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International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B2. Istanbul 2004
6.1.1 Contour lines: The history of the contour lines shown
on Danish topographical maps goes back to the middle of the
19" century. At that time maps were an army responsibility. It
was decided that the surveying should be carried out by plane
table measurements at scale 1:20.000 and with 5 Danish feet
(1.57 meter) contour interval. From 1889 the contours were
measured with an equidistance of 2.50 meters. In the middle of
the 1960's photogrammetry was introduced to produce 1:10.000
map manuscripts. (Nielsen et al., 2002).
However, only a small part of the country was re-measured as
regards the contours. The major part of the existing contours
was interpolated by hand from 5 feet to 2.50 meter. The
contours were digitised and labelled (index) automatically.
The first digital elevation model of Denmark was constructed
solely from these contours in the late 1980ies (Frederiksen,
1987)
6.1.2 Database Objects: Certain objects from the geometry
database are per definition linked to the terrain surface. Road
centre lines and lakeshores are examples of such objects. The
geometry database holds all three co-ordinates of these objects
that are photogrammetrically measured during the revision
procedure. So, it is obvious to include such objects as individual
points to the basic data from which the elevation model is
generated.
6.1.3 Spot Heights: A number of spot heights are identified at
KMS and delivered to the photogrammetric companies together
with all other material necessary for the revision. That is points
with local maximum and minimum elevation and points that
support the description of the terrain undulations. Typically,
400 — 500 points are identified pr. 100 km?. The (X,y) co-
ordinates are given to the producer who's task it is to measure
the elevations photogrammetrically
6.2 DEM Construction and Revision
The elevation model is an in-house product calculated from the
above-mentioned 3 types of data. It is believed that the contours
describe the shape of the terrain surface quite well while tests
have proved that the absolute accuracy varies from one part of
the country to another. The photogrammetrically measured
objects and the spot heights are included for two reasons:
Firstly, to create coincidence between the TOPIODK objects
and the elevation model and secondly to ensure the correct
absolute level in the model.
Initially, two terrain models are created, one from the terrain
points. of the geometry objects and spot heights and another
from the points describing the contours. The height difference
between the two models is low-passed filtered revealing the
significant differences between contours and geometry
objects/spot heights. The contours are draped on the geometry
data by subtracting the filtered data from the original contours
preserving high frequency information and eliminating
inexpedient trends from the contours. In this way the modelling
takes advantage of the contours as a shape-describing
component. Kriging is the predominant estimation method but
also bilinear interpolation is applied (Ekholm, 1996).
The revision of the DEM is planned to follow the 5 years
revision of the geometry objects. At the moment, the revision of
the DEM is out of sync with the TOPIODK geometry revision
cycle due to first priority on the production of 1:50.000 maps
for the Danish Army. These maps include up-to-date calculated
5-meter contour lines.
When the geometry revision has passed the various check
procedures and stored in the database, relevant objects for the
Figure 4. Digitale elevation model of Zealand, Denmark
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