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GIS Guide to Good Practice |
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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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The right of Mark Gillings, Peter Halls, Gary Lock, Paul Miller, Greg Phillips, Nick Ryan, David Wheatley, and Alicia Wise to be identified as the Authors of this Work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. All material supplied via the Arts and Humanities Data Service is protected by copyright, and duplication or sale of all or part of any of it is not permitted, except that material may be duplicated by you for your personal research use or educational purposes in electronic or print form. Permission for any other use must be obtained from the Arts and Humanities Data Service(info@ahds.ac.uk). Electronic or print copies may not be offered, whether for sale or otherwise, to any third party.
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