Scale is the ratio between the size of a feature and its representation on a map. Scale is displayed in the format:
1:scale_value [e.g. 1:10000]
where scale_value is the relative size of a real feature compared to a map feature 1 unit in length. Scale_ value is sometimes called scale denominator since it represents the denominator of the scale ratio (the numerator is 1). A large scale map has a large scale ratio, meaning scale_value is small. A small scale map has a small scale ratio, meaning scale_value is large.
Large scale maps cover small areas in detail. Small scale maps cover large areas in less detail. A large scale map might have a scale of 1:500 while a small scale map might have 1:250 000.
The scale is important in GIS because it:
• aids in selecting the resolution at which data is stored
• determines what detail can be presented on a digital map
The stated scale of the map is only true for the area where the line of tangency, or line of intersection, used in the projection touches or intersects the earth's surface represented by the map. Everywhere else the map scale will be slightly different from the stated true scale.
Scale factors for the UTM grid.
The lines of the UTM grid are uniform in spacing so that the printed squares on a map are exactly the same in dimension. However, no map printed on a flat sheet can preserve a truly constant scale in all parts of the map. Consequently, the fact must be accepted that the 1000-metre grid square does not everywhere represent a ground square of exactly 1000 x 1000 metres. On the grid lines, the scale factor is 1.0 but on the outer edge of the grid zone, the scale factor is 1.001. These scale errors may appear trivial, but a GIS is designed as an extremely accurate mapping system and it will accommodate this kind of information. Correct data entry for the header is important because it permits greater maximum locational accuracy when integrating files and executing GIS analysis on spatial data.