1-6 Scale, Accuracy and Resolution

Scale of map refers to the ratio of distance on a map over the corresponding distance on the ground. The scale is represented as 1: M or 1/M, where M is called the scale denominator. The larger the scale, the more the detail described by the map and with higher accuracy.

In GIS, largest scale of map would be 1/500, that is used in cadastre survey. The smallest scale would be 1/1,000,000, that is used in the world map and global study.

Accuracy is defined as the closeness of measurements or estimates by computation to true values. Accuracy is generally represented by standard deviation of errors, that is difference between measurements and the true value.

where _ : error of measurements

n : number of measurements

In GIS, errors result from the map itself, map digitizing and coordinate transformation, which will sum up to about 0.5 mm on the map.

In digital GIS database, there is no concept of scale but resolution, expressed as pixel size (interval or dot per inch), grid cell size or grid interval, ground resolution for satellite images and so on.

There is a rough relationship between scale and resolution, as follows.

grid interval

where M : scale denominator

Table 1.2 shows the relationships between scale, accuracy and resolution. Height accuracy is usually one third of the contour interval according to international standard. Most of pixel size of the scanned raster data will be 200 ~ 400 d.p.i. (dot per inch) or 0.1 mm interval on maps.