Custom Coordinate Systems
To add your own coordinate systems, you need to edit the proj4_projections.txt file that is located in the same folder as the Coordinate Master application itself. You can do this by pressing the edit button at the bottom left of the main page of Coordinate Master.
The easiest way to create a new coordinate system is to copy a similar one which already exists and then to modify the parameters as required.
There is a section containing a category for user coordinate systems at the end of the file, but you can add coordinate systems anywhere in the file, modify or delete existing ones, or even add your own coordinate system categories.
An example of a coordinate system is shown below:
Name: DHDN / 3-degree Gauss-Kruger zone 2 (E-N)
Ellipsoid:6377397.155,299.1528128
ToWGS84:598.1,73.7,418.2,-0.202,-0.045,2.455,6.7
Proj4:+proj=tmerc +lat_0=0 +lon_0=6 +k=1 +x_0=2500000 +y_0=0 +a=6377397.155 +rf=299.1528128
Affine:1,0,0,0,1,0
The meaning of each of these lines are as follows:
- Name: The name you want to refer to this coordinate system by.
- Ellipsoid: The semi-major axis (a) and inverse flattening (1/f) for the ellipsoid used by this coordinate system. (These values are usually also duplicated in the Proj4 section for projected coordinate systems).
- ToWGS84: The 7-parameter datum shift values. (If you only have 3 datum shift parameters, just set the last 4 values to 0).
- Proj4: The PROJ4 library command for this projection, or "LATLON" if this is a lat/lon coordinate system. A full list of the available PROJ4 projections and their parameters is available here.
- Affine: This line is optional, but if present contains the 6 affine parameter values. Affine parameters modify the map coordinates and are often used to create local grids such as those used at mine sites.
The ToWGS84 parameters represent a shift (DX,DY,DZ), rotation (RX,RY,RZ) and scale(S). They have the following units:
- DX: metres
- DY: metres
- DZ: metres
- RX: seconds
- RX: seconds
- RZ: seconds
- S: parts per million
Notes:
- Coordinate Master uses the "coordinate frame rotation" convention for the rotations in the "ToWGS84" parameters. This convention is commonly used in Australia and the US. But in Europe the "position vector rotation" convention is commonly used. The only difference between these conventions is that the signs of the three rotation values (RX, RY, RZ) are reversed.
- If you want to compute 1/f and you know the semi-major axis (a) and semi-minor axis (b) of the ellipsoid, the formula is 1/f = a/(a-b).
Once you have set up a new coordinate system, it is recommended that you test it with a point whose coordinates you know in both lat/lon WGS84 and in the new coordinate system.
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