Creating a custom spatial reference string for a truncated UTM grid

If you have your data in a truncated UTM (or MGA, etc) then you will need to use a custom proj4 string to define the spatial reference system. The instructions below describe the way to create the required string.

Assuming you have the following parameters for your truncated UTM system:

Truncation East = TE

Truncation North = TN

For the projection, it is necessary to use tmerc (rather than utm) to avoid the default adjustments made by the utm projection. You will need to determine the central meridian for the UTM zone that you are using (This site might be helpful). We will refer to this as CM.

For a UTM projection, the false easting is 500,000m. The false northing is 0 in the northern hemisphere and 10,000,000m in the southern hemisphere.

For the truncated system, the key is to reduce the false easting and false northing values by the amount of the truncation, so:

FE = 500,000 - TE

Norther Hemisphere UTM:

FN = -TN

Southern Hemisphere UTM:

FN = 10,000,000 - TN

Proj4 string template

So, with the values computed as specified above, here is the template proj4 string

+proj=tmerc +ellps=WGS84 +lat_0=0 +lon_0=CM +k=0.9996 +x_0=FE +y_0=FN +units=m +no_defs

You can substitute your values for CM, FE, and FN to generate the required string to enter into the Pointerra custom proj4 string.