I am a Marine vet turned academic historian working with MGRS data from the Vietnam War. I have an overlay georeferenced 1:12500 map of Hue, Vietnam (NIMA Reference L909XHUE) with an imagery basemap. When I use ArcMAP MGRS geocoding, my feature points appear significantly off in the newly created feature layer.
When I use this mapplet to geocode, I get a spot on location. The mapplet appears to use the following datum to successfully plot the grid:
Datum: Indian 1975, Thailand
Ellipsoid: Everest (India 1830)
Equatorial Radius (m): 6377266.345
Polar Radius (m): 6356075.413
Datum Transformation (local to WGS84): dX (m) 210, dY (m) 814, dZ (m) 289
The mapplet produces the correct point from MGRS translation as follows:
UTM 48Q 777400 1822600
For example, 48Q YD 764266 (Hue, Vietnam) should be at the edge of the Perfume River (known in 1968 as the LCU ramp). However, ArcMAP places it significantly SSE of the location just above a stadium. The incorrect ArcMAP geocoding data transformation (WGS84 or any similar Indian 1975, etc. datum I can find) displays as 48QYD7739522600 MGRS, 107°35'53.094"E 16°28'8.44" or 48Q 777395 1822599 UTM
I have tried without success to find/construct a custom datum in ArcMAP to replicate the above datum. Only mapplet datum plots correctly, but I need to geocode nearly 500 point features.
Any suggestions on how to build a custom datum or is their a native ArcGIS/ArcMAP datum I should use that would be identical?
Suggestions welcome. If you need any more info, please ask.