So I am working on a project that is approx. 80,000 acres, 20 miles wide. It has only been surveyed with a total station so all maps are ground distances, and now I am trying to work on the project with a gps system. I understand the concept of scale factor for the most part, where I am lost is how to figure it out for a project this size.
Is it small enough I can just use one scale factor?
Is it large enough that since curvature of the earth was not taken in when the initial surveying was done, is that why my points are not matching up.
All border poles are quite old so I don't trust that they are accurate. I measured 3 points with a total station and closed the loop to make sure it matched up. I tried using that to get my scale factor, but for every line I got a different scale factor. The longest line was 3500 feet. Is that a wright way to figure out scale factor?
I am marking border lines and replacing boundary poles in the area, so I would like to be as accurate as possible.?ÿ
Any advice would be greatly appreciated.
Many TS work was done with curvature of the Earth factored in and can work well with GPS results.
It would depend upon where it is and to what state system is in works and to what directions are the longer direction.
I am fortunate to live where data can be collected and used in WGS84 and scale factor is not enough to overcome Static or RTK accuracies and it is possible to work 1 = 1.
Take your problem to another surveyor. Don't be ashamed to get a bit of help. In person.
N
That's awfully big for one Combined Scale Factor, and even worse if you have a lot of elevation change. You may be able to make a scaled projection work. Since I see you are in Belize, you will have less change in convergence angle to deal with than someone with that size of project at a larger latitude.?ÿ But it could be a case where you need a Low Distortion Projection set up for the project in order to easily convert GPS readings into something that matches ground well enough.
Use whatever software you have to find Combined Scale Factor for some projection (UTM?) at points in the middle and extremes of your project.?ÿ Look at how much difference you get if you use an average factor over that distance, and compare to the error tolerance you are trying to achieve.
Since it appears to be about 3 times as wide on one axis than the other, the variation in scale factor will depend in part how well the long direction of your project matches up with the state plane projection.?ÿ If it is a Transverse Mercator zone, and the long direction is east-west you will have more distortion than if the long way is north-south.?ÿ Opposite for Lambert.
If all else fails, would it be acceptable to do it in sections, with a common line between each piece and the next?
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Not familiar with your location, intended accuracy or purpose. That said, I would heed Jim Stem??s advice in:?ÿ https://www.ngs.noaa.gov/library/pdfs/NOAA_Manual_NOS_NGS_0005.pdf
I copy the relevant page below:?ÿ
Thanks for the advice. I will try setting it up in sections.
The elevations for the area range from 10m to 160m.Is that enough of a change to scale for that as well?
The TS used for the initial survey did not compute curvature of the earth. At what range would there be a 2cm difference because of earths curvature at this elevation?
If you scale all your GPS measurements for the mean of those two elevations, 85 meters, a 2 km line that is all at one of those elevations will be on the order of ~2 cm off ground length.?ÿ I didn't compute it exactly, but that should give you a sense of it.
Figuring out what earth curvature does is more complicated, depending on choice of projection.
?ÿ
No problem with a site that size, you can get very close across it. I would set it up in your software with some "real world" locations from Google and figure out a scale factor. I do those size areas often, there is no reason you can't.
The important issue is to compare ground vs grid across the entire site, with the small changes in elevations there shouldn't be an issue to construct a scale that gets you around?ÿ15 ppm at the outliers. Earth curvature is taken care of by applying a Geoid model to the GPS Bill mentioned you are in Belize, I can't say what Geoid would work well there, are you trying to mesh with a bearing system?
What you are doing with a scale factor is setting a grid projection to a surface that mimics your real world ground surface, you will never be exact.
I have to say that Belize sounds interesting this time of year,,,,,need any help? ??ÿ
Just a comment. A project of that size, localize on perimeter, and tie a few in the middle, for your localize. There's always enough possibility with total sta to have cumulative error. Whereas, gps is usually non cumulative. The more points you have in common, the better your relationship between ts and gps. (Generally speaking).
N
I'd strongly recommend studying up on geodetic control methods. You're overall results when done properly on a project of this size will be far superior in accuracy to any TS work. With this size I would recommend breaking it up into as many zones or sectors to enable your RTK system to cover it all. Then establish your initial control on whatever datum you end up choosing and setting control and locating monuments using no scale. Then I would scale each of my separate zones accordingly. You would have a different scale factor for each zone but in the end it would all match your TS "ground" measurement data. "Geodesy for the layman" is a good start for reference I formation. I also like the USCOE manual on "Control Surveying". Depending on where you are will dictate the details on setup, but the basics are the same. Yes, elevation is a factor due to it adding to the radius of the datum. Think of it like this, "Is there an arc length difference between to curve with the same angle but different radius lenghts?" Yes very much so. The arc length is relatively similar to the "ground" distance. Where as the Chord would be more like the "grid" distance. So absolutely size and Elevation matter. I hope this helps, and feel free to contact direct with any further questions on this. I personally love it. Best of luck on your project!
Many TS work was done with curvature of the Earth factored in and can work well with GPS results.
It would depend upon where it is and to what state system is in works and to what directions are the longer direction.
I am fortunate to live where data can be collected and used in WGS84 and scale factor is not enough to overcome Static or RTK accuracies and it is possible to work 1 = 1.
Here's an extract from the Geodimeter 600 (later became Trimble 5600 series) which seems to imply that the corrections are automatically made by that particular instrument. In the later Trimble ACU manual with Survey Controller software, its manual had a warning buried in it about double dipping on the corrections.
Thanks everyone for the advice. I think I got it now