Contour lines

On a Korean 1:25,000 map a contour is drawn every 10 metres of height, and every fifth one is drawn thick.

A line that stays at one height

A contour line joins points of equal elevation. Picture an island in a lake that is slowly draining. Each time the water drops by two metres, you trace the new shoreline. The tracings never cross, they never split, and they nest inside one another like rings. That set of shorelines is a contour map. In the scene, press Lift contours and the brown lines on the map float up to the heights they stand for, wrapping the model like the waterlines on that island.

Contour interval and index contours

The height difference between neighbouring lines is the contour interval, and it is printed in the margin. The USGS picks it to suit the land: a fairly flat area may have an interval of 10 feet or less, a mountainous one 100 feet or more. To make counting easier, every fifth contour is drawn thicker and labelled with its height. This is the index contour. The thinner lines between are intermediate contours, and extra dashed lines called supplementary contours are sometimes added on very flat ground.

Korea uses the same idea with metric numbers. The NGII’s drawing rules for its 1:25,000 and 1:50,000 maps set the main contours at 10 metres on 1:25,000 sheets and 20 metres on 1:50,000 sheets. The index contours come at every fifth main contour, so they mark every 50 metres on a 1:25,000 sheet and every 100 metres on a 1:50,000 sheet. The NGII lists two more kinds for gentle ground, drawn as dashed and dotted lines between the main ones.

Reading heights between the lines

A point that sits between two contours is higher than the lower line and lower than the upper one. If it lies halfway between the 440 and 460 metre lines, its height is about 450 metres. This is an estimate, which is why surveyors add spot heights on summits. Change the interval in the scene from 50 to 20 and then 10 metres. The shape of the land does not change, but the map fills with lines and your estimates get tighter. Too many lines, though, and the map becomes hard to read, which is the trade-off every map maker faces.

Hills, hollows, valleys and ridges

A set of closed rings getting smaller toward the middle is a hill. But closed rings can also surround a hollow such as the crater in the scene. To tell them apart, USGS maps draw depression contours with short ticks, called hachures, pointing downhill into the hollow. Where contours cross a stream they bend into a V, and the point of the V always aims upstream. Where they cross a ridge they bulge into a rounded U pointing downhill. Switch the landscape to Hills and a valley and follow a blue river: the brown lines crossing it all make Vs that point back toward its source.