GIS and map scale

Learn why scale matters in GIS – and what could trip you up.

When looking at a paper map, map scale is important. This is the relationship between the dimensions on the paper to the real distance on the ground.

If a building is 13 metres long in the real world and a map depicts this length as 13 mm, the scale is 1:1000. Multiply the distance on the map by the scale factor to get the building’s real-world dimensions.

In the world of GIS and computerised mapping, things are more complicated. A description of scale can lose its meaning – the scale of the image on screen can depends on your monitor’s size.

Scale of capture

GIS software lets you to zoom in on your map data, so you can’t say it has an exact scale. Even so, all topographic data has a scale, whether the source was from a paper map or an aerial photo.

You need to understand the source scale of your data. This is because it will only look right if viewed within a certain range of magnification. And combining two or more datasets only works if they use the same scale of capture.

Generalisation

Detailed mapping, which depicts the outline of walls and fences, is known as large-scale data. The positional accuracy of features shown on this type of mapping is high. But if you zoom out, the view becomes cluttered because the map is so detailed.

The maps most of us recognise have been simplified. A cartographer creates readable maps by selecting information from a larger-scale source. Features are combined, smoothed out or omitted altogether. Otherwise, if a road atlas tried to show every building in the country, it would be unreadable.

To make space for map symbols, some features might be moved from their exact geographical position. And to make roads and railways stand out, maps show them much wider than in the real-world. The science of small-scale map production is known as generalisation.

Be careful with scale

Many small-scale GIS data products originate from generalised map sources. They’re useful for simple, overview maps but don’t make sense if you zoom in too far. And displaying them with large-scale mapping magnifies the effects of generalisation. So while you can view data at any scale in a GIS, use each product within its suggested scale range for best results.

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