Thanks again for the ideas. I have been looking more closely a dump trucks recently. When looking at the side of the truck, it was pointed out to me that there is a gap between the cab and "box" about even with the door handle. I am thinking of using a ultrasonic or laser sensor to find this opening and work off of it to find the front edge of the truck bed. I am thinking of using a second sensor located near the silo discharge cone to determine distance from the cone. If the distance is in a preset range, the truck will be assumed to be positioned in such a way that the asphalt should dump into the truck bed. Any distance greater than the maximum would suggest the truck has moved past the "dump zone". Any distance less than the minimum may suggest dumping on the cab. I'm thinking that, if the reflected signal is not too dispersed by the surface of the piles of asphalt in the truck bed, this scheme should work for all three dumps.
Am I going to have trouble with the reflected signal?