Greyscale elevation works like this.
You have 255 colors.
Each color is a step in elevation starting at see level which is black (rgb 0,0,0). The highest elevation is white (rgb 255,255,255). So one step above sea level is 1,1,1 and so on.
However, you have to realize the distance of the steps depends on your max elevation that you set when you import the grayscale elevation map. Even though it says 64000 is the max you can set actually it doesn't take anything above 32,000. So lets say you set your max at 20,000 ft. that is then divided by 255. So each gray scale change after 0,0,0 is a 78.4 ft increase in elevation ending at 255,255,255 which would be 20,000 ft.
If you set your max elevation when you import at 10,000 then each gray scale step would be 39.2 ft.
In the case of the map I was working on the highest mountain was 9,772 ft. so each step is 38.3 ft increase in elevation.
Although I think HTC code rounds the numbers so my examples really would be 78ft for 20K, 39ft for 10K and 38ft for 9772.
So for the U.S. find the highest mountain height for the continental U.S. and use that as your max elevation when you important your grayscale map.
