We have a technical limitation here. Ground textures are limited to 1 pixel per ground vertex. The reason for that lies in the limited amount of textures you can assign to each baseboard. So, for the Trainz 5 m grid we are able to get 1 pixel per 5m, for the 10 m grid, it is one pixel per 10 m. That means, ortho-imagery will generally not work. Instead, you should apply a map to ground textures. For the 10 m grid, 1:50000 will be okay, and for the 5 m grid, 1:25000 or, in the US, 1:24000 will be fine. We are also limited in the number of colours. The default TransDEM ground texture set is optimized for maps, not ortho-imagery. It only has 20 colours, however, for maps, that is enough.
Now, when it comes to larger scale, including ortho-images, TransDEM offers extra texture carrier objects, called UTM tiles. If you have done the "Muengsten" tutorial, you will already have seen these tiles, painted with a 1:5000 map. If you can't remember, or skipped that chapter, do that tutorial again. UTM tiles can carry textures of up to 4096 pixel for a 500m UTM tile object. theoretically resolving objects down to 10 cm.
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