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GIS Guide to Good Practice
Section 3 - Spatial data types

3.5 Scale and Resolution

Scale is the ratio of the distance measured on a map to that measured on the ground between the same two points. For example a quoted scale of 1:50,000 implies that a distance of 1 cm on the map translates to a distance of 50,000 cm (or 500 metres) on the ground. Often, the difference between large and small map scales is confused. The larger the ratio, the smaller the map scale. Therefore, a map of the world would have a very small scale, whereas a map of a town centre will have a large scale.

Resolution is the smallest distance that can be usefully distinguished on a map with a given scale, for example on a 1:10,000 scale map the smallest distinguishable distance is 0.5 mm which equates to a distance of 5 m on the ground. It is worth noting that the accuracy of a map cannot be 'better' than its resolution, but it can often be much 'worse'.

The larger the map scale, the higher the possible resolution. It is very important to be aware of the scale of a given spatial data source as the degree of simplification and reduction involved in the representation of spatial features tends to increase as scale decreases. As map scale decreases, resolution diminishes and feature boundaries must be smoothed, simplified, or not shown at all. This process is referred to as generalisation. To give an arbitrary example, a map of an area of rural Greece produced at a scale of 1:5000 may show villages and towns as discrete areas, whereas at a scale of 1:500,000 they will be portrayed as little more than dots.

It should also be noted that the wider usability of any co-ordinate system is partly a function of the resolution in which it is quoted. For example, the widely used six figure OSGB grid references are only valid within the hundred kilometre grid square for which they are quoted - they repeat in every other hundred kilometre grid square and they also only address to the nearest kilometre, even though the OSGB National Grid per se can support references to the nearest metre (or even sub-metre, if given as a decimal number).

In a GIS where spatial data sets from a range of sources are integrated and the spatial resolution of a given data set can be altered at will, it is vitally important to be aware of such issues and not to analyse spatial information at a scale greater than that of the data source (see DeMers 1997: 56 for discussion).


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