Geography Reference
In-Depth Information
speed of processing may, in many cases, be markedly increased if the database is
well structured. Burrough and McDonnell (1998) and Worboys and Duckman (2004)
provide more detailed descriptions of database systems. Conventionally, attributes of
vector spatial data are ot en stored using a relational database system. Raster data are
ot en stored as self-contained objects. However, as noted before, in cases where a ras-
ter grid is composed of integer values, an associated table may be used that records the
number of instances of each value.
2.3.1
Database management
Data stored in a database are accessed using a database management system (DBMS).
A DBMS of ers facilities for updating the database and extracting data in a l exible
way. h e DBMS includes facilities to import data into the database, to manage user
access, to update the database structure and content, and to conduct queries to extract
specii c information. h e querying of spatial databases is the subject of Section 2.11.
A good summary of DBMS is provided by Longley
et al.
(2005a).
2.3.2
The Geodatabase
h e term 'Geodatabase' is sometimes used to mean a spatial database in general. More
specii cally, the Geodatabase is the core means of storing and managing spatial data in
the ArcGIS™ environment. h e Geodatabase of ers various benei ts over conventional
GIS i le formats such as shapei les or coverages (both widely used and well-established
vector i le formats). In particular, the Geodatabase combines data into a single inte-
grated database rather than storing each layer in discrete i les. h e Geodatabase has
built-in rules which help to maintain the integrity of the database and reduce database
maintenance.
1
Referencing systems and projections
2.4
h e location of spatial objects is usually recorded using some kind of spatial referenc-
ing system such as longitudes and latitudes, or eastings and northings using some kind
of national grid system. Given that the surface of the Earth (as well as other bodies) is
not l at, one of a variety of projection systems can be used to transform locations on a
sphere to features on a l at surface. It is essential that data are projected appropriately,
and a brief summary of some key issues is provided for context.
A projection can be dei ned as comprising the ellipsoid, the datum, and the projec-
tion. h e ellipsoid is the smooth approximate shape of the Earth and an ellipsoid can
be selected which best represents the surface of the Earth for the whole planet or for a
given area. h e surface which represents deviations from the ellipsoid is termed the
'geoid' (Clarke, 1999). h e datum is the origin or centre and rotation of the ellipsoid.
1 http://www.esri.com/news/arcuser/0701/migrating.html
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