Just yesterday I was trying to help find a bug in a hobby GPS software, where the map projection was off by a bit, resulting in surfaces that were off by 5% compared to Google Earth. Someone had written a little library for matlab and gave it away on the mathworks website.
Being able to test that library without matlab, confirm it was working, then using Octave to debug the translated code was invaluable.
After debugging and confirming both version gave the same result, we were closer to the Google Earth solution, but still off by a bit. That’s when we learned the projection used by GE isn’t very good to compute surfaces, and ours was actually doing a better job. Octave turned day+ job into a one hour session.
Massive thanks to the authors, and the author of that matlab library!