Update: Check out NaturalEarthData.com for a new version of the World Relief Map and matching vector data.
What are cross-blended hypsometric tints?
They offer a partial solution to a long-standing map reading problem: many people misidentify hypsometric tints (elevation colors) as vegetation or climate regions. Conventional hypsometric tints often show green in the lowlands, yellow in mid elevations, and red in the highlands. These map colors incorrectly suggest to the untutored that forests cover the parched Persian Gulf region and a hot desert climate prevails on the Greenland ice cap.
Cross-blended hypsometric tints instead use modified elevation colors for regions that people presumably associate with the natural environment. The Sahara is dusty brown, northern Russia is boreal forest green, the Mekong delta is jungle green, and Antarctica is icy blue-grey. As in nature, the map colors gradually blend into one another across regions (x and y axis) and from lowlands to highlands (z axis), hence the name cross-blended hypsometric tints.