GIS Files 2: Geographical data

2.4: What does GIS data look like? (3)

Changing the appearance of raster data

As explained in Section 1.2 (Maps in bits), the nature of raster data inherently defines how the mapping should appear. The raster map is an image of coloured pixels, and the fact that a road is depicted comes from a visual interpretation of adjoining pixels of the same colour, not from any information saying this is a road in the data structure itself. The only data entities are the pixels themselves, and the only intelligence stored about those pixels is their colour.

Having said that, it is still possible to alter the appearance of raster data in a GIS. In simple terms you could make all red pixels appear blue, for instance. Usually this is not a good idea, because the image was designed with the most appropriate colours in the first place.

However, in some circumstances this facility is useful because you can tone down the colour scheme of the raster image to allow other information to be placed on top and made more readable. The graphic below demonstrates some different ways in which the appearance of a raster image can be adjusted.

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