Generally the data storage in hybrid GIS without 
concern of map sheet boundaries is referring to a 
fundamental homogeneous coordinate system, UTM 
or similar. In contrast to this map sheet orientated GIS 
are possible, in order to an easily realization if single 
scanned maps build the first data base, where each 
map is stored on a separated optical disk for example. 
In such map sheet individual systems transformations 
between different meridian stripe coordinates need 
not to be performed. Each map is stored with its own 
geometric reference system. This leads into large 
problems if such single stored map archives shall be 
used as a database for applications in areas covering 
two or more map sheets. So hybrid Geographic 
Information Systems should be stored without 
concern of map sheet boundaries, especially if 
additional data from remote sensing systems or 
similar are included in the database. 
But with regards to the user requirements map sheet 
orientated access also has to be available besides the 
standard access via free coordinate windows (x, y, Ax, 
Ay). So far map sheet numbers or names should be 
stored as non graphic information in the continuous 
GIS to enable this user access. 
It is quite evident, that such a continuous GIS based on 
UTM e.g. is handling a large amount of data. This is 
the reason why a practical hybrid GIS realization 
without substantial degradation of retrieval perfor- 
mance needs a special data structure. For this purpose 
an internal separation of the raster database in small 
submatrices (128-128 pixel e.g.), using header and index 
management, is necessary. So any user defined 
window will be found in the database in a very fast 
manner. 
3.2 Data Reduction 
Another particular problem in hybrid GIS in context to 
the large amount of raster data represents the disk 
capacity. By that data compression methods have to be 
integrated. The worldwide available compression 
algorithms are divided in two main groups: Bit level 
reduction methods achieve a decreased need of bits per 
pixel. Compression techniques of homogeneous data 
transform the data from pixelmatrices to pixel coun- 
ting structures. 
Examples of bit level reduction are: binary image gene- 
ration, reduction of gray levels, and calculation of gray 
value differences. For cartographic purposes binary 
images are used for most of the scanned map foils, and 
reduction of gray levels - from 8 bit to 6 bit e.g. - may be 
used for the hill shading data. Many compression 
techniques of homogeneous data are available, like 
run-length-, quad-tree-, or chain-coding to store 
scanned map data. Compression methods are nor- 
mally not very helpful if image data from remote 
sensing or photogrammetric systems are included and 
their full information content should be preserved. 
However, it must be pointed out that hybrid GIS need 
data compression methods. Depending on the appli- 
cation different compression methods have to be 
applicable simultaneously. 
692 
3.3 Layer Structure 
Independant to the amount of geographic data another 
database structure principle is performed by sub 
layering of the continuous GIS according to the appli- 
cations. Even analog topographic maps are organized 
in different layers respectively map foils. For example 
the foils of a topographic map 1:50.000 (TK50) of the 
Federal Republic of Germany include: 
- planimetric, 
- script level, 
vegetation, 
- waters, 
- contour lines, 
- hill shading 
- special level for hiking, cycling, touristic 
institutions and so on. 
Such a layer concept represents the fundamental 
graphic data structure of an hybrid GIS in general. Like 
shown in figure 5 GIS layers consist of the topographic 
database and the application database. The topographic 
data include digital maps (vector data), scanned maps 
(raster data), digital elevation models, also digital 
orthophotos, and rectified satellite images. Results of 
image interpretation and image classification (land 
cover e.g.) belong to the application database as well as 
geological data, soil types and temperature, land use 
(forestry data, water quality data e.g.), population 
statistics, administrative data, and much more. 
  
Fig.5: GIS layer structure 
(GOEPFERT 1987) 
4. INTEGRATED HYBRID GIS 
Additionally to the introduction of the basic data 
structure the principle system architecture of an hybrid 
Geographic Information System will be discussed in 
the following. It is shown how the different input data 
are connected to the database itself and also to the 
output data as results of applications with GIS. The 
complete system is separated into five subsystems: