Trimetrogon is an aerial photographic survey method that involves the use of three cameras in a single assembly. One of the cameras is pointed directly downwards, and the other two are pointed to either side of the flight path at a depression angle from the horizontal of 30 degrees. The images overlap allowing the use of stereographic interpretation of topography.
The name comes from the Metrogon cameras used in the original montages.
References
Trimetrogon Wikipedia(Text) CC BY-SA