Georeferencing describes the process of locating an entity in 'real world' coordinates. For example, you would georeference your house by determining its latitude and longitude coordinates.
Georeferencing has become more prominent in recent years due to the increasing availability of Geographic Information System (GIS) tools designed to facilitate this process. The Reach Indexing Tool (RIT) is an example of such a tool. It is designed to facilitate the location and identification of surface water entities, which have associated attribute data stored in a database, and to georeference them to EPA's National Hydrography Dataset (NHD)
Surface water entities are sets of hydrologic features (such as streams, lakes, or shorelines) that are grouped together for management purposes. Examples of surface water entities could be fish consumption advisories or no discharge zones. Surface water entities are also important for Clean Water Act reporting, such as the Assessment Units used for Section 305(b) water quality reports.
Once a surface water entity has been georeferenced, the relationship between the entity and its 'real world' location can be used to map and display information about the entity. Frequently this information is stored in a database that was designed without consideration for how the data could be used with GIS. Effectively displaying attribute data in a database through a GIS requires some design considerations in addition to those used in standard database design. The inability to accurately map data in a database is called spatial indeterminacy.
The following discussion of spatial indeterminacy and possible solutions will use the national database that has been designed to track assessments of surface water quality under the 305(b) Clean Water Act as an example.