When reading any map the scale is important to understand. On United States Geologic Survey (USGS)Â Maps there are two scales: the fractional scale and the bar scale.
Fractional scales can get a bit confusing as mathematics are involved, but please donâ€™t stop reading; I promise it is not that bad!
First of all, the name fractional scale may be confusing because it is almost always printed as a ratio. Remember ratios are really just fractions so if you see 1:24,000 you are really working with 1/24,000. But what does that mean?
Let us assume we have a map with the fractional scale of 1:24,000. 1:24,000 is the ratio between measurements on the map with the distance it represents in real life. Example: If you measured one inch on the map it will equal 24,000 inches in real life. The units are trivial; you could use any units such as centimeters, inches, frog hops (however you measure that!), etc. â€“ 1 on the map = 24,000 in real life.
Maps with larger fractional scales (1:24,000 = 1/24,000) cover less area, but offer more detail â€“ think of them as local street maps. However, maps with smaller fractional scales (1:250,000 = 1/250,000) cover more area â€“ think of these maps as state highway maps that cover the entire state but lack the fine detail of local street maps. Remember that the fraction 1/24,000 is a larger number (thus a larger scale) than 1/250,000.
Here are examples of the three most common scales that are used by the United States Geologic Survey. Starting with small scale (1:250,000) and working to large scale (1:24,000) we will focus on Mount Mitchell: elevation 6,683 feet and the highest point east of the Mississippi river.
Can you find Mount Mitchell? This map is designed to have a small scale resulting in more area covered but with less detail. Â Great for highway travel or a general overview for the area but not good for trail maps.
The 1:100,000 map offers more detail and Mount Mount Mitchell can be easily found. If you are hiking on well maintained trails such as the Appalachian Trail 1:100,000 scaled maps are sufficient. They offer you generalÂ terrainÂ information and give you a good idea as to what is around you.
A 1:24,000 map is a large scale map that offersÂ plenty of detail. These maps are great for most trail conditions and off trail travel. 1:24,000 maps are also called 7.5 minute quadrangle maps because they cover 7.5 minutes of longitude wide and 7.5 minutes ofÂ latitudeÂ high, but that is for another post. For now just remember 1:24,000 = 7.5 minute quadrangle = large scale map = more detail. I choose this older map to show that some times the scale ratio can be expressed as a fractions.
The bar scale, as the name implies is composed of bars with a set unit of measurement in reference to the map. Bar scale compares map distances with ground distances in defined units usually miles or kilometers and make it easy to calculate distances in the field. Using a topographic map and the bar scale you can measure the distance between points of interest on the flat surface of the map, but you also must take the contour of the land into account.Â A 3 mile trail would be an easy hike, but if the trail gains 2,000 feet in elevation that is a whole different story.