The basics of scale
When looking at a paper map, probably the most important thing to bear in mind is the map scale. This is the relationship between the dimensions on the paper to the real distance on the ground.
If a building is 13 m long in the real world and a map depicts this length as 13 mm, the scale is 1:1000. Multiplying the distance on the map by the scale factor gives you the real world dimension.
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 depend on the monitor size.
Scale of capture
All GIS packages enable you to zoom in and out on the map data as much as you like. Again, this means that you cannot say that the map data has a particular scale. However, all topographic data has a scale of capture – that is the source data was captured at a particular scale, whether this was a paper map or an aerial photo.
It is important to understand the source scale of your data for two fundamental reasons:
- Data from a particular scale should only be viewed within a certain range of magnification for it to make sense visually. Combining two or more datasets together is only appropriate if they have an equivalent scale of capture.
Very detailed mapping, showing the outline of individual objects such as walls and fences, is known as large-scale data. The positional accuracy of features shown on this type of mapping is very high but there is so much detail that if you zoom out the view becomes very cluttered.
The mapping that most of us recognise has been deliberately simplified. A cartographer creates these simple, readable maps by selecting information from a larger-scale source. Not all the detail from the source map can be shown. For example, a road atlas that attempted to show every building in the country would become far too cluttered, so some features are aggregated, smoothed out or omitted altogether.
Sometimes it may be necessary to alter a feature's true survey position slightly to make space for the map symbols. Furthermore, the thick red lines of an A road are shown much wider on the map than the actual road is on the ground. This science of small-scale map production is known as generalisation.
Be careful with scale
Many GIS data products are created from generalised map sources – they are very useful for simple, quick to draw overview maps. However, you can view the data at any scale once it has been incorporated into a GIS – as we have seen, this can lead to data no longer making sense if you zoom in too closely. Worse still, the effects of generalisation will show up if this data is viewed against other more large-scale mapping.
These principles may seem straightforward, but it is alarming how often the real benefits of GIS are lost through using inappropriate combinations of data. For example, you may find an accurate road layer being shown against a less accurate coastline, which can give the (false) impression that the restaurant you are looking for is actually an underwater one!