Compass recognizes two kinds of backsights: Normal Backsights and Redundant Backsights.
Normal Backsights. Normal backsights are sometime called reversed shots and they occur when the measurements are taken from the "to" toward the "from" station. In the diagram, A2 to A3 is a normal backsight.
Redundant Backsights. A redundant backsight occurs when you make two measurements of the same shot. One measurement is a foresight and one measurement is a backsight. The purpose of a redundant backsight is to catch blunders and reduce errors.
How The Compiler Processes Backsights. In order to process a backsight, the computer reverses the compass and inclination angles. This is done by adding 180 degrees to the azimuth and by changing the sign (+/-) of the inclination. With the redundant backsights, you have the option of having the Compiler average the two values. In cases where either the foresight or backsight are missing, the Compiler will use the values that are available.