404 Grid Overlay

Using the Grid Overlay to Figure Areas
The grid overlay is a simple method to calculate areas on a map. It is not as accurate as
using a planimeter or geographic information system, but it produces results accurate
enough for Community Rating System impact adjustments.
In this example, the community has a park on Wildhorse Creek. The park boundaries are
from Darwin Street to the corporate limits between 1st and 5th Streets. The CRS
Coordinator wants to know how much of the park is in Zone A.

Step 1. Highlight the area to be measured. Mark the outline of the area so that it will be
easy to see the boundaries through the overlay.

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Step 2. Select the overlay to use. For larger areas, Grid Square # I will take less time
because there are fewer squares to count. However, the smaller squares (Grid Square #2)
should be used to calculate smaller areas. These instructions use #2 because the example
area is small.
Step 3. Lay the overlay over the area to be measured and count the number of crosses.

Count every other cross that is on the boundary. In the blow up below, the crosses to be counted
are circled. Cross"A" is on the boundary. It is counted but the next cross on the boundary, Cross
"B" is not. In the above illustration, there are 111 crosses.

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Step 4. Lay the overlay over the area at a different angle and count the number of
crosses. In the example below, there are 113 crosses in the area. If the number is not:
close to the previous count, repeat step 4 at a third angle.
Step 5. Average the count of crosses. In this case 111 + 113 divided by 2 is 112.

Step 6. Determine the scale of the map. In this case, the map scale is 1" = 400'

Step 7. Find the conversion factor. At the bottom of the grid squares are the conversion
factors for the more common map scales. For Grid Square #2, 1" = 400, the conversion
factor is .103 acres per square and .00016 square miles per square. If you are using a map
with a scale not listed on the grid squares, see the instructions below.
Step 8. Multiply the average count of crosses times the conversion factor. The result is
the size of the area measured. For the example illustrated, the area of Wildhorse Creek
Park in Zone A is 112 x 0.103 = 11.54 acres.
Instructions for Step 7 if you are using a map with a scale not listed on the grid squares: Grid
Square #2 has six squares to the inch. Divide the number of feet per inch on the map scale by
six and square the result. Example: 1" = 600’; 600/6 = 100. 1002 = 10,000. One grid square is
10,000 square feet.
There are 43,560 square feet to an acre. Divide the size of the grid square by 43,560 to
determine the number of acres per grid square. Example.: 10,000/43,560 = 0.23. One grid
square is 0.23 acres.
In this example, the conversion factor for 1" = 600’ is 0.23 acres/square. If you are using Grid
Square #1, use the same approach but divide the map scale by 3 instead of 6. If you are
working in square miles instead of acres, divide the size of the grid square by 27,878,400
instead of 43,560.
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