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 map’s 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 don’t 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).
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