Geoscience Reference
In-Depth Information
2.4.2 Contour levels
The most verisimilar contour system is based upon equal intervals, as
stated by Imhof (1982). But it's only scales greater than 1:1 000 000 that
allow equal intervals. Beginning from 1:1 000 000 it's almost impossible
to prevent equal interval everywhere. Here the geometric progression
proved to be most useful.
Despite the fact that lots of great contour systems have been developed,
they need adaptation for screen visualization. The smallest recommended
distance between contours on paper maps is 0.25 mm. At the same time,
the smallest clearly visible distance between lines on screen is 1.5 pixels or
0.375 mm (for 0.25 mm screen cell) when using antialiasing, as was ascer-
tained by Jenny et al. (2008). If antialiasing is not used, the distance would
be even greater — 2 pixels or 0.5 mm. This means that contour interval
has to be extended in some mountainous areas where contours are very
close to each other.
While mapping relief in large and middle scales for great territory contour
system should inevitably be more flexible than on paper maps and interval
would increase up to even 4 times in mountainous areas and decrease by 2
times in flat regions (
Table 3
).
Additional interval
Scale
Main interval
Plains
Mountains
1:10 000
2,5
1
5/10
1:25 000
5
2,5
10/20
1:50 000
10
5
20/40
1:100 000
20
10
40/80
1:200 000
20
10
40/80
1:500 000
50
25
100/200
Table 3:
Example contour intervals for large and middle scales (in meters)
The problem of changing interval is easy in topographic maps, where
every map sheet has fixed interval. At the same time, multiscale maps have
no sheets and intervals should be changed within whole terrain forms and
height levels. Traditional intervals offered by Imhof (1982) for small
scales usually need no adaptation and can be used directly.
