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Help with Geographic Transformation

Question asked by Lynn_Carlson on Nov 6, 2011
Latest reply on Nov 9, 2011 by MKennedy-esristaff
I have been provided a file of waypoints that were collected several years ago on a Garmin eTrex GPS. I need to convert into a shapefile.  I've been told that a custom datum was created in the Garmin, so to make sure the point locations fall in the correct place, I need to duplicate this custom datum.  The information I have about this custom datum is that it is a "modified WGS_84". 

The modifications are:
DA = 112
DF = -0.5475071
DX = -134
DY = 124
DY = 316

I've determined that I can use the Create Custom Geographic Transformations tool to generate the transformation I need.

However, in order to apply the transformation, I think I also need to create a custom spheroid?  I believe that the DA parameter is to change the semi-major axis of WGS-84 by subtracting 112. However, I'm not 100% sure what to do with the DF parameter of -0.5475071?

It seems that it is supposed to be the difference in the WGS_84 flattening, so I think I need to create a custom spheroid with a semi-major axis of 6378025 m (6378137 - 112) and a flattening of 298.804730663 (298.257223563 - -0.5475071)?

I've applied this to my point shapefile by initially defining it as WGS-84, then Projecting to my new sphere (as defined above) using the Geographic Transformation I created.  The points have definitely shifted to more reasonable locations, but not completely (these are point locations of bird nests, and a couple of them are still appearing off the coast in the water -see attached graphic).

Can anyone help with what to do with DF, or provide guidance on what else I am doing wrong?

Thanks very much!

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