Digital Elevation Models and Texture Overlays as Elements of Virtual Landscapes
A landscape documentation can just be an abstraction of the real world. No matter which method is
used, only a selection of the landscape’s features can be surveyed, recorded and visualized. In order to
describe the morphologic features of a landscape, elevations of single points are recorded. This data set,
where spot elevations are attached to selected vertices are referred to as Digital Elevation Model (DEM).
It may consist of single raster points arranged in a regular grid pattern, single points in an irregular
pattern (e.g. from tacheometric measurements) or contour lines (from existing maps). Since elevations
for points between these data have to be interpolated, there should be enough points in the DEM to
describe the topographic surface adequately. If sharp edges are present, the DEM should include break
line information to avoid smooth interpolation across these lines.
To give information about the features covering the topographic surface, a texture image has to be
draped over the DEM. Raster type images photographed with digital cameras or scanned from maps or
photographs can be used (Fig. 1). As long as this image is presented in vertical parallel projection, a
simple map type 2D representation of the landscape is achieved. Shading may help to experience the
third dimension; contour lines are more precise but not as easy to conceive. If all data are available in
digital form, perspectives can give a much better impression of the landscape and its topography (Fig. 2).
Animations with smoothly changing relative positions of camera and landscape result in a better three-dimensional
perception and allow a good interpretation.
Fig. 1: Combination of a DEM and a map to produce a perspective view of a
part of the Rhine River valley (Emmel 2000).
Surveying and Imaging Techniques
Digital Elevation Models can be derived by several different methods. For detailed DEMs, tacheometric
surveys or stereophotogrammetric measurements from aerial photographs should be used. Larger areas
can be surveyed using aerial photographs of smaller scale or even GPS measurements. Some Earth
observation satellite sensors supply images suitable for stereoscopic vision by taking images from two
different locations of an orbit, looking forward and backward towards the same ground location (in-track
stereo) or by taking images from neighboring orbits (across-track stereo). These images from digital
sensors or scanned photographs allow automatic DEM generation by matching techniques, thus
supplying a very powerful and
economic method to create
DEMs for large areas.
Texture overlays are always
needed in raster format.
Nevertheless, vector information
from maps, plans CAD or GIS
systems can be used if scanned
or converted to raster data (Fig.
1). Scanned aerial photographs
or digital satellite images are
ideally suited to generate
overlays (Böhler et. al. 1997,
1999). Their potential has hardly
been recognized anywhere in
cultural heritage documentation.
In the meantime, raster data
with one meter pixel size are
provided (SpaceImaging 2000).
So, textures of high quality can
be used (Figs. 2, 3 ), even when
aerial photographs are not
available.
Fig. 2: Perspective view showing a part of a Tang emperor’s mausoleum
in Shaanxi, China. IKONOS data have been draped over a DEM derived
from a local tacheometric survey. Vector data from archaeologic
prospecting (findings, tumuli, location of an ancient wall) are
superimposed.
Fig. 3: Comparison of different satellite image resolutions.
(From Böhler, Heinz 1999, complemented with an IKONOS image).
Upper row from left to right: IKONOS PAN, KVR-1000, SPOT PAN
Lower row from left to right: IRS-1C LISS III, SPOT XS, LANDSAT
Accuracy of data can become a critical issue when different data sources have to be combined. All data
have to be geo-referenced in the same coordinate system. Small scale maps, for example, show
intended displacements when several symbols are very close together. Therefore, map information may
not coincide with other information (see railway line at the left valley side in Fig. 1). If high resolution
images are used, the information intended for combination has to be of corresponding accuracy. In the
case of one meter IKONOS satellite images, we found out that the DEMs previously used with LANDSAT
and SPOT images did not have sufficient quality.
Geographic Information Systems
Geographic Information Systems (GIS) are an ideal means to manage data of cultural landscapes or
cultural heritage objects in landscapes (Heinz 1997). Quite a few World Heritage landscapes and objects
are already documented (and managed) using GIS-techniques (Roessler 2000). Special guidelines for
the use of GIS in cultural resources management were developed (UNESCO 1999). A GIS provides tools
to store, manage, analyze and visualize spatially related data. If a GIS with a temporal dimension is
chosen, the evolution of a cultural landscape can be modeled and studied. Also, future developments
and planning alternatives and their consequences can be explored in the same way.
