One of the primary roles of GIS is to use the
Geographic Information System to perform analyses for the various County Service Groups,
and to teach them how to perform such analyses themselves.
The term spatial
relationship can be best explained through an
example. Consider the question "How many wells are in Volusia County?" This
query is nonspatial in nature. The answer does not require knowledge of the physical
location of the wells nor does it describe where the wells are in relation to one another.
On the other hand, a question that asks "How many wells are in the County that are 10
inches in diameter and are 1000 feet apart?" is spatial in nature. To answer this
question, one must have the ability to determine the location of each well, measure the
distance between the wells and examine their attributes (e.g., diameter). A GIS can
readily provide such information.
A GIS can do this because it has the ability to link
spatial data with information (facts and figures) about a particular feature on a map. The
information is stored as attributes, or
characteristics, of the graphically represented feature. For example, without a GIS, a
street network might be represented by simple street centerlines, in which case the actual
visual representation of the road would not yield much information. To obtain the
information about the road, such as type (paved, gravel, etc.), you would have to take the
street name from the map and use it to query a separate database. As shown below, the GIS
allows you illustrate the database on the map by using different symbols to draw the
roads; such as by drawing heavy line, a regular line, or a dotted line to show whether the
road is paved, surface-treated, or unpaved. The result of such a display is that the user
can determine information about features on the map simply by looking at them.
In the County's ARC/INFO
GIS software the 'ARC' part of the software handles the features, while the 'INFO' part
handles the feature descriptions. A map depicting the County paving plan was produced by
importing TIS mainframe data into 'INFO' and merging it with the GIS street centerline
file stored in 'ARC'.
Types of spatial analysis
Frequently on this page, reference is made to "spatial
analysis" or "geographic analysis." Performing such an analysis is a
complex task performed by GIS personnel only after a great deal of planning and research.
This is because "spatial analysis" is an umbrella term covering many different
procedures. Descriptions of several types of spatial analyses
performed by the GIS staff can be found by following the hyperlinks below:
Many GIS projects involve a combination of the various
types of geographic analyses. For example, a project completed for VOTRAN to assist in
route planning and target marketing utilized spatial overlay, buffering and network
analysis. As shown in the illustration,
six layers of data were overlaid to determine relationships between existing routes,
low-income areas and the locations of medical facilities. Network analysis was performed
to generate the routes. Then 3/4 mile buffers representing the maximum distance VOTRAN
thought a person would walk to a bus stop were created around the routes. Following the
analysis, routes were adjusted and a direct mail campaign was launched within certain zip
codes to target potential new customers.
Rarely are all the data required to perform the above
described analyses available from one source. And more often than not, the data are not in
an immediately usable format. Therefore, a great deal of GIS staff time is devoted to data
preparation and the integration of various datasets. Common data types incorporated into
the GIS include tabular or statistical information from sources such as the mainframe or
PCs, CAD files, Global Positioning System
(GPS) coordinates, satellite imagery, and even video
If the data to be used are not already in digital form,
that is, in a form the computer can recognize, various techniques can be used to capture
the information. Maps can be digitized, or hand-traced
with a computer mouse to collect the coordinates of features. Electronic scanning devices
can also convert map lines and points into digital data. Identities of the objects on the
map as well as their spatial relationships must then be specified.
Once all the information is in the GIS it must be
manipulated so that it registers, or fits, with information gathered from other sources.
So before the digital data can be analyzed, they may have to undergo other manipulations,
especially projection conversions. A projection
is a mathematical means of transferring information from the Earth's three-dimensional
curved surface to a two-dimensional medium, such as paper or a computer screen. Different
projections are used for different types of maps because each projection is particularly
appropriate for certain uses.
The integration of data which may have been obtained from
various sources, computerized at various scales, and based upon different projection
systems, is a complex task and remains a major challenge.