I don't know much about Leica or SurVCE software, but hopefully there is some way to check and verify that you are meshing your ground distances to your surface coordinate system. I do know there are some routines in Data Collectors that make you think it's giving ground distances but it's actually inversing along the ellipsoid just like UTM coordinates do. It's important to find a way to check your system numbers against real measured ones.
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Here?ÿis a screen shot of an inverse between two located points (#57 to #85)?ÿin a recent job:
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This an LDP projection so the grid azimuths are close to Geodetic, and for these two points the grid distance and ground distance are very close. Because there is almost 300 ft in elevation change it's more luck than anything else that those distances are so close, there is 7PPM to be expected in that large of an elevation shift.
You want to be sure and?ÿverify that the numbers you are?ÿusing actually check to what you are telling everyone you are doing.
And as you can see there is .4' in Geoid height difference in a short distance which really illustrates the need for a Geoid model.
Ok so by reading this grid to ground scale factor from the rover head, it puts my coordinates into ground. If i'm post processing for a PPP solution or I guess the american equivalent, this step is not necessary. I just need to remember that I need to process the survey to ground once i'm back at the office. Then I can import the processed file into my civil database to have grid utm coords and local utm coords for the client.
Thanks for all the responses guys, I definitely need to do more research on the subject and get my head wrapped around it more before being completely comfortable!
Point of order here:
There is no such thing as a "local UTM (or SPC) coordinate" (at least within the context that we are discussing here).?ÿAs soon as you scale, truncate, rotate, WHATEVER...a UTM or SPC Coordinate, it ISN'T a UTM or SPC Coordinate anymore. Depending on how you massage the REAL (true) UTM/SPC Coordinate values, you can certainly maintain the "grid bearings," but the "coordinate values (N/E)" are NOT UTM/SPC (except maybe ONE of them if you do a "local scaling"), and you should make sure that the resulting coordinate values do NOT "look like" REAL UTM/SPC values (always truncate).
In any case, although I despise the whole modifrickingfied UTM/SPC mind set, I do understand that many clients WANT this kind of geodetic vandalism as a finish product, so you do what you have to do. Just be damn sure that you supply enough metadata (how ya done it), so that the next guy/gal can duplicate the?ÿcoordinate graffiti down the road.
Something that needs to be understood. There is no combined scale factor for an area. Each point has a combined scale factor, and each line has one. Think of a point with a particular lat, long, elevation. It has a state plane coordinate fixed to that lat, long, it has a grid scale factor fixed to the long, and a elevation scale factor fixed to the elevation (ellipsoid height). There are an?ÿ infinite number of combined scale factors for that lat, long depending on the elevation of the point. The receiver on the tripod has a different combined?ÿscale factor than the spike?ÿdriven in the?ÿground that is being measured.
When the DC prints out a scale factor it is usually taking the values associated with the control point and showing the combined scale factor for that point. To illustrate how quickly they can change a combined factor of .9997779 on the control point driven into the ground would change to .9997781 on the receiver sitting on the tripod.
Point being?ÿthere is nothing sacred at all about the combined scale factor shown in the DC for a control point, you may wish to use it for something or not. If you know where you are going for a project, and it's limits, nothing keeps you from figuring all this out before stepping out of the office. You can figure out a mean elevation and mean latitude longitude for your project and use those numbers for an?ÿadjustment factor, put it into your computer, develop a projection, print it out, place a hard copy of the meta-data in the file, send it to your DC and when you begin in the field it's all set up. Then never, never, never change it!!!!!
You want all the stake holders in the project to be cool with what you are doing.
Of course in a purist sense nearly everything said so far is true. Perhaps a better way to say it would be:
I generate local coordinates by multiplying SPC northings and eastings by the inverse of a combined factor that introduces minimal distortion. The process can be reversed by dividing the local northings and eastings by the same number.?ÿ
You see, Surveying involves knowing how to achieve results that get us close enough without blowing every budget overdoing it. If I can do that and maintain a known relationship with a common datum and projection when needed it's a good thing. The plain truth is NOBODY has ever expressed a valid reason why I shouldn't do it this way.
I work from the coast to the divide and encounter just about every distortion you can imagine. Until the CADD gods give us back our custom projection and true geodetic capabilities I'll keep doing it that way.?ÿ
One last rant.. I left my office doir open last night. It qas comforting to see none of my local coordinates escaped and attacked my SPC files...
