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)


  • 2
    WGS84 is short for WGS 1984, which didn't exist back in those days, neither did GRS80 reference ellipsoid or even WGS 1972, UTM did, maybe WGS 1960 ngs.noaa.gov/PUBS_LIB/Geodesy4Layman/TR80003E.HTM. However it is entirely possible that assumptions being made about what datum the points are recorded in are incorrect; having georeferenced historic maps it can be quite tricky to know what parameters to use unless specifically printed in the marginalia. Commented May 15, 2019 at 7:49
  • Hi. Thanks for your comment. The info on the original maps is thus: *Spheroid = Everest; *Grid = 1000m UTM Zone 48; *Projection = Transverse Mercator; * vertical datum = mean sea level at ha tien: Horozontal Datum = Indian Datum 1960.....
    – Vin
    Commented May 21, 2019 at 5:40
  • This is the exact map area I'm working on... legacy.lib.utexas.edu/maps/topo/vietnam/…
    – Vin
    Commented May 21, 2019 at 5:58
  • Please help, this is super important to me 😁
    – Vin
    Commented May 21, 2019 at 5:59

1 Answer 1


There is a NW/SE axis offset between WGS-84 MGRS GC and ING-A MGRS GC in Southeast Asia, somewhere between 300 and 450 m off the top of my head.

If all you are to using to find places is the old maps, you’re fine.

If all you’re using to find places is a GPS unit, the revised mapping, and the WGS-84 datum, you’re fine.

If you try to use your old maps and a GPS unit and you don’t adjust the unit from the default datum setting (WGS-84), anticipate the systematic error I mentioned at the start.

There is no Indian 1960 (ING-A) datum setting in a handheld GPS unit, but Garmin’s Indian Thailand will get you in the ballpark. It seems to be slightly more weighted to INF-A (Indian 1954) than ING-A but at that point you are close to the EPE for a handheld GPS unit regardless. Other GPS brands seem to offer similar hybrid datum’s for that area.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.