International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Vol XXXV, Part B4. Istanbul 2004 
determines the Steiner points, adds both to the data set. Since 
the Delaunay criterion is re-established in the preliminary 
triangulation, the shape of the integrated TIN may deviate 
somewhat from the one of the initial DTM. The methods have 
in common, that inconsistencies between the data are neglected 
and thus may lead to semantically incorrect results. Rousseaux 
& Bonin (2003) focus on the integration of 2D linear data such 
as roads, dikes and embankments. The linear objects are 
transformed into 2.5D surfaces by using attributes of the GIS 
data base and the height information of the DTM. Slopes and 
regularization constraints are used to check semantic 
correctness of the objects. However, in case of incorrect results 
the correctness is not established. A new DTM is computed 
using the original DTM heights and the 2.5D objects of the GIS 
data. 
2. SEMANTIC CORRECTNESS 
2.1 Consequences of non-semantic integration 
In our investigations a digital terrain model (DTM) is 
represented by a triangular irregular network (TIN). Bridges, 
vertical walls and hang overs are not modelled correctly 
because it is a 2.5D representation. The topographic vector data 
we consider are two-dimensional. The topography is modelled 
by different objects which are represented by single points, 
lines and areas. The integration of the data sets leads to an 
augmentation of the dimension of the topographic objects. 
Figure 1 shows two examples of the non-semantic integration of 
a DTM and 2D topographic vector data. The height values of 
the lakes do not show a constant height level (left side of Figure 
1). Several heights of the lakes near the bank are higher than the 
mean lake heights. 
At the right side of Figure 1 the roads are not modelled 
correctly in the corresponding part of the DTM. The slopes 
perpendicular to the driving direction are identical to the mean 
slope of the corresponding part of the DTM. There are no 
breaklines on the left and the right borders of the roads. Also, 
some neighbouring triangles of the DTM TIN show rather 
different orientations. 
2.2 Correct integration 
If we divide the topography into different topographic objects 
(road, river, lake, building, etc.), like the data of a GIS, there 
are several objects which have a direct relation to the third 
dimension. These objects contain. implicit height information: 
For example, a lake can be described as a horizontal plane with 
increasing terrain at the bank outside the lake. To give another 
example, roads are usually non-horizontal objects. We certainly 
do not know the mathematical function representing the road 
surface, but we know from experience and from road 
construction manuals that roads do not exceed maximum slope 
and curvature values in road direction. Also, the slope 
perpendicular to the driving direction is limited. 
Of course, all other objects are related to the third dimension, 
too. But it is difficult and often impossible to define general 
characteristics of their three-dimensional shape. For example, 
an agricultural field can be very hilly. But it is not possible in 
general to define maximum slope and curvature values because 
these values vary from area to area. 
The objects containing implicit height information which need 
to be considered for the semantically correct integration can be 
divided into three different classes (see Table 1). The first class 
contains objects which can be represented by a horizontal plane. 
The second class describes objects which are composed of 
several tilted planes. The extent of the planes depends on the 
curvature of the terrain; the planes should be able to adequately 
approximate the corresponding part of the original DTM. The 
last class shown in Table 1 describes objects which have a 
certain relation to other objects. Bridges, undercrossings and 
crossovers contain a certain height relation to the terrain or 
water above or beneath. 
  
Object Representation 
Sports field, race track, Horizontal plane 
runway, dock, canal, lake, 
pool 
Road, path, railway, 
tramway, river 
Bridge, undercrossing, 
crossover 
  
  
Tilted planes 
  
Height relation 
  
  
  
  
Table 1: Some topographic objects and their representation in 
the corresponding part of the terrain 
To integrate a DTM and a 2D topographic GIS data set in a 
semantically correct sense, the implicit height information of 
the mentioned topographic objects has to be considered. That 
means, after the integration process the integrated data set must 
be consistent with the human view of the topography and the 
height representations as shown in Table 1 'have to be 
represented correctly. 
3. AN ALGORITHM FOR THE SEMANTICALLY 
CORRECT INTEGRATION 
The aim of the integration is a consistent data set with respect to 
the underlying data model taking care of the semantics of the 
topographic objects. 
Topographic objects which are modelled by lines but which 
have a certain width, are first buffered. The buffer width is 
taken from the attribute “width” if available, otherwise a default 
value is used. Thus, the lines are transformed into elongated 
areas, the borders of which are further considered. The next step 
of the algorithm is a non-semantic integration of the data sets. 
  
Figure 1: Results of the integration of a DTM and a 2D vector data set without considering the semantics of the topographic objects, 
left: lakes, right: road network 
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