4 
id 
  
complete coverage exists at this scale, maps at scales of 1:250.000, 14500, 000 
and 1:1 000 000 are successively derived from the preceding larger-scale map. 
Where 1:50 000 maps do not exist, the derivation process starts with the 
1:250 000 map. 
During this derivation process, smaller lakes are dropped and outlines are 
generalized or exaggerated in order to provide line separation and legibility 
as given in the following table. 
MINIMUM SIZE OF LAKES & ISLANDS 
  
  
  
  
  
  
  
  
  
  
MAP SCALE LAKE DIAMETER ISLAND DIAMETER 
1:50 000 
Topographic 20 m 155m 
1:250 000 
Topographic 300 m 75 m 
1:500 000 300 m 
Aeronautical or 
Chart 450 m long (exag- 
gerated width) 
1:1 000 
International 1250 m 500 m 
: Map of World 
  
The lake outline is drawn from aerial photography in the original compilation 
and this outline is carried through, with modification, to the derived smaller- 
scale maps. This map derivation process, as it affects the portrayal of water 
is shown in figure 1. The stages of simplification are quite obvious; what is 
not so obvious on such a small sample, is how the character of the land can be 
lost in this process. Such simplification assumes a uniform distribution of 
random sized lakes. If there are a very large number of small lakes the carto- 
grapher faces the problem of either portraying the area as having no lakes 
(since all are under minimum size) or selecting a few lakes to exaggerate and 
carry through to smaller scales in order to indicate that lakes exist in the 
area. He is also constrained by the labour of outlining a multitude of small 
features. There is an understanable tendancy to use cartographic symbolization 
in such difficult areas. 
Landsat imagery has provided a means of examining how well, or poorly, the 
small-scale maps portray the nature of the land as it pertains to water areas. 
Figure 2 shows such a comparison, in which one finds it is very difficult to 
actually find on the image, the lakes that are "mapped" at 1:1 000 000. 
A CARTOGRAPHIC ALTERNATIVE:  LANDSAT IMAGERY 
Water areas are well-defined on Landsat Infra-red bands 6 and 7, leading to a 
relatively straightforward thematic extraction of water areas by selection of 
the appropriate density slice in either of the bands, although band 7 is to be 
preferred. 
Geometric correction of the Landsat digital image has been achieved to the accu- 
racy level required for small scale maps in the Digital Correction System (DICS) 
967