Extent

Extent has several different usages with regard to geographic information and analysis and is variously used synonymously with terms such as coverage, scope, area, and other related concepts. The extent of a study in time, the extent of an area represented on a paper map or digital display, and the extent of a study area (analysis extent) have critical implications for the information they contain and portray. An additional use of the term extent relates to the horizontal and vertical dimensions of a geographic feature or collection of features.

Temporal extent of a geographic data set expresses the time period for the data and includes the frequency of the observations used to create the data set. In the Federal Geospatial Data Committee (FGDC) and International Organization for Standardization (ISO) metadata standards, temporal extent is defined by beginning and end dates. Temporal studies, especially those of a social nature, often apply to a sample taken at a moment in time, with beginning and end dates equal. The relevance of information derived from temporal studies often requires consideration of temporal extent. For example, when using census data, it is essential to know the date of collection.

[Page 143]Vertical extent is defined by the maximum and minimum elevation or height values for a data set. In the metadata standards, vertical extent is defined by maximum value, minimum value, and the ascribed units of measure.

Geographic, or horizontal, extent is generally expressed as the latitude and longitude of diagonally opposite corners of a rectangle that encloses all objects in the data set. This is often called the minimum bounding rectangle.

Analysis extent is defined by the smallest bounding rectangle surrounding the area in which spatial analysis occurs. Analysis extent and overall spatial extent of a geographic data set or data sets may be different.