DOT GRID

Dot counts are the most widely used method of estimating area on aerial photographs. A dot or square grid works well with an irregular shape area on a map or an aerial photo. It does not require special equipment other than an overlay and it is a good method for estimating area totals. Dot grids allow us to calculate area measurement by counting the number of dots per square inch or square centimeter. We then convert these tallies to acres or hectares.

The method is simple. Place the overlay on top of the area you want to measure. Count each dot according to the type class in which it falls. Count all full squares and all squares that fall halfway within the area. Tally the total number of dots in each type class. Then tally the total of each type class in the survey area for an area summary measurement.

Procedure:

1. Place the dot grid over the photo. Be careful not to align the grid for "best fit" with the boundary of the area to be measured. Tape the grid in place.

2. Count all the grid cells that fall completely within the area to be measured. Convert to total dots. Record the number.

For example, a 1:20,000 scale dot grid has 5 dots per grid cell. 4 cells at 5 dots per cell = 25 dots. Another example is 4 cells at 25 dots/cell = 100 dots.

3. For the remaining grid cells, count all the dots that fall within the area being measured. Record the number

4. Count all the dots that touch the boundary of the area being measured. Divide by 2. Record the number.

5. Add dot counts from steps 2, 3, and 4. This cumulative number of dots represents the total area being measured. Multiply the total count by the area represented by one dot to arrive at the area measurement.

Procedure modified from Fundamentals of Remote Sensing and Airphoto Interpretation by Thomas Avery and Graydon Berlin.

Here add three pages of grids -- scan from your technical manual