Types of GIS Analysis

Information Retrieval:

With a GIS we can point at a location, object, or area on the screen and retrieve recorded information about it from the DBMS, which holds the information about the mapís features. In order for a GIS to answer the question "what is where?" we need to carry out retrieval. Retrieval is the ability of the DBMS or GIS to get back on demand data that were previously stored (Clarke, 1997). As Clarke put it "Geographic search is the secret to GIS data retrieval"

Searches by attribute

Most GIS systems include as part of the package a fairly basic relational database manager, or simply built on the exiting capabilities of a database system. All DBMS include functions for basic data display. Searches by attribute are then controlled by the capabilities of database manager. Find is the basic attribute search (Clarke, 1997). Find is intended to get a single record. Find can be browse or by searches. Examples include show attributes, show records, generate a report, find, recode, select, renumber, sort, compute allows the creation of new attributes based on calculated values, restrict, join, replace; all are examples of data reorganization. Attribute queries are not very useful for geographic search as they donít or difficult to indicate location; so they just work as humble assistants in our geographical searching needs.

Searches by geography

In a map database the records are features. The GIS spatial retrieval is the generating maps, which allow searching for information visually and highlights the result, (Clarke, 1997). For example to generate a report; the spatial equivalent would to produce a finished map; the spatial equivalent of a find is locate. Spatial equivalents of the DBMS queries result in locating sets of features, or building new GIS layers. These include: Spatial searching, browsing the map and picking features, Spatial sorting to identify features that result from attribute sorting, Recoding features spatially, that is changing the scope of their attribute, is equivalent to spatial merge, Spatial select is to extract specific features. The form of select used most is buffer operation. Buffering is a spatial retrieval around points, lines, or areas based on distance, a join operation is the cross-construction of a database by merging attributes across flat files, in spatial terms it is called overlay. Thus overlay is a spatial retrieval operation that is equivalent to an attribute join. Combinations of spatial and attribute queries can build some complex and powerful GIS operations, such as weighting e.g. dominant ethnic group in an area. Entire suites of geographic searches are searches and tests by relations of points, lines, and areas. Typical GIS searches are point in polygon, line in polygon, and point distance to line (Clarke, 1997).

3.1.1.1  The query interface

The user must interact with the data in appropriate way, to do that, we need the query interface. GIS query is usually by command line, batch or macro (Macro are files containing commands to be executed, one at a time) (Clarke, 1997).The batch-type interaction with the data is usually linked to working with the operating system, the physical management of disk. This type of interaction dates back from the punched cards, in that all processes had to be thought out in advance and a file (or stack of cards) produced that could execute the different commands one at a time. When interactive computing become commonplace, the command line as a query vehicle for data query took over. Commands were typed into the computer one at a time, under the control of the DBMS itself, and the software responded by performing the computations one at a time while the user waited for the command to be completed. Most GIS packages are fully integrated with the WIMP (windows, icons, menus, and pointers) and use the GUI (graphical user interface) of the computer's operating system, such as windows to support both a menu-type query interface and a macro or programming language. And fairly recent trend is that most GISs also contain a language or macro tool for automating repetitive tasks; e.g. ArcView's Avenue, MapInfoís MapBasic, and Arc/Info's AML (Clarke, 1997). SQL (standard query language) has been developed to be a standard interface to relational databases and is supported by many GISs. These use interfaces have specific characteristics (Montgomery, 1993 and Korte, 1992)

3.1.1.2  Spatial overlay

One basic way to create or identify spatial relationships is through the process of spatial overlay; Spatial overlay is accomplished by joining and viewing together separate data sets that share all or part of the same area.

 

3.1.1.3  Buffer analysis

Buffer analysis is used for identifying areas surrounding geographic features. The process involves generating a buffer around existing geographic features and then identifying or selecting features based on whether they fall inside or outside the boundary of the buffer. This process is used to identify neighborhood for a specific ethnic group.The purpose of this; is to cluster households of the same ethnic group. The process involves representing each household on the map as a point. Each point is coded with information that pertained to the type of ethnicity, as well as the other demographic characteristics.

3.1.1.4  Neighborhood Operations

Neighborhood operations can evaluate the characteristics of the area surrounding a specific location: Identify all cells with value 5 (say people under 5 years old). Neighborhood operations include the following: Search (Average, Diversity, Majority, Maximum/Minimum, and total), Topographic, Interpolation (interpolation involves using known cell values to predict predicting the values of intermediate cells), and Contour Generation

3.1.1.5  Connectivity Functions

Connectivity functions involve traversing an area and accumulating values: Contiguity measures, Proximity, Network functions, Spread, Seek and Stream functions

Find operationA raster GIS offers numerous ways to find items (DeMers, 1997). A simplest is to create a new coverage that eliminates all unnecessary data. To do this, all the other portions of the database are masked through a simple reclassification process e.g. reclassify everything else in the coverage as background, then output the result in table that allows to count the target points (grid cells) and background. Generally this will also allow to output percentage directly as well making possible comparisons of the amounts of the coverage occupied by the selected feature.Network AnalysisNetwork analysis is used for identifying the most efficient routes or paths for allocation of services, and for evaluation of it. Identifying an efficient route or path is finding the shortest or least-cost manner in which to visit a location or a set of locations in a network. GIS can handle complex network problems, such as road network analysis. A GIS can work out travel times and the shortest path from A to B. This facility can be built into more complicated models that might require estimates of travel time, accessibility or impedance along a route system Network analysis can also be used to optimize the allocation of resources. Such allocation is performed by identifying and creating areas of influence or service zones based on certain criteria. It is accomplished by assigning portions of a network to a location based on impedance. Digital terrain analysis, GIS can build three dimensional models, where the topography of a geographical location can be represented with an x, y, z data model known as Digital Terrain (or Elevation) Model (DTM/DEM). The x and y dimensions of a DTM represent the horizontal plane, and z represent spot heights for the respective x, y coordinates.

Network (TIN), The data sets derived from a Digital Terrain Model can be used to analyze environmental phenomena or engineering projects that are influenced by elevation, aspect or slope. The visualization (display) power of the computer allows the terrain data to be visualized in three-dimensional form, often from any angle of view (this is known as point-of-view analysis).

3.1.1.6  Selection of Demographic variable

Selection Demographic variable: Here, the question addressed, which is always a problem for planners, is which demographic characteristic to be considered for which planning analysis. Selection of demographic variables (characteristics) to use in a planning analysis is a taxing issue, which is the basis of the results you get from the analysis. Statistical packages (like SPSS) provide partial solution by giving the correlation between the variables. But this leaves some unaccounted relation as it considers only the aspatial correlation and some times the variables may be more spatially correlation.


Last modified 25 September 2000 by Wadembere Mugumbu