If you're stuck on a desert island with just Bryce and no image editor, you can still make heightscale maps by doing a render from top view and assigning greyscale materials to primitives, imported objects, or 2d objects. Just go to top view, make your ground plane black, turn off sun and moon shadows in the skylab, and move the sun to the bottom of the sun position dial so you wont get any unintended shadows in your render. I like to make my document the same size as terrain/lattice resolution. Then when you're done export the render from topview and import it as a picture in the terrain editor. After you generate the terrain it helps to smooth a few times, or if you do have an image editor just apply a slight amount of gaussian blur. This bridge was made using several 1024x1024 lattices using heightfields designed in this manner. The advantage of using this method over an altitude render is that it doesn't matter how thick an object is, just what tone of grayscale it is. You could have a white cylinder and subtract from it by overlapping it with a black or gray 2d square. it's kind of like working with layers.