ll data 
à large 
tiguous 
vel 1). 
le size. 
1b-sub- 
hierar- 
n cares 
. DTM 
ganiza- 
/ 
f a GIS 
Oo Some 
y within 
existing 
1as two 
’t exist 
Figure 9 shows a possible GIS data model for height 
attributing, 
   
  
      
terrain 
and 
situation 
  
    
Figure 9: GIS data model for height attributing 
The generation of a DTM or, in other words, of an object 
"height model" whould be possible but does not make 
sense; the heights are supplementary information for the 
objects they are attributed to and therefore in most cases 
not suited for a sufficient description of the earth sur- 
face. In consequence, the query space for data analyses 
is reduced to a 2.5D query space for the situation objects 
only. 
4.1.2. DTM interfaces 
  
DTM interfaces extend the methods of GIS data presen- 
tation such that also DTM products in vector and raster 
form can be some layers of a GIS. The underlying data 
structures are not changed; there is also no link between 
the situation data of the GIS and the terrain data of the 
DTM (figure 10). The worse approach in this case is a 
separated data management in two different data base 
management systems (DBMS) when data consistency is 
not guaranteed within both systems.Up to now there 
exists two realisations for DTM interfaces (Ebner et 
al.,1990): 
- data file interface; the instructions of the GIS are 
sent to the DTM program package via files. In the 
same way the results, produced by the DTM package 
are sent to the GIS. This data file interface can be 
realized without much effort because almost nothing 
has to be changed in the two existing program sys- 
tems. A disadvantage, however, is that the operator 
has to work with two different user interfaces. Ad- 
ditionally the data exchange between the two pro- 
grams via files is rather slow. 
501 
- subroutine interface; in this case all data processing 
and transfer is done in main memory without any 
disc access and the user works with one user inter- 
face only. This interface is much comfortable and 
faster than a data file interface, but requires a higher 
programming effort. 
  
     
  
    
terrain 
and 
situation 
GIS 
aa EE 
e; et 
DTM program package 
   
  
    
  
  
Figure 10: Data model for DTM interface 
4.1.3. Total integration of DTM 
  
Total integration of DTM into GIS means at first the 
recovery of data by only one DBMS. The underlying data 
structures can be 3D. However, terrain data must not 
neccessarily be merged with situation data in one posi- 
tional data set, if they are only essential for the descrip- 
tion of the earth surface; with regard to an accelaration 
of data access it is in most cases suitable to have situation 
and terrain data as separated data sets. But for an exi- 
sting relation between situation and terrain data (e.g.a 
lake) a combination of both should be possible to avoid 
redundant data storage. In case of separated geometry 
the terrain elements (nodes, edges, areas) should con- 
tain further thematic identifiers to allow for separated 
questions in terms of situation and terrain, Figure 11 
indicates possible thematic attributes for a DTM edge;