Question and Answer
How do I understand and use map scales?
Map scales are usually given in the catalog as a representative fraction, such as 1:100,000. A map series for a larger area may include the area you are interested in; so be sure to check for maps of larger areas such as countries or continents.
Scale is defined as the ratio of the distance on a map to the corresponding distance on the surface the map represents. Use this guide to understand the different scales found on maps in our map collection.
Three forms of scale
1 inch equals 16 miles
This example tells us that 1 inch on the map represents 16 miles on the surface of the Earth. This is the easiest scale to understand because it generally uses familiar units.
Graphic or bar scale
The bar scale is particularly important when enlarging or reducing maps by photocopy techniques because it changes with the map. If the bar scale is included in the photocopy, you will have an indication of the new scale.
Representative fraction (RF) or natural scale
1:1,000,000 (this is the same as 1/1,000,000)
The RF says that 1 of any measurement on the map equals 1 million of the same measurement on the original surface; for the example above 1 foot equals 1 million feet or 1 cm. equals 1 million cm. A good quality map should have both the RF and Bar Scales.
Large & small scale
When we speak of large scale maps we are saying the RF is large, i.e. the RF's denominator is small. 1:10,000 and 1:62,500 maps are large scale. Small scale maps have a small RF. 1:500,000 and 1:1,000,000 maps are small scale.
Converting between scales
Often when using cartographic materials it is useful to convert from one form of scale to another. If you have a good understanding of the concept of scale, the techniques are fairly simple. Here is an example of converting from verbal scale to RF. Remember, the RF has the same unit of measurement on both sides of the colon.
1 inch = 10 miles --> 1 inch = 10 x 63,360 inches (1 mile = 63,600 inches); RF= 1:633,600
To convert from RF to verbal scale you convert the fraction to familiar units of measurements; for example:
1:250,000 --> 1 inch equals 250,000 inches (63,360 inches = 1 mile); Verbal scale = 1 inch equals 4 miles
Here is a list of RF scales commonly used our map collection and their equivalent verbal scales.1:24,000 - 1 in. = .379 mi.
- 1:250,000 - 1 in. = 4 mi.
- 1:62,500 - 1 in. = .986 mi.
- 1:500,000 - 1 in. = 7.891 mi.
- 1:100,000 - 1 in. = 1.578 mi.
- 1:1,000,000 - 1 in. = 15.783 mi.