TerrainPage texture splatting

Is there a way to create a detail texture alpha map that exactly corresponds to a region of a TerrainPage's height map?

Suppose I have a TerrainPage with a 513x513 image-based height map, and that there is a "road" on the height map, represented by a straight, 2-pixel line from, say, (250, 10) to (250, 240).  Is there a way to create an alpha map to put a "road-ish" texture on that exact portion of the terrain?  With a 512x512 alpha map, the texture scales to blur part of the road into the surrounding terrain.  Is there a better approach?

Yes, 3D Terrain Editing software that creates the heightmaps creates alphas for textures that you've applied. What program are you using?

I'm currently just using an ImageBasedHeightMap created from a BufferedImage that I've drawn a road on (with some noise, blur, etc, to give it some interesting features).  So, I have a 513x513 BufferedImage for the height map, and a 512x512 texture for the alpha map.  Is there a way to apply the alpha map to a given region of the terrain, so it doesn't scale, or maybe a way to apply multiple alpha maps to cover the full terrain page?

I would prefer to be able to draw the features on both the height map and alpha map using the AWT painting methods (e.g., Graphics2D's drawLine(…)) on the corresponding BufferedImage instances.  I won't know the road positions until runtime, so I need a way to dynamically create the height map and apply the appropriate textures.

Should I be using TerrainBlock (2N) instead of TerrainPage (2N+1), to match the alpha texture size (2N)?

I'll play with a terrain generator to see what the output looks like.