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WHAT IS GIS

By Mary A. Tsui, copyright 2000

The definition changes as time goes by and computers and software mature, but we can safely say the following: GIS, as it is understood at present, is a computer-based mapping system that is derived not from drawings but from spatial data. As with most definitions of 20 word or less, this is the tip of the iceberg.

We might first want to ask, "Why would anyone want computerized maps?" The simple reason is that hand drawn maps are obsolete almost as soon as they are drawn. They reflect a snap-shot of the property covered at a given point in time. Change is always occurring. To reflect all changes, a map must be updated or redrawn continuously.

A more complex reason is the subject matter of maps. Generally we try to convey one or two issues on a map: roads and physical features, property lines and population, etc. At some point we usually want to combine the issues on one map with different issues on another. In the manual drafting world, this generally requires the generation of a new map or at minimum, the creation of transparent overlays. The idea that each issue is represented by its own overlay is seminal to the concept of GIS: if one could have different map coverages for the same geographic area and call them in at will, life with maps would be much more productive.

In addition to the notion of overlay, the need to move between map scales is also critical to GIS.

Hand drawn maps are created at a scale that focuses on the subject at hand; if we want to focus on an urban area, for example, we create a map that shows the features larger than they would appear on a state-wide map. Eventually, we will want to overlay one map coverage with another and will usually find that the scales are different, thus preventing us from creating the overlay and getting on with our analysis.

Properly prepared, GIS will allow map scales to be altered so that maps can be made to coincide and provide the overlay view. In general, large-scale, urbanized coverages can be scaled to fit the broader area county or statewide coverage, retaining all the original detail.

We’ve all seen rooms full of maps created at varying scales and illustrating different topics. We keep them and use them as best we can because of the important information contained in them, yet that use is clumsy at best. Frequently we find maps, created by other entities, that we would like to include in our maps and find that impossible to do well.

GIS has the advantage of pulling the most amount of value from these maps, keeping them current, and allowing them to be combined with other map coverages at varying scales. If all is done well, we no longer are faced with the dysfunctional map room and catalogue.