Geographic spatial analysis

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.

Spatial relationships

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. A Geographic Information System allows you illustrate the database on the map by using different symbols to identify unique attributes present, like road type (paved or dirt) or building type e.g. city hall or police department. 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:

• Spatial overlay
• Boundary analysis
• Address matching and geocoding
• Proximity analysis
• Buffer analysis
• Network analysis

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.

Data conversion

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 images.

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 that 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.