I travel regularly to sites around Vietnam reported in declassified Australian military documents. All grid coordinates are reported in abbreviated MGRS and are usually quite accurate.
In recent years a university has created a large resource intensive map of most of the same sites which appear on Google Maps different positions. The explanation I had from the uni was that the war era coordinates were using Indian 1960 datum and they've had to correct this to fit into WGS.
However, back in those days they were not using GPS, but reading from topo maps with a MGRS grid overlay. I'm no expert GIS wiz, just an end user, but I would think that the MGRS grid would be the same as WGS / UTM and the grid coordinates presented in the declassified documents would not need transformation.
In all my years of using the grids to overlay georeferenced sketch maps onto Google Earth etc the landscape features have always matched up.
Below I present one example of a single point reference.
Can anyone explain the discrepancy with the university map?
I'd hate to think I'd been out of calibration all these years.
The original quoted location is YS508549. Naturally this is an abbreviated MGRS.
If I expand this I believe it should look like 48PYS5080054900;
Or 750800.00 m E 1154900.00 m N (10.439367°N 107.290986°E decimal degrees)
When I plot this onto Google Earth it appears just south of Duong Bach Mai school, some 4-500m away from the location on the university battlemap (diagonally opposite side of the main hwy 44 /Vo Thi Sau road)