Ordnance Survey – Great Britain's national mapping agency
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1.5: The significance of scale (4)
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.
The illustration shows what can go wrong. The coloured lines are generalised map data captured from a road atlas. When you zoom in, the deviation of the simplified features from their survey position is apparent when the large-scale data becomes visible. The generalised data is not wrong – it is just being magnified more than was ever intended.
- Small-scale map data
- Small-scale map data zoomed in on box
- Small-scale map data overlaid over large-scale map data
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!
Getting to grips with GIS has introduced some very fundamental principles of how map information can be stored in GIS.
The next section - Putting the Gi in to GIS - looks at how to capture and view geographical data.