What does the gray area of topographic map mean?

The basic principle of generating gray topographic map technology is to use MxN grid to represent the terrain, and at the same time provide corresponding gray maps. According to the gray level of each pixel in the gray map, the elevation of the vertices in the grid is determined. Black pixels (the value of each color channel in RGB is 0) represent the lowest altitude position, and white pixels (the value of each color channel in RGB is 255) represent the highest altitude position. In actual development, the elevation of pixel vertex is calculated by the following formula:

Actual elevation = minimum elevation+maximum height difference x pixel value /255.0