Area Determination


A common task in Aerial Photogrammetry is to determine the area of an object or polygon on a photograph.  This method can be applied to aerial photographs or photocopied maps that may have changed scale from the original.

Area determination is usually determined by the use of a dot grid. Several conversion tasks are necessary. You must first determine the area representation of a single dot in at the scle of the photo or map you are using.

Example dot grid

Figure 1. An example of a portion of a dot grid.

Given the dot grid you choose to use you must determine:

  1. The dot spacing in units of your choice (eg. inches)
  2. The spacing units are multipled by the Photo Scale Reciprical (PSR) to determine the ground distance between the dots in units from above (eg. inches).
  3. Ground unit are usually converted into more convienent units (eg. feet).
  4. The units are squared to determine the area of the square.
  5. If the reporting units are different from the calculating units they need to be converted (eg. from square feet to acres).

Example dot grid with box

Figure 2. An example of a portion of a dot grid with a are box.

Now the dot grid is randomly places on the images and the dots within the area of interest are counted. These are multipled by the reporting units to determine the area.

Example dot grid area box

Figure 3. An example of a portion of a dot grid with the represented are mark with a box.

This process is the same as rasterizing a vector map in a computer based system.

Created by David R. Larsen July 2, 2004
Last Updated: August 6, 2012