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GISDataDepot - Map Scale Considerations

GISDataDepot > HelpDesk > Map Scale

Map Scale

Almost every map has a scale. And once you know and understand a map's scale, you can evaluate its detail and accuracy. On a paper map, scale is a simple idea. If we say a map has a scale of 1 to 100,000 (expressed as 1:100,000), we mean that a distance of 1 inch on the map represents 100,000 inches in the real world. The idea is straightforward because the size of a paper map never changes (as long as the user doesn't reduce it on a photocopier).

A map with less detail is said to be of a smaller scale than one with more detail because the scale expression is actually a fraction: 1/24,000 is larger than 1/100,000. Cartographers often divide scales into three different categories.

Small-scale maps have scales smaller than 1:1,000,000 and are used for maps of wide areas where not much detail is required. For example, a small-scale map of Europe (1:12,000,000) would fit on a single page (typically 8 inches by 11 inches).

Medium-scale maps have scales between 1:75,000 and 1:1,000,000. A medium-scale map of San Francisco (1:300,000) would fit on a single page.

Large-scale maps have scales larger than 1:75,000. They are used in applications where detailed local maps are required. A large-scale street map of a small town (1:24,000) would fit on a single page.

When you know a map's scale, you can estimate its likely contents. Usually, a small-scale map will be more general than a larger-scale map because many of the larger-scale map's features will not be included on the small-scale map. Consequently, a small-scale map will typically be less accurate than a larger-scale map because ground positions can be measured with more precision on the larger-scale map.

Scale in Digital Maps

With digital maps, the traditional concept of scale in terms of distance does not apply because digital maps do not remain fixed in size. They can be displayed or plotted at any possible magnification. Yet we still speak of the scale of a digital map.

In digital mapping, the term scale is used to indicate the scale of the materials from which the map was made. For example, if a digital map is said to have a scale of 1:100,000, it was made from a 1:100,000-scale paper map.

However, a digital map's scale still allows you to make some educated guesses about its contents because, generally, digital maps retain the same accuracy and characteristics as their source maps. So it is still true that a large-scale digital map will usually be more accurate and less general than a small-scale digital map.

Because the display size of a computer-based map is not fixed, users are often tempted to blow up maps to very large sizes. For example, a 1:100,000-scale map can easily be plotted at a size of 1:24,000 or even 1:2,000-but it usually is not a good idea to do so. It encourages the user to make measurements that the underlying data does not support. You cannot measure positions to the nearest foot if your map is only accurate to the nearest mile. You will end up looking for information that does not exist.

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