The FGDL contains two types of graphical data models: VECTOR and RASTER.

The vector data model represents geographic features by points, lines and polygons. Points represent geographic features too small to be depicted as lines or areas; lines represent geographic features too narrow to depict as areas; and polygons represent geographic features that have area. In a vector data model, each location is recorded as a single x,y coordinate. Points are recorded as a single coordinate. Lines are recorded as a series of ordered x,y coordinates. Polygons are recorded as a series of x,y coordinates defining line segments that enclose an area.

The FGDL contains ESRI's ArcView shapefile format.

Shapefiles require a minumum of three files to store the related geographic and attribute information. These three files can be easily recognized by their last three letters:

• *.shp - the file that stores the geometry of the feature
• *.shx - the file that stores the index of the feature geometry
• *.dbf - the dBASE file that stores the attribute information

NOTE: Free software for viewing ESRI's ArcView shapefile format is available at http://www.esri.com/

The raster data model is based on a regular grid of dots or pixels. In the raster data model, each location is represented as a cell. The matrix of cells, organized into rows and columns, is called a grid. Each row contains a group of cells with values representing a geographic phenomenon. Cell values are numbers, which represent nominal data such as land use classes, measures of light intensity or relative measures. Like the vector data model, the raster data model can represent discrete point, line and polygon features. A point feature is represented as a value in a single cell; a linear feature as a series of connected cells that portray length; a polygon feature as a group of connected cells portraying shape.

The FGDL CD-ROMs contain raster-based data in the following formats: ERSI GRID format for thematic data, and MrSID image compression, or ERDAS IMG format for satellite imagery and aerial photography. (For more info regarding the MrSID compression format see http://www.lizardtech.com/

Note: The FDOR TAX Data is not in vector or raster format. This data is in dBASE format which is not a graphical format. According to the website PC Webopaedia, dBASE is:

A popular database management system produced by Ashton Tate Corporation. The original version, called Vulcan, was created by Wayne Ratliff. In 1981, Ashton-Tate bought Vulcan and marketed it as dBASE II. Subsequent versions with additional features are known as dBASE III, dBASE III+, and dBASE IV, all of which are currently owned and developed by Borland Corporation.