LEE
The base length as well as the gap between flight paths
are designed to comply with sheet DGK5 (German base
map 1:5,000). In this way an orthophoto covers a
complete map sheet. This arrangement is not necessary;
it does, however, simplify processing.
2.2. Aerotriangulation
Aerotriangulation takes place in analytical equipment with
subsequent bundle block adjustment using the BLUH
program system (bundle block adjustment of the
University of Hanover). Horizontal and vertical control
points are taken from the DGK5 for this scale. With this,
all the following data can be acquired in the Gauf$ Krüger
coordinate system.
2.3. Scanning of the aerial photographs
Scanning of the aerial photographs is carried out on a
photogrammetric scanner in a resolution of 1000 dpi
(approx 25 um). This equates in our example to a pixel
size on the ground of approximately 32 cm. In order to
produce colour photos every second aerial photograph is
scanned in three channels.
3. CORRELATION OF DTM AND RELIEF MODEL
A 1 m point grid is automatically produced with a
computation speed of 200 points per second through
model stereo matching. The terrain model (DTM) is
produced by a specially developed finishing process on a
raster base, whereby interference such as bushes or
buildings are interpolated.
In order to acquire the relief model (DEM) we return to
the original model.
It should be remembered that sharp edges are not
recorded. The absence of sharp edges leads to smooth
transitions where the earth's surface is characterised by
hard edges. Despite this, the landscape structure can be
modelled in great detail due to the high point density.
Data storage and processing of the large grid models is
done using ERDAS IMAGINE. Again, this level allows
morphological parameters (e.g. exposition, landscape
gradients) to be derived via suitable processing methods,
despite very high volumes of data.
3.1. Orthophoto generation
The computation of orthophotos takes place using a
generalised form of DTM in OrthoMAX. The tilt of the
building remains but the visible vertical masonry is
represented faithfully.
The ground resolution of an orthophoto pixel is about 30 -
35 cm for the above- named image scale of 1:12,500.
3.2. Mosaicing
Mosaicing the individual orthophotos produces a
complete picture. radiometric equalisation around the
edges avoids hard transitions between the individual
aerial photographs. The mosaics can be output in various
pixel resolutions.
319
4. GIS OPERATIONS
Digitalization of all buildings is carried out in the
orthoimage. At the same time reference is made back to
the true height data (DEM) during digitalization so that
measurement in the stereo model is not necessary.
4.1. Merging and Editing
Automated combination of the matched heights with the
building polygons cannot be carried out using standard
software. The programmes used for this purpose are a
joint development of IPI (Institute for Photogrammetry and
Engineering surveys of the University of Hanover) and
PHOENICS GmbH.
The following processes run automatically:
e Allocation of all measured building heights to the
relevant building polygons,
e Calculation of a statistic relevant to each building and
calculation of spot heights,
«Establishment of buildings whose relief must be
remeasured manually,
e Attributing raster and vector data sets with the heights
above sea level for each building,
* Calculation of the absolute building height through
calculation of differences of the building grid and the
DTM.
Any conspicuous buildings (statistics) are marked
additionally in the stereo model in order for subsequent
measurement to be carried out if required, and to allocate
the new height to the data set.
One of the results is the building model in figure 3.
The acquisition of trees corresponds to an analysis
which results from the intersection of the DTM with the
DEM taking all buildings into consideration. Vegetation
with a height of » 3 m high can be acquired with a
probability of > 95%.
4.2. Accuracy / Data control
The evaluation of the images is performed with the
above-named photo scale with a horizontal accuracy of
dx 7 dy « 1,5m. The height accuracy is dz < 1,0m.
Verification of relief data is done by reference
measurements with analytical equipment.
4.3. Data output
The vector output of the buildings is in ARC/INFO export
format. These are closed and attributed polygons.
Data output in DXF format or in ASCII is also possible as
output in various other standard formats.
The raster files of buildings and DTM can stored with 16
bit depth and 1m or 5m grid width as IMAGINE, GRID,
TIFF, SUN-Raster or other formats.
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996
