STL (STereo Lithographic) files describe 3d objects as mesh objects. Here, we assume that the 3d object consists of a 3d surface on the top of a rectangular, equi-spaced 2d grid. We further assume, that each node of the STL file describes the x-y location of an actual measurement. These measurements are then converted into the surface matrix of an x3p object. The resolution is derived from the distance between consecutive x and y nodes.
Examples
if (FALSE) {
# the website https://touchterrain.geol.iastate.edu/ allows a download
# of a 3d printable terrain model. For an example we suggest to download a file from there.
gc <- rgl::readSTL("<PATH TO STL FILE>", plot=FALSE)
x3p <- stl_to_x3p(gc)
}