A METHOLOGY FOR CREATING ANALYTICAL HILL-SHADING BY COMBINING
DIFFERENT LIGHTING DIRECTIONS
D. Loisios1, N. Tzelepis2, B. Nakos2
1 - Hellenic Military Geographical Service, Athens, Greece
2 - National Technical University of Athens, Greece
jbm@mail.ntua.gr
Hill-shading
images for cartographic use are mostly produced considering the light coming
from north-west, as this has been visually approved to help all map users,
experienced or not, to perceive the forms of earth’s surface relief immediately
and accurately. This condition is deeply rooted in the hill-shading process,
due to subconscious, positive understanding of graphic elements under
upper-left light in our everyday experience. In some cases, when slopes that
face to opposite north and south directions or to northwest and southeast are
dominating, the acceptance of alternative lighting directions (usually from
west or south) might be carefully utilized for better shading results. Also,
the idea of south lighting on three-dimensional shaded maps of areas in the
northern hemisphere had been strongly supported by some cartographers, more
concerned for the clear presentation of earth’s surface covering, than for the
correct sight of the relief.
Scepticism arises from the fact
that, a strict adherence to only one, optimal light direction for the whole
map, would be rather difficult to accentuate all the relief forms that exist in
the portrayed area, having several, different main directions. The well-known
method of local adjustment of light direction has been developed to deal with
this problem, but it works in a rather local way by adapting within a limited
range the azimuth and slope angles of light around the local terrain. The
problem still remains for entire sides of ground formations that are not properly
lightened. What is more, steep slopes that don’t face the light, are shaded
with dark grey tones that create optical confusion, finally leading to the
encryption of underlying relief information or any other superimposed
cartographic symbol. In larger scales with more detailed relief, the above
phenomena acquire more attention, because steeper slopes occur, variability of
aspect is higher and local adaptation is less effective.
This study experiments with ways to
create an analytical hill-shading image where the above deficiencies are
possibly omitted. Two images of shaded relief are produced, an initial one
based on a standard light direction, and an enhanced mixture of shading images
derived from different lighting directions, where each one participates with a
specific weight, either calculated as a function of aspect or even empirically
defined in order to give bonus to the traditional directions. The cells of the
initial shading that correspond to steep slopes are partly replaced by the
values of the enhanced shading, and the amount of partial replacement depends
on slope value. The final combined hill-shading image lacks of highly dark
tones in steep slopes and allows the relief details in these areas to be
revealed, still keeping the beneficial influence of north-west or other optimal
standard lighting.