10-03-2013, 01:41 PM
The white crosses are used during aerial photography. After the photos have been shot, the crosses, which show up in locations whose coördinates are known, allow the photogrammetrist to mosaic together the individual photo frames. Overlapping frames can then be examined using a stereo plotter to derive a 3D view and plot contour lines or a mesh of known elevations.
The typical local government aerial survey, for cadastral mapping with building roofprints, only needs a white cross "target" every couple of miles or so, and these days it's usually accurate enough to locate those with GPS devices. Putting several down in a mile, plus the other fieldwork you see with the devices on tripods, probably indicates work for a road widening or similar project where they have to figure out how much earth will be moved, what the resulting slopes will be, and how high the retaining walls must be.
The typical local government aerial survey, for cadastral mapping with building roofprints, only needs a white cross "target" every couple of miles or so, and these days it's usually accurate enough to locate those with GPS devices. Putting several down in a mile, plus the other fieldwork you see with the devices on tripods, probably indicates work for a road widening or similar project where they have to figure out how much earth will be moved, what the resulting slopes will be, and how high the retaining walls must be.