Why Are the Mountains Brown on That Map? Colour Standing In for Height

Contour lines carry the actual information about height, and reading them requires finding a labelled line and counting from it, which is slow and demands attention. Filling the space between successive contours with a colour assigned to that height band lets the eye take in the pattern of high and low ground at a glance, without reading a single number. The colours are therefore a visual aid layered over the real data rather than data in themselves, and a reader who wants a height must still go to the contours or to a spot height.

One sequence has become near universal and is worth stating:

• •Greens for the lowest land, darkening towards sea level
• •Yellows and light browns for intermediate ground
• •Deeper browns and reddish tones for high ground
• •White or pale grey or purple for the highest peaks
• •Blues for water, deepening with depth below sea level
• •The bands are not equal in height, being narrower where relief is greater

The convention has a serious flaw that map users fall into constantly, which is that it invites the colours to be read as ground cover rather than as height. Green suggests vegetation and brown suggests bare earth, so low-lying desert appears green and a forested plateau appears brown, which is exactly backwards as a picture of the landscape. The Netherlands appears lush and the Sahara depression below sea level appears greener than a wooded hill. Cartographers have known this since the scheme was standardised in the nineteenth century, and alternatives using a single hue that simply darkens with height have been proposed repeatedly and have never displaced it.

Colouring depth below sea level raises a difficulty that the land scheme does not have. Depth data is far sparser and far less accurate than land elevation, so a smooth sequence of blue bands can present a level of confidence the underlying survey does not support, particularly in areas mapped only from satellite gravity measurements. The bands used are also frequently on a completely different scale from the land ones on the same map, since the ocean is far deeper than most land is high, which means a reader comparing a blue band with a brown one is comparing quantities that were coloured by unrelated rules.

Modern relief mapping combines this with other techniques rather than abandoning it. Shaded relief, which simulates light falling across the terrain from an assumed direction, conveys the shape of slopes far more intuitively than banded colour and is usually laid over it. That convention assumes light from the upper left, and reversing it makes hills appear as hollows to most viewers, an illusion that is hard to resist once noticed. Digital elevation data allows continuous colour gradients rather than discrete bands, which removes the false impression of steps in the landscape, and allows the colour scheme to be changed instantly for any purpose.

Colour filling the bands between contours lets the eye read the shape of the land at a glance, while the contours carry the actual heights. The standard sequence runs green through yellow and brown to white, which invites the colours to be read as vegetation and makes low desert look lush and wooded uplands look bare. Shaded relief assuming light from the upper left is usually laid over it.