GIS relies heavily on databases of text and numbers. The relationship between such information and geography is discussed later in the chapter. For now, you simply need to know how text and number data sets are stored.
For this discussion, a database is a collection of discrete symbols (numbers, letters, and special characters) located on some physical medium with at least one principal underlying organization or structure. An old-fashioned library card catalog is an example of a database with a single underlying structure: an alphabetical list of authors; the medium is 3 x 5 index cards, and the data are the symbols on the cards describing books and their locations in the library. Most libraries have substituted computer-based catalogs with the advantage that a user can search and find not only authors but titles and subjects as well, so a number of organizing themes may underpin a database.
Another example is a “hard disk” that has recorded the most common type of soil found in a specific acreage in a county. The disk is the physical medium, the codes assigned to a soil type constitute that data, and the location of each acre—as understood from the position of each datum on the disk—could be the underlying structure.
Existing general purpose databases usually: