The first feature was route planning. Adding really big pins into the map in 3d seemed a good idea.
The ability to name and annotate the route pin in 3d was also a nice idea
As was printing of arbitrary maps and routes
And adding in Garmin GPS support for uploading and downloading routes.
I tested this out in the Lake District and worryingly I was rarely where I thought I should be ! The route logs taken by the Garmin device at 1 minute intervals can be downloaded into the software to see where you've been, and the planned routes uploaded to the device for it to tell you where to go.
A phonic based searching mechanism for the Ordnance Surveys 250,000 place name gazeteer seemed a good idea too ! You can find places by name, the click on their name to take you there in 3d.
Once I'd got that data there was no reason why I couldn't have the signposts corresponding to the place names visible in 3d as well.
All wrapped up in a simple user interface, which included continuous Easting and Northing and Longitude and Latitude, and height readouts.
And once I'd added all these features, I was still getting 32 FPS for the whole of the UK. I then sent it out for testers to try out - a few walking friends of mine gave it a bash and after a few tweaks it was ready.