Mighty's World Health Site

The Choice Of Scale

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All maps are characterized by the scale chosen to represent the geographical distribution of the disease of interest. The scale of a map determines the extent of aerial coverage possible and the degree to which spatial structures will be observable on the map. The scale determines the visibility of the geographical distribution of the disease in question.

The first consideration in choosing a scale is the relation of the map scale to the study area. If a fixed area is to be used, then the scale employed should represent the variation of the disease within the fixed study region. If, on the other hand, the area to be mapped can depend on the scale chosen then some trade-off between scale and area may be useful. In either instance, the scale chosen must represent the variation within the study area defined. The close relation between scale and area can be seen in Figures 2.1 and 2.2. Figure 2.1 shows the map of lung cancer mortality for Falkirk for the period 1978-83.

Figure 2.2. Falkirk: single region thematic map

The mortality is displayed as varying counts within enumeration districts which are classified by shade. At this scale, the overall distribution of lung cancer mortality within Falkirk is clear. If we change the scale of the map to increase the resolution (we zoom in to see more detail), we inevitably reduce the geographical area which can be mapped. If, on the other hand, we zoom out to a larger aerial extent (for example, the whole of Scotland) we lose resolution and display less detail about the geographical distribution. Zooming represents a change in scale and concomitant with that change, there are likely to be changes in what we can interpret. Zooming-in allows the examination of smaller areas but this must be coupled with a change in resolution if it is to be of significant benefit. For example, if the 06AL05 tract area of Falkirk was examined, with the same resolution (but with different scale) as the whole map of Falkirk then there would be little information gained from this process (see Figure 2.2). Note also the close relation between the mathematical concepts of differentiation/integration and resolution level.