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storing the raster and vector maps due to their basic differences. Further, the vector
data are often available organized in relation databases even though they are exported
into file groups (e.g., ESRI shape or Mapinfo tab files). Consequently, the raster maps
need a database structure completely different from that of the vector maps.
12.3.1 Storage of Raster Maps
In most cases, the raster maps can be stored in a table structure in which one record
represents one map with all its descriptive data as shown in the table below.
map_id
Title
Sheet number
Scale
...
ยจ
denburg
3,455
P48
300,000
...
...
...
...
...
...
The present large capacity database managers allow the binary storing of raster
files, graphical data and various kinds of data in BLOB type fields. BLOB is an
abbreviation of binary long object, which term expresses that it contains the original
information converted into a series of bytes. To display the content of these fields is
not as simple as that of the usual alphanumerical fields. However, a great advantage
is its secure storage, because the authentication system of the database manager
automatically protects the BLOB type data too. This is very important for the
information systems that have a high risk of security.
However, the map collections do not belong to the category of high risk.
In addition, it may also happen that the raster files of the maps must be modified
(e.g., due to noise filtering, improving the quality of the image, georeferencing),
when the storage in the BLOB fields would cause problems. Therefore, the maps are
stored in another way. The descriptive data are stored in data tables, while the maps
are stored in file systems outside the relation database. The descriptive tables
contain the name of the maps only, which is a reference in the
map_id
field.
Therefore, if the raster file is modified in any way, it will be immediately visible
in the system without changing the database.
The raster map data are stored in two data tables. One stores the groups of maps
(table of
groups
, Fig.
12.2
), the other one stores the data records of maps
(
rastermaps
, Fig.
12.3
). Individual maps are sorted into groups by referring to the
table of
groups
.
This simple relation (of
1:n type
) link describes the grouping of maps. Several
other field values may be chosen for this purpose from the collection (e.g., scale),
but as the present project is a pilot plan, we decided to have a simple structure first.
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