I think what he is doing is using the magic wand tool in photoshop to select a grey scale area, and thus a grey scale "value" that winds up being the road. He then "fixes" the path that this selection creates by erasing the non-essential bits. Now, if you used the magic wand tool to choose the original "path" of the road you should theoretically be able to use the eyedropper tool to choose the exact grey color that corresponds to that height....the color grey you'd use for the road. I know, I know...that probably STILL makes no sense unless you know photoshop, but it was the best I could come up with on short notice.