Must all topographic maps have a contour line that represents 0 m of elevation?
I'm not sure about this question, but I don't think you do need a line of 0 m because each contour line is exactly the same apart from each other, so you could just subtract that amount to get zero. Am I right?
No, that is not correct. In topographic maps, contour lines are used to represent different elevations on the land. Each contour line represents a specific elevation above a reference point, typically Mean Sea Level (MSL).
It is not necessary for all topographic maps to have a contour line representing 0 meters of elevation. The contour lines on a map depend on the range of elevations present in the area being depicted. If the area being mapped does not include any points at or below sea level, there may not be a contour line representing 0 meters of elevation.
To determine the presence or absence of a 0 meter contour line, you would need to consult the specific topographic map of the area you are interested in. The contour interval stated on the map's legend will give you the spacing between each contour line, allowing you to calculate the elevations of the lines you see. If the contour interval includes non-zero value lines but doesn't include a zero line, it means that there are no points at or below sea level in that area.