Which map feature is used to measure actual ground distance?

The bar scale is the correct choice for measuring actual ground distance on a map. It is a graphical representation that shows the relationship between a specific distance on the map and its corresponding distance on the ground. By using the bar scale, individuals can accurately determine how far apart locations are in real life simply by measuring the distance on the map and comparing it to the scale provided.

Contour lines indicate elevation and terrain features but do not provide information on horizontal distances; they represent changes in elevation rather than direct ground measurements. Grid lines serve as a coordinate system for navigation and locating specific points on the map but do not convey distance values. The legend provides essential information about the symbols and colors used on the map but does not itself measure distance. Thus, the bar scale stands out as the feature specifically designed for measuring actual ground distances.