August 20, 2008

An actual assignment, how unusual. Let's see what I learned today about coverages, shapefiles, and geodatabases.
Coverages were created in the 1970s when GIS were first developed. As a result, many agencies continue to use this data format despite its limitations. The largest drawback is the multi-step process required to create polygons. The user must first individual arc segments, which connect to form a shape, and then he must perform a "build" command to create a polygon. Today, the geodatabase format allows a user to simply draw a polygon and the software creates the necessary data in the associated attribute table. It is a one-step process.
On the other hand, coverages support topology, which is a significant benefit when performing data analysis related to adjacency, connectivity, or containment. A coverage file forces linked nodes and lines to remain connected when editting. Should you change one layer, you must change any associated layers to keep the topology. This is one important function that was lost with the creation of shapefiles.
Shapefiles do not maintain topology, which means the area and perimeter of objects in a layer will not be updated if editting occurs. This is a significant problem for agencies using this format, as the attribute table could provide incorrect data. Despite the speed of shapefiles and the ability to use them in many GIS software programs, the lack of topology functions limit their abilities.
The best of both worlds is the geodatabase, which was introduced by ESRI with their ARCGIS 9.2 update. A geodatabase stores topology, can create polygons without additional commands, and updates the shape of individual features during editting sessions. They are quite useful in the world of GIS and will hopefully become ubiquitous in the near future. Until then, only GIS specialists using ESRIs software have the ability to create this data format.