Using AusDatumTool to convert old miltary map grid references

How to compute the datum shift parameters

  1. Get a physical copy (or hi-res scan) of both the old miltary map and the new GDA94 map for the area that you're interested in.

  2. Choose three test points that are spread over a wide area. Find identical points on the old map and the new map for these features (e.g. Avoca Trig, Tangy Dangy Trig, Mangrove Trig). Write down the eastings and northings for these points (e.g. 441000 yE, 866321 yN for Avoca Trig on old map, and 351563 mE, 6297600 mN on new map). You will probably need to use a ruler on the map to get accurate values.

  3. Use AusDatumTool to convert these eastings and northings to lat/lons (e.g. -33.4528865,151.4032235 for Avoca Trig on old map, and -33.4507546,151.4029191 on new map). Select ANG -> ANG when doing the old map conversion from easting/northing to lat/lon, and GDA -> GDA when doing the old map so that the correct ellipsoid params are used. (No datum conversion is performed if input and output datums are the same).

  4. Run the ComputeDatumShift program and enter the appropriate ellipsoid params for the old miltary map (ANG datum). i.e. Major axis = 6378293.639, Inverse flattening = 294.260680

  5. Enter the lat/lons for all three test points on both old and new maps in the tool. See an example here. (Avoca Trig is the second test point in this screenshot).

  6. Press the Compute button.

For the Gosford area test points, you will get these values: 
DX = 10129.213597
DY = -5546.401364
DZ =  7245.237032
RX =  24.6181860670
RY = -22.4770369791
RZ =  23.4644208914
M  = 2128.40907448686


Once you have these parameters, you can use AusDatumTool to convert miltary map grid refs from old documents into modern coordinates as follows:

How to convert an old miltary map grid reference to modern coordinates

  1. Run AusDatumTool and choose ANG as the input datum and GDA94 as the output datum.

  2. Click the properties button which appears just to the right of the input ANG datum. Enter the 7 parameter shift values computed in previous set of steps. You also need to make sure you're using the right ANG zone. It should be 8 for Gosford.

  3. For each miltary map grid ref e.g. 341 629, add the appropriate prefix digits for the area that you're working in. For the Gosford area, you add a "4" to the start of the easting and an "8" to the start of the northing, and also add two zeros at the end of each. e.g. In this case, the full easting in yards is 434100 and the full northing is 862900.

  4. Press the red arrow button to process the input and produce the corresponding GDA94 easting and northing. In this case 345296 E, 6294342 N or -33.4792458,151.3349484 in lat/lon.

  5. Click the Google Earth button if you want to view this location on Google Earth.


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