by Jody Barnes

Created on: May 23, 2008   Last Updated: May 30, 2008

When you are planning a trip into the backcountry, a good topographic map is an essential item to bring. However, topo maps come in different scales, so you need to carefully choose the map you print or purchase, because if you choose the wrong scale your map will be more valuable as campfire tinder than as a navigational tool. The scale of the map refers to how a distance on paper relates to a real life distance. This is typically reported as a ratio number, such as 1:24,000, and scale is also shown as a graphical bar on the map. The ratio is often used to describe a map, and understanding how the system works will help you choose topographic maps that are useful.

In the case of a standard 1:24,000 scale map, the ratio indicates that 1 unit of measurement on the map equals 24,000 of that same measurement in the real world. It doesn't matter if you use inches, meters, or furlongs: the ratio remains the same. Map scales can be confusing to some people because the smaller the number on the right side of the ratio, the larger the map scale. "Large" or "small" map scale refers to the detail that can be depicted by a map. Large scale maps have more detail, because they are zoomed in on an area. Small scale maps are just the opposite; map features will appear much smaller on these maps, but the maps will probably cover a greater area. Just remember that "small numbers equal large scale" and you'll be able to keep this all straight.

For a concrete example: our 1:24,000 map will have more detail than a 1:62,500 map, but to show the same physical area as the smaller scale map, we would need four of the 1:24,000 maps.

Theoretically, a map could be at almost any scale, and with the advent of GIS software you can make custom maps at the scale of your choice; however, there are some standard map scales. These include 1:24,000, 1:62,500, 1:100,000, and 1:250,000.

The ratios used by map scales might seem random, but they actually were purposefully chosen. The United States Geological Survey (USGS) started mapping the entire country in 1879. To create a uniform system for the maps, they divided an area that was 1 degree of latitude by 2 degrees of longitude, and then subdivided that into smaller sections. The smallest unit was a rectangle 7.5 minutes of latitude by 7.5 minutes of longitude, which happens to be at the 1:24,000 scale, and these maps are often called "7.5' quads." The 1:62,500 maps are four times the size of the 7.5' quads, with the length of each map border at 15

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