Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 23

Relational Databases
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 topics 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:

Result from some sort of project; some individual or team constructs it—frequently going to
considerable effort.

❏

Need to be updated (modified and corrected as time progresses) if they are to continue to be of value.

❏

Contain errors regardless of size, care of construction, simplicity of data, or quality or physical
medium used.

❏

Serve a function when allied with some process. The function may be as simple as supplying a tele-
phone number from a physical phone directory (structure—alphabetical by name; medium—cheap
bound paper; data—phone numbers, in very small type; process—looking up a name, finding the
adjacent number). The function served by the database might also be quite sophisticated—support-
ing far-reaching land use decisions, for example.

❏

Numerous databases are used to solve problems at all levels of government. Access to these databases is
achieved through the use of referencing schemes. Following are some examples:

Referencing Scheme Examples of Data Contained

Names of people Salary, Social Security number, medical history, criminal record

Auto license plate number Car color, owner, serial number

Street address House value, lot size

Job title Person employed, duties, salary

Transaction number Money received, paid, transferred, invested

Events Schedules, orders, crimes, accidents

Many schemes exist for presenting or storing data. Suppose parents want to find a name for a newborn
child. A list of potential names in random order might be provided. It would probably be more useful if
the list were divided into girl's names and boy's names. Another improvement could be to alphabetize
the list, or present it in terms of current popularity of names.