You are running into a computer classic graphics problem. The way that Panda implements shadow mapping is very much optimized for a spot light and how one would use a spot light in the real world. It is no intended for large outdoor scenes.
To use shadow mapping to do larger outdoor scenes, you need to get a little smarter and it is generally not possible to use the auto shader generator.
This is a very hard problem that many modern game engines have to deal with in a smart manner. Many game engines optimize things by drawing the shadow at a high resolution for things close to you and at a much lower resolution for things further away. Certain game engines don’t render shadows at all for things far away.
There’s been some work done on this in Panda before. Check out the work on Parallel Split Shadow Mapping.
[Parallel-Split Shadow Maps (PSSM): Shadows for large scenes)
I have to warn you though. This isn’t for the faint of heart. To properly solve these types of problems requires a pretty good understanding of computer graphics and rendering.
I will say that we’ve been working on a system to improve this over all and make this stuff easier to build and access but that’s still a far time coming.