Therion relies on the proj4 library for coordinate transformations. Besides the UTM WGS84 the other supported coordinate systems are listed in the esri and epgs files in the subdirectory externd/proj4/nad of the therion sources. If your data are in an unsupported coordinate system, or you rather use different parameters for the coordinate conversion, you can change one of these files and enter the parameters for your coordinate transformation.

The easiest way is to pick an entry in one file that you are confident you will "never" use, and change it. It is advisable to make a copy of the line into a comment (put the number sign '#' in front of the line). You must leave the code number unchanged, because you will use it to refer to your new coordinate transformation.

For example,

# my custom coordinate transformation
<26593> +proj=tmerc +lat_0=0.0015 +lon_0=8.99964 +k=0.999600 \
+x_0=1500000 +y_0=0 +ellps=intl +units=m +no_defs  no_defs <>

Alternatively you can enter a new line, with an unused code, followed by the parameters of your transformation. Again, write a comment to remind you about what it that transformation, because the next time you will look at the file you will not remember what you have done. If you decide to add a new line, you must modify also the therion sources, in particuilar the files thcsdata.h thcsdata.cpp adding the proper entries in the enumeration and array for the coordinate system. Be aware to increase also the size of the array.

You must compile the library proj4, and therion, before the changes take effect.