Work out contour interval instantly with clear inputs, formula shown and shareable results.
Contour interval is the elevation difference divided by the number of gaps, which is one less than the number of lines counted. 250 m across five index contours gives 50 m intervals. Conventional intervals scale with the map: about 10 m at 1:25,000 and 20 m at 1:50,000, though relief and terrain type shift the choice.
Contour interval
interval = elevation difference / (contours counted - 1)
Map distance
1 km on the ground = 100000 / scale denominator cm on the map
Because five fence posts have four gaps. Counting lines instead of intervals is the classic off-by-one error in contour reading.
Steep ground. The horizontal distance between lines is the run for a fixed rise, so tightly packed contours are cliffs.