recoordinating raw gps files; correcting for the wrong AH

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recoordinating raw gps files; correcting for the wrong AH

Post by mlbrack » Fri Jul 28, 2006 2:49 pm

I got in a jam where I setup my base station to elliptical height and in rebooting FG accidientally set my rover to NAVD. So for part of my data, I have a Z error equal to the geoid separation.

Possible to recoordinate the GS records?

Pretty much the same problem is created when the wrong antena height is used. How is this corrected in the raw and coordinate values?

With a total station if an incorrect HR is used, an LS record can be inserted and the raw file recoordinated. Easy enough.

How do you do the same thing with GPS?

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Jason Poitras
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Post by Jason Poitras » Mon Jul 31, 2006 3:31 pm

Yes this can be done with FieldGenius. In our next service pack 2.0.5 which is currently being beta tested you will be able to do this with just a few clicks. For now you will need to manually edit a record in your raw file before processing.

In the raw file you should be able to find the following record:

VA,PV3,N 0.0000,E 0.0000,LZ0.0000,SO0.00000,SA0.00000,GNUnited States NAVD88

Make sure you edit the last VA record in the raw file and change the GN data type to Ellipsoidal:

VA,PV3,N 0.0000,E 0.0000,LZ0.0000,SO0.00000,SA0.00000,GNEllipsoidal

Once you complete this, you can go to Settings | GPS Local Transformation and set the Origin N & E, Trans N & E, Slope N & E, and rotation to zero. Leave the scale set to 1.

Now press the Adjust Points button and FieldGenius will re-process the GS records based on the EP records. During reprocessing it will use the mapping system defined in the GN data type. I suggest noting the elevation of a couple of points before processing so you can compare the new elevations. As a check, the delta elevation should be roughly equal to the geoidal seperation.

You might be asking why you have to manually edit the raw file? Well that is because the datum settings only get written to the raw file upon a connection to a GPS receiver. This has been changed in 2.0.5.
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