Scale describes the ratio in size between something in the real world and its representation on a map, diagram or model1. In other words, if a map has a scale of 1:50 000 then a distance of 1cm on the map represents 50 000cm (500m) in the real world.
Units of measure don't come into play because one inch represents 50 000 inches in the real world, just as one cubit2 represents 50 000 cubits in the real world.
It's All Subjective
Two terms are frequently used when talking about maps: large scale and small scale. Although these terms are subjective, they tend to be applied to ranges of scales of maps.
A map is considered to be a large scale map when features in the real world are portrayed larger on the map than on a map of a different scale representing the same thing.
So, if a distance of 1km in the real world is represented by 10cm on the map (scale of 1:10 000) and by 1cm on another map (scale of 1:100 000), the 1:10 000 scale map is a larger scale map than the 1:100 000 scale map.
A small scale map, on the other hand, is one where real world features are relatively small when represented on the map. A map with a scale of 1:10 000 000 would be a small scale map because many features are so small if represented on the map they wouldn't appear on the map.
Common Map Scales
There are a number of common map scales used by national mapping organisations. One reason for this is to allow people who have maps for both sides of a border to align them seamlessly. Map series that use imperial units commonly have a scale of '1 inch to the mile' or 1:63 360. Map series that use metric units commonly have a scale of 1:50 000, which means 2cm equals 1km. Common scales for smaller scale maps include 1:250 000 and 1:1 000 000.
You may have used a map that had the scale printed on the map. It might have said 1:2 000 if it was a detailed map of your neighbourhood or 1:500 000 if it was a road map. This is useful if you have time to measure the distance with a ruler and do the calculations. It's not so useful if you're driving along trying to figure out if you should buy petrol now or at the next petrol station indicated on the map3. Completely useless if you should happen to photocopy or scan the map. The moment you enlarge or reduce the map, you have changed the scale.
To deal with this problem, cartographers (people who make maps) put scale bars on the map. There are numerous styles of scale bars, but they all share some common features. They consist of a line with divisions on it indicating how long the division is. They do this using tick marks along a straight line, alternating black and white blocks (or other contrasting colours).
When you reproduce the map and enlarge or reduce it, the dimensions of the scale bar are preserved.
How to Use a Scale Bar
One of the simplest ways to use a scale bar is to take a piece of paper and mark a tick on one edge near the corner. Place this tick at your start point and align the edge with your destination. Place a tick on the paper at your destination.
Next, align the first tick on the zero end of the scale bar and read the distance where the second tick is. If the distance between the two ticks is longer than the scale bar, mark a tick at the end of the scale bar and shift that tick to align at zero. Keep doing this until the tick for the destination is on the scale bar. Count the number of intermediate ticks and multiply that by the length of the scale bar. Add the last portion and you should have the distance from your start point to the destination, as the crow flies - in other words, the straight line distance.
Another way to use a scale bar is with a set of dividers. A set of dividers is a device consisting of two arms with a fine point at the end of each arm. The arms are joined so that they can spread open.
To use a set of dividers to measure the distance between two points on a map, first connect the two points with a straight line. Next, place the end of one arm of the divider at the zero on the scale bar. Spread the arms apart and place the end of the other arm at some point along the scale bar that is a distance you'll be able to multiply easily.
Without moving the arms of the divider, place the end of one arm on the original and 'walk' the dividers along the line by placing the other end down and then swinging the first end around and so on. Count the number of times you can do this completely. For the last piece of the line, adjust the arms so that the arm being swung around is on the destination point. Measure that distance against the scale bar and add to it the number of times you swung the arms around completely multiplied by the distance you first measured off. This is the length of the line.
If you don't have a set of dividers handy, you can use other objects in their place. You can use a pencil if you carve the scale bar on it, a piece of paper or even parts of your body.