A technique for determining the area of features on air photos and maps by using dot grids. A transparent overlay with a grid-cell pattern is overlaid on the air photo. By counting the number of cells or dots that occur within a feature, it is possible to estimate its area, knowing the area for each dot or cell.
Dot grid. In this example, it is assumed that the dot grid overlay fits 9″ x 9″ size of an airphoto which has a representative fraction (RF) of 1/15840; that is, 1″ on an airphoto is equal to 15840″ on the ground. This scale can also be restated as 1″ = 1320′ or ½ mile. If 4 cells on the grid have a length of 1″ then 1 cell has a length of ½″; the area of cell is 330 ft.