Functions of GIS – Geography – UGC NET – Notes

• A GIS (geographic or geospatial information system) allows you to record a base map with a geospatial referencing system such as longitude or latitude and then to add additional layers of other information. Importantly that information is identified using the same geospatial referencing.
• The GIS then allows the individual layers, or themes as they are called to be linked. Analysis of the information can then be undertaken using the statistical and analytical tools that are provided as part of the GIS. By providing spatial analysis of suitably coded data it is possible to provide striking, visual representations of data. These representations can often reveal patterns and trends that might otherwise have gone unnoticed without the use of GIS techniques.

Data Pre-processing and Manipulation

• Under this heading we will consider a large number of functions which a GIS may be required to perform in order to get the digital mapped data into the desired format so as to obtain requisite map output or to confidently allow for any subsequent data analysis.
• Essentially, this means that the original digital data may need to be changed in some way, i.e. either by correcting it, updating it, refining it or by altering it in some desired way. It is possible that many of the functions can be performed using other types of software, e.g. image processing packages.
• The capacity of a GIS to perform pre-processing means that the user has a huge opportunity to “interactively experiment” with the available data, thereby allowing for the appropriate data to be derived according to the task in hand. The efficiency in which individual GIS’s perform manipulations will depend upon the particular algorithms which they use and the way in which the data is structured.

Data Validation and Editing

• In essence this function represents the checking and revising of any data which has previously been captured, with the obvious aim being to minimise errors. In the case of digitised data, it is often possible and desirable to perform editing immediately following data capture, i.e. as a final stage in the digitising process, but it is important to note that many GIS software programmes allow for the detection and correction of digitising errors as a pre-processing function.
• GIS software also contains programmes for verifying the correctness of all geometric, topological and attribute data, e.g. making certain that all graphical data is suitably defined, that attribute data does not exceed expected ranges and that impossible combinations of attributes do not occur.
• Data may be copied, deleted, moved, joined, altered, etc. Any of these data editing functions should be capable of being performed on both the graphical and the textual data. If data is not carefully verified, and errors remain, then manipulations of the data at a later processing stage will cause error propagation and multiplication, thereby invalidating, or at least making less useful, any final GIS output.

Structure Conversion

• For many manipulations it may also be preferable to convert data from a raster to vector structure, or vice versa. This is necessary since there are still no truly integrated GIS’s which are able to handle both raster and vector data with equal ease.
• It is important to note that in the vector to raster conversion (rasterising) there will be an inevitable loss of accuracy, a factor which would be exacerbated both with increasing sinuosity of the lines and with increasing raster cell size.
• In the raster to vector conversion (vectorising) the GIS software programme performs a vectorising process which “threads” a line through groups of pixels using a special “thinning” algorithm. There will be a consequent need for topological information to be constructed and for individual features to be identified.
• These latter requirements can call for considerable operator intervention, but there are GIS functions which automatically compute new nodes and links and compile topology tables.

Geometric Conversion

• When performing manipulations on mapped digital data, it is important that, if the data is to be merged in any way, then it should all conform to the same geometric reference system. Latitude and longitude co-ordinates are frequently used in small scale mapping, although the most frequently used co-ordinate system in GIS is the Universal Transverse Mercator (UTM) system.
• Nearly all GIS software allows for the possibility of converting the map referencing system used to a wide range of possible map projections or from one co-ordinate system to another. Such processes are sometimes called transformations or rectification.
• Transformations are based on the mathematical relationships that exist between the various map projections, i.e. relative to angles, areas, direction and distances. A more basic type of geometric conversion is called registration. This is simply changing one mapped view to line up with another, i.e. irrespective of any referencing system.

Scale changes are easily accommodated via a simple multiplier function and maps can easily be rotated to particular orientations. A more complex function which most GIS’s can achieve is the correction for distortions (rectification). These distortions may occur in the original source data for a number of reasons, e.g.

1. Aerial photographs or RS satellite images have varying scales due to platform tilt and the curvature of the Earth.
2. The angle of view or relief differences also cause variations in scale.
3. Photographs or maps variably shrink with age.
4. Maps on paper can easily suffer from stretching.
5. Distortions in the optic systems being used.

Generalisation and Classification

Under this general heading a large number of manipulations can be performed, all of which are designed to change the data in some way such that it can be more easily used for a particular purpose, e.g.

1. Adding to data, or deleting undesirable data.
2. Aggregating or disaggregating numerical or attribute data.
3. Classifying or reclassifying data into user defined, or GIS suggested, classifications. This usually involves deciding upon attribute value classes or changes to existing classes.
4. Using data reduction algorithms to generalise or smooth linear data, e.g. to thin out coordinates in digitised lines (in order to greatly reduce the amount of data storage).
5. Lines can be deleted or “dissolved” in order to simplify mapped surfaces.
6. New attributes can be assigned to spatial points, lines or polygons.
7. Annotations can be added to maps using labels, text, legends or cartographic symbols.