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Within this procedure the grey values of the slope aspect image are transformed in a way, that the grey value 
indicates the difference from the northwest direction. Theoretically, certainly a sine function would be at hand, 
which in our case has been approximated linearly. 
A visual inspection shows that the exactly northwest exposed slopes are frequently completely white. This, 
however, does not significantly disturb the total image, since the respective sector of the conic test figure is 
only very small. 
- A "lithographic method" ("Bergplattenverfahren") based on a practical print-technical approach: High quality 
shading prints frequently contain a so-called "mountain plate" increase at an increase of the normal shading. 
This type of shading does not show any grey values on the north-western slopes any more. In order to reach this 
result, the north-west exposed slopes have been brightened to an extent that the above effects have been 
reached (Fig. 3). 
  
  
  
  
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Figure 3: Combined hill shading. "Lithographic method". 
- An 'interpolation method" with comparatively free designed possibilities: Based on an spatially adaptive weighted 
addition operator, masks of a comparatively "hard" oblique shading have been generated with various contrast 
factors. This type of combined shading results in darker tones on the illumination side and thus a better depiction 
of the geomorphological characteristics of the north-western slopes. The interpolation method results in the best- 
balanced representations (Fig. 4). 
Aerial perspective effects did not yield any problems during their generation. In this case the well-known approach of 
logarithmic contrast changes has been used (cf. Brassel 1973 and Lukas 1994). Furthermore, the approaches using 
linear contrast changes and linear brightness changes also yielded satisfying results. 
Local rotations of the illumination in order to avoid tangential illumination turned out to be problematic. It is undis- 
puted that local directional changes are very helpful for the manually shading cartographer. During many digital at- 
tempts, however, the model with its stringent mathematical background was sort of hindering. Evidently rotations of 
illumination need further theoretical studies (Fig. 5). 
Eventually, tests have been performed to reduce the effects of the unnatural northwest illumination (in north-oriented 
maps of the northern hemisphere). In this case partial solutions through changes of the illumination angle seem pos- 
sible for monochrome hill shadings. New possibilities are offered by different variations of colour for polychromatic 
shading. This approach, however, has reduced application value due to a clash with colours of other map elements. 
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 629