I have seen big symbols (X, +, V, T) painted along state trunklines. What are these symbols, and what are they used for?
The X, +, V, and T are symbols used for aerial surveys and are painted on or alongside the road large enough to be legible in survey photographs.
The symbols are ‘targets’ an aerial photograph; there is no meaning to the different shapes or configurations of the markings.
A survey crew is dispatched to survey the location of these targets and assign coordinates to each.
When the survey is completed, the spatial relationship between each of the targets is known, such as the distance between them and the difference in elevation. This information is provided to an MDOT “photogrammetrist,” an aerial mapper.
The photogrammetrist loads two aerial photos into a machine called a “stereoplotter.” Each pair of photos overlap each other so that each inch of roadway is photographed from two positions to create a 3D view. (Similar to toy stereo viewmasters where two photos are inserted in the viewmaster to make a 3D picture.)
When the photogrammetrist views the targets in the 3D view, he can assign the coordinates given to him by the surveyor. Once this is complete, the relationship between the ground distances and the distances on the photo are known.
Once the relationship between the ground distances and the distances on the photo are known, any distance measured on the photo can be equated to its true distance on the ground, and a true scale map can be made of everything that can be seen on the photo.