What is a GIS?

A GIS is a computer-based technology used to incorporate, display, analyze and output various types of geographically-referenced data in conjunction with their associated attributes.

The Structure of a GIS Comprises Geographic
Features As Well As Linked Database Attributes

At the most basic level the GIS consists of two types of data: graphical and tabular. The graphical data encompass features that represent geographic things which fall into 3 classes: points, lines and polygons (or areas). These "things" are simply referred to as the features of a GIS. Some examples of features are topographic lines, rivers, lakes, political boundaries, and cultural features such as roads, buildings, parks, etc.

Examples of point data or features can be sample locations, outcrops, epicenters, craters, etc. Examples of line data can be geologic contacts, faults, seismic lines, roads, etc. Examples of polygons are geologic units, vertical projections of fault surfaces, city or park boundaries, building footprints, craters, outcrops. Some features, such as cities or parks, can be portrayed as either point features or polygon features. The scale of the display (or map) within the GIS has a lot to do with whether a feature is to be treated as a point or a polygon. There is also a close - topological - relationship between polygons and the lines that delineate the individual polygons. (Similarly there is a relationship between the nodes, which represent the end-points of lines, and the lines which the nodes terminate.) Topological data are stored within the GIS for each of the lines surrounding a polygon. These data identify the polygon, or pair of polygons, each line helps define. The significance of this is discussed below.

In addition to the point, line and polygons features, annotations - though obviously not representing a geographic "thing" - can be placed in proximity to the geographic features to help identify those features. But all of these features are only half of the data-representation side of the GIS. With just points, lines, polygons and annotation, we have the core of a CAD (or Computer Aided Drafting) system, such as AutoCAD. What distinguishes a GIS from a CAD is the incorporation and tightly linked database which supports the attributes of each of the geographic features.

Some examples of attributes assigned to features could be sample site id, data collector's name, sample date, type of feature (such as type of fault), rock age, earthquake magnitude, depth, etc.

The database attributes and the graphical features they support are tightly linked and, together, they provide the means of entering and extracting information about the features. This capability, singularly, is the core of the GIS. With this structure, users of the system can impose queries that can lead to greater understanding of the data and, potentially, result in the generation of new or refined sets of data.

The GIS Amalgamates Diverse Data and Resolves
Issues of Varying Map Scale and Map Projection

Data Entry is the process of incorporating similar to potentially disparate forms of data into the GIS for collaborative use. Data used within the GIS can be of various types and in various formats. Data resident with the GIS is obviously in digital form. But the sources of graphical data can be digital (both vector and raster) as well as paper maps which can be scanned and converted to digital form. Data can also be entered through manual digitizing of features on a map using either a digitizing board or through on-screen digitizing.

On the database side, attributes of the graphical features can be brought in as tables (ASCII files) or the attributes can be typed in manually.

Data coming into the GIS from various sources and various scales can all be accessed, viewed and manipulated within the same view because of the ability of the GIS to perform projections on the data. This brings data, potentially in various map projections, into one consistent projection for the view. The result is that features are displayed spatially in the same reference.

Graphical data come into the GIS in various projections. The source map projection is often chosen to take advantage of some map characteristic. Commonly maps with different subject matter - yet which cover the same geographic region - will have different projections. Within the GIS some of the data must be re-projected to come into conformance with a chosen projection. Once this is done, regardless of the scale of the original source, the newly projected data can be viewed and manipulated in conjunction with other data of the same projection.


11/08/99