it…Which points do you use as control for your adjustment…
Let’s say you ran a closed traverse using arbitrary points (i.e. N: 10,000 E: 10,000) and came back and located three of the traverse points with the GPS (VRS/RTK). One of the three will be the Point of Localization. Do you hold the one GPS point (localized point), scale the other two from grid to ground using the combined grid factor, and then use those three scaled GPS points for your adjustment’s control?
OR
Do you use the three GPS points as they were collected (no scaling yet), run the adjustment using these “grid” coordinates, then scale all the points from your chosen Point of Localization?
OR
Run the adjustment using only the raw total station data first. Using the same 3 (arbitrary 10,000, 10,000) points that you will later GPS as control. Then, translate your adjusted points to your GPS control Point of Localization, rotate the adjusted points using a bearing from one of the 3 GPS points, then scale everything?
OR
Something completely different using a closed traverse, GPS control points, a least squares adjustment, and Point of Localization?
Hopefully this made sense! Any help or insight will be much appreciated!
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5 Replies
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It kinda depends on what you want for the final product. Do you want final data to be spc, or a ground local system? How big of an area do you need to map?
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There are 10,000 answers to this question. And, 10,000 ways to solve it.
You will want someone to discuss this with. It’s like going to California, via the southern rout, or via the northern route.
A basic understand of projections and the average csf of the target area will be needed.
Also, the needs of your client.
Example, average csf in my area is 100 PPM, or 1000.10′ in grid coords = 1000′ ground. That’s 0.01′ of difference, per 100′. Or, 0.528′ per mile….
Gotta go..
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And.. I said that wrong. Grid is scaled up to ground. Grid = 1000.00′ and ground = 1000.10′
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Combining GPS/GNSS vectors and terrestrial measurements in a simultaneous adjustment is something that can be done quickly a routinely using any of several software packages. I use, and prefer, StarNet. Without he benefit of such software you might hold the GPS coordinates and apply the appropriate scale factors to your terrestrial measured distances.
Remember that GPS is not a black box coordinate generator. It is always measuring a 3d vector between a base and a rover. In some circumstances the base is a virtual one, but there is always a vector between a base and a rover. Your data collector uses that vector to compute coordinates. Those measurements can be used in adjustment just the same as any others.
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Thanks for the input so far! I understand there are several ways to skin the cat that is kind of why it is a bit confusing. To add to and answer some of your replies…
Their one-point localization using an averaged grid factor goes into the map whether it is a 0.5 acre boundary survey, a 20 acre topo being sent to the engineering department, or a 4 mile long boundary retracement with a linked, closed traverse.
The final product is tied to SPC using the point of localization but then scaled out to ground (i.e. local system?).
The average CGF of the area is 0.9998….
I know the differences in lengths are rather small sometimes and mostly around 0.10′, but it is their uniformed way (right or wrong, I suppose?).
The adjusting software is CIvil3D using raw data collector files and Fieldbooks.
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