On a topographic map, 0 represents the elevation of the datum, which is a reference point used to measure elevations.

Here's a breakdown:

* Datum: A standard reference point or surface used as a basis for measuring elevations. The most common datum is the North American Vertical Datum of 1988 (NAVD88).

* Contour Lines: Lines on a topographic map that connect points of equal elevation.

* Contour Interval: The difference in elevation between adjacent contour lines.

Therefore, on a topographic map with a contour interval of 20 feet, a contour line marked with 0 would indicate an elevation of 0 feet above the datum (NAVD88).

It's important to note that:

* The datum is not necessarily sea level. It's a defined reference point that may or may not coincide with sea level.

* The elevation of 0 may not be found on all topographic maps. Some maps might focus on areas with higher elevations.

Let me know if you have any other questions about topographic maps!