Contour lines are imaginary lines that connect places at the same height above sea level.

These are recorded in brown on the survey maps. These documents appear to be closed curves. The height difference between the two adjacent contour lines on the survey map is 20 m. This is called contour interval.

As the distance between the contour lines on the map increases, so does the height. In hanging places the distance between the contour lines will be reduced.
Hill refers to the contour curve that decreases in size as it enters one another. When referring to a cliff, many contour lines are very close to each other.

The difference in height between two positions is called the vertical interval OR VI and the horizontal distance between them is called the Horizontal equivalent OR HE. VI can be calculated using contour records. HE is the distance calculated using a map. Dividing VI by HE gives a gradient or slope. Paths with less gradient are more comfortable for soldiers.

Each location can be accurately detected using advanced tools. The map will have a black dot and a number next to it to show the places where the elevation was found. That number shows the elevation above sea level of that location. This sign is called Spot height. In some places the elevation is recorded and they are called Bunch mark. The map is marked BM on the map to indicate those locations.

The locations used as observatories are called triangulation stations. A small triangle on the map to indicate them and the height of the area adjacent to it can be calculated.