The bar scale is an integral part of any useful scale map. It's the part that tells how much distance an inch (or other measurement) corresponds to on the territory the map represents. Like many such projects, the process of drawing a bar scale is neither complex nor difficult. It does, however, have a low margin for error. Getting this detail wrong can render an otherwise excellent map useless. Methodical work and double-checking are necessary parts of this process.
Determine the scale of the map for an even measure of distance, such as 100 meters, 1 kilometer or 10 miles. If you don't have that information, it is usually available from the printer of the map.
Measure the distance manually if necessary. You can do this by finding landmarks an appropriate distance from another. Measure the distance on paper between those landmarks.
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Draw a straight line of the appropriate distance on a blank or otherwise less vital part of the map. Traditionally, bar scales are located in in a corner of a map, often near the map key and compass rose.
Mark the scale the line indicates near the line.
Make a larger scale line next to the original line. For example, if your original line was 1 cm long representing 1 km of distance, you could draw a 5 cm line representing 5 km.
Double check your data and work both before and after drawing your bar scale. Scale is vitally important to mapmaking and an error, as mentioned above, can make your map useless for many purposes.