Designing a Database

In designing a GIS data base, the first decision is the geometry to use.  In part this will depend on the scale; something that would be a point or line at small scale could become a polygon at large scale.  Most databases can hold only a single geometry type, and data symbolization would become very difficult when mixing multiple geometries.  KML files in Google Earth can mix geometries, but have little flexibility in changing the symbolization.

A point database can be much simpler, as a single, simple table can hold both the coordinates and their attributes.  It can be edited in a spreadsheet with little chance of corruption.  In contrast, a line or area database usually requires multiple files because each record can have wildly different number of coordinate pairs, from two for a simple straight line or three for a triangular polygon, to tens or hundreds or thousands of points for complicated feature like the outline of a coastal state.  Because there are multiple files, you cannot simply edit the database with impunity because deleting a record, inserting a record, or reordering the records must include changes to all the files; all edits must be done in a GIS program which coordinates all changes in each of the files.

The most common format for GIS databases, the shapefile, uses at least three files.

If you have a choice, picking a point database will make your life simpler.  The coordinates can be included in the dBase table, and if required, the GIS software can easily turn that file into a complete shapefile.

Polygons can be even more complex if donuts are allowed; these will have exterior boundaries, and interior outlines of the "holes".  Most programs keep track of this by the direction of digitizing, clockwise or counterclockwise.

Once the geometry is chosen, the user must define the fields.  Considerations here include:

 Newer databases do not have some of these limitations, but because databases are stuck with the dBase table used in shapefile, any attempt to use a new standard will encounter huge incompatability issues.

Last revision 8/29/2015