I'm an avid cross-country skier, and traditionally daily trail reports are done by hand by the maintenance staff after they're out all night working on the trails.
I had the bright idea of putting GPS tracking devices in grooming equipment and creating the "what's been groomed" report automatically, in real-time.
It took about 4 seasons to really get it right, and there was no appreciable income for that period. Lots of lessons learned about equipment (antennas, good wiring practices in vehicles, power cleanliness in big equipment, etc), good ways to present the data, map projections, how to deal with messy data, dealing with non-technical users, cross-border shipping tarrifs, mobile-network provisioning rules, the list goes on. I did it alongside my full-time job for the first 4 years.
It's a tiny niche, and one I never expect to get all that big, but it looks like I'll be able to make it my sole income source next season.
Which is great, because it'll let me go skiing more.