|Geographic Information Systems|
Definition (after Duecker): A geographic information system consists of a database of observations on spatially distributed features, activities or events which are definable in space as points, lines or areas. A GIS manipulates data about these points, lines and areas to retrieve data for ad hoc queries and analysis.
Conventional databases may or may not contain spatial information but they do not use it as such. e.g. the familiar DB containing information about parts and suppliers may store the address of the supplier, which is spatial data because it locates the supplier. However, if a map is not included as part of the system, it would be very difficult to answer a query such as 'Find all the suppliers within a 5 mile radius of our store'. With a GIS, it should be straightforward to answer such a query.
Example. A GIS was used to identify landscape patterns associated with Lyme disease risk in a county of New York state. Remote sensing data from Landsat was used to map vegetation and a land-cover map, showing house and forest areas, was prepared to find areas where houses and forest were contiguous. The incidence of Lyme disease was added. The result proved a link between forest contact and the disease, giving guidance to health planners working against the disease.
DBs and GIS
An advantage of this is that the data management of the two components can be optimized separately. However, the spatial data are handled outside the DB and cam-lot gain from standard DB functions such as integrity, security etc.
GIS systems should have the following functional
We have space here to consider only some of these, where the~~ are particularly important to GIS.
Data Capture and Storage
The earth is an ellipsoid and converting such a 3-dimensional object to 2 dimensions involves choosing a suitable projection and scale. There are several projections in common use e.g. transverse Mercator~(which preserves shape and direction), equal area (which preserves area). Then, a co-ordinate system must be selected such as latitude/longitude or the military grid system. A good GIS should be able to convert maps in different formats to a standard one.
Georeferencing This involves finding control points for aerial or satellite images so that these can be converted to the co-ordinate system that you are using for your maps. This is not as easy as it might appear at first glance.
Raster and vector
Raster grids are easy to understand, capable of rapid retrieval and analysis, good at representing continuous field variables such as topography. However, they require a lot of memory because every grid cell must have a value and for a large uniform area many cells may have the same value (although compression techniques are available). Rasters do not produce such accurate pictures as vectors since the cell size determines the resolution of the data. Vectors can follow features very closely and are very good a representing features that are shown on maps as lines e.g. rivers, roads, boundaries. They are much more efficient than rasters since an outline map could be drawn with perhaps only a few thousand points, far fewer than the number of raster grid cells that would be needed.
There are a number of retrieval operations involving spatial data which a GIS should be able to perform.
Some of the above operations may involve some degree of analysis and, of course, there is a large set of analytical operations which can be performed on data stores in a GIS which there is no space to consider here.
This brief account of GIS is designed to give you a flavour how such Dbs differ from others which you have studied.