How to Calculate Acreage on a Map
Land size is defined by acreage in the U.S. An acre is defined as being 66 feet wide and 660 feet long. There are exactly 640 acres in a square mile. Depending on the scale of the map, square or rectangular measurements can be easily transformed into approximate acreage with simple mathematics. Maps with metric scales can also be used to approximate acreage with the appropriate mathematical conversion. Irregular areas can be calculated with an analytical balance.
Things You'll Need
- Map with a scale
- Ruler or calipers
- Pencil
- Tracing paper
- Fine scissors
- Analytical balance
Instructions
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Square and Rectangular Acreages
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1
Find the location or area of interest on the smallest scale map possible. Read the scale and note how many feet or miles per inch for the scale.
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2
For rectangular or square parcels, measure the sides of the parcel in inches.
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3
Convert the inches to feet or miles using the map's scale. For instance, if the parcel is one inch by two inches on the map, and the scale is one inch to one mile, then the parcel is two square miles in area.
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4
Convert from square miles to acres. Divide the square miles by 640. That is the number of acres. For areas expressed in square feet, divide by 43,560 to get acres.
Irregularly Shaped Parcels
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5
Trace the area of interest on plain white paper or tracing paper if necessary. Cut out the area of interest--for instance, an island. Use fine scissors to get the outline as perfect as possible.
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6
Draw a similar-sized square on the same piece of paper using the map's scale. For instance, if the scale is in feet and the irregularly-shaped parcel is approximately 2,000 feet square, make a scaled-down 2,000 foot square.
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7
Cut out the scaled-down square.
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8
Weigh the square on an analytical balance. The weight of the square is proportional to the land area within the square. For instance, 2,000 by 2,000 is four million square feet, and the piece of paper weighs 0.0312 grams.
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9
Weigh the irregular shape on an analytical balance. Divide the weight of the irregular shape by the weight of the square shape. Now multiply by the number of square feet represented by the square shape. For instance, if the irregular shape weighed 0.0263 grams, divide 0.0263 by 0.0312 to get 0.843. Now multiply by the area represented by the square (4,000,000 square feet) to get the area in square feet of the irregular parcel. Divide by 42,560 as in Step 5 in Section 1 to get acres. To convert square kilometers or meters to acres, see the conversion chart in Resources.
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1
Tips & Warnings
For irregular shapes, it may help to enlarge the area using a copy machine. You can cut out the shape more precisely and you will get a more accurate estimate of the actual acreage. The method of using paper weight to determine area was used by early analytical chemists to determine area under complex curves before the advent of modern computers.
References
Resources
- Photo Credit drawing compass on a map image by Nikolay Okhitin from Fotolia.com
