Three Types of Map Scale.
All maps or charts are drawn to a specific scale. The scale is the relationship between a distance on the map versus the same distance in the real world. A large-scale map shows a small area in great detail, while a small-scale map covers a large area but with little detail. Selecting a map with an appropriate scale is therefore a compromise between coverage and detail, and depends on your intended activity. For example, climbing a mountain with complex topography requires a large-scale map, while a small scale map may be a better choice for a long canoe trip. The scale that you see written on the map is just an approximation. It is never completely constant across the whole map, but varies according to the projection that was used to create the map. The larger the area that a map covers, the more susceptible it is to scale variations. The scale can be expressed as a verbal scale, a ratio scale, or a bar scale.
Verbal scale
A verbal scale uses words to express the relationship between a distance on the map and the corresponding distance in the real world.
Example of verbal scale
The following is written in the margins of your map: “4 cm on the map represents1 km on the ground.” Here it is implied that a distance of 4 centimeters on the map is equal to 1 kilometer in the real world. This relationship allows you to calculate any other real world distance from the map. For example, a measured distance of 15.6 cm on the map represents (15.6 cm)/(4 cm) x 1 km = 3.9 km in the real world.
Ratio scale
The scale of a map can be expressed as a ratio between a distance on the map versus the same distance in the real world. Some people find ratio scales confusing because distances are not tied to a specific unit. You can use any unit you want as long as you stick to the same unit during the conversion between map and real world distances. After doing the conversion, you can of course convert the resulting distance into other units.
Example of ratio scale
A map with “Scale 1:50 000” written in the margins means that the distance between any two points is 50 000 times longer in the real world than on the map. For example, a distance of 8.5 cm on the map would be 50 000 x 8.5 cm = 425 000 cm = 4250 m = 4.25 km in the real world.
Bar scale
A bar scale shows real world distances directly on the map. Bar scales is the most common type of scale, and are easy to understand because of their graphical nature. A common practice is to show several bar scales on the same map, each representing a different length unit. Bar scales are often shown together with ratio or verbal scales.
Examples of bar scales
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