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How to measure distances on a map

Maps simply represent the characteristics of a region. They can have different shapes, present different types of information, and one piece of information that can be obtained from any map is the distance between different sites represented. Let’s see how to do it.

Once we have the map and have identified the points between which we want to measure the geographic distance, we can measure the distance on the map using a ruler. If we are interested in measuring a path between two points that does not follow a straight line, we can take a string, superimpose it on the path whose length we want to measure, and then measure the length of the stretched string with the ruler.

Next we must convert the longitude measured on the map into the geographical distance between the points. For this we use the scale of the map, that is, the equivalence between longitude on the map and geographical distance. The scale is usually printed in one of the corners of the map or on the bottom or top edge. The scale can be expressed with an equivalence in numbers and words, for example 1 centimeter equals 3 kilometers. Another way to express the scale is with a fraction that represents the direct conversion between longitude on the map and geographic distance. For example, 1 / 200,000, which can also be noted as 1 : 200,000, means that 1 centimeter on the map represents 200,000 centimeters of geographical distance, that is, 2 kilometers.

If the scale is expressed by a numerical equivalence, to obtain the geographic distance, simply multiply the measured length by the equivalence. In the previous example, if the length measured on the map is 2.4 centimeters, we will have a geographical distance of 7.2 kilometers by multiplying 2.4 by 3. If the scale is represented by a fraction type equivalence, it is multiplied by the denominator and, considering our measurement with the example of the previous paragraph, we would have a geographical distance of 4.8 kilometers: by multiplying 2.4 by 200,000 centimeters we would have 480,000 centimeters, which is equivalent to 4.8 kilometers.

A usual way of expressing the scale on a map is with a graphic scale on a bar with one or several sections, where the equivalence of the length of the bar is expressed. The following figure shows the political map of Mexico, and in the upper right part the graphic scale. The length of the full bar represents 450 kilometers on the map, while each of its sections represents 150 kilometers.

Political map of Mexico. Political map of Mexico.

In the event that the scale is represented graphically, to make the conversion we must measure the length of the bar or sections of the bar; then we divide the measured length between the places on the map that we are interested in knowing the distance between the length of the bar and multiply the result by the equivalence expressed under the bar. In this way, in the map of the previous figure we can see that the geographical distance between Mérida and Cancún is about 300 kilometers.

The graphic scales, although they can be more uncomfortable to convert, unlike the other scales, they maintain the proportion when the map is enlarged or reduced. In the example above, we could enlarge the image on the screen to see it better and the graphic scale will be enlarged by the same amount, so it is still valid. If it were a direct equivalence, the scale would no longer be valid.

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Edward Dalmau. The why of the maps . Debate, Barcelona, ​​2021.