The ruler on a map used to convert ground distance to actual distance is called a scale bar.

Here's how it works:

* Representing Distances: A scale bar shows a specific distance on the map, corresponding to a real-world distance. It can be linear (a straight line) or graphic (a series of bars).

* Measurement: You measure a distance on the map using the scale bar, then use the ratio of the scale bar to determine the actual distance on the ground.

* Example: If a scale bar shows 1 cm = 10 km, then 2 cm on the map represents 20 km in real life.

Why is it called a scale bar?

The word "scale" refers to the ratio between the map distance and the real-world distance. The "bar" is simply the visual representation of that scale, making it easy to use.