We’ve discovered many prospective customers lack a clear idea on how to approach the evaluation of GIS polygon data. Increasingly, companies are faced with a variety of choices when acquiring or licensing data, including the ever-tempting-but-fraught-with-peril do-it-yourself option. Choosing poor quality or unmaintained data sets has implications for your budget, ongoing resource commitment dedicated to loading and maintaining that data, and of course the quality of the final map products.
And so, where to start?
Here are six steps you can take to make better decisions.
- Overlay the digital data onto the source map. This seems like an obvious step, but we often find people comparing digital data sets one to another. This really doesn’t tell you anything since you are not considering the original regulatory or authoritative data source. For do-it-yourselfers, you need to refer to the original documents, not free public data.
- Verify the availability of metadata. Metadata means “information about the data” and should at least consist of a description of the source of the data, i.e. a hard copy map and the name of the authoritative agency.
- Check for granular modification date history. Verify that each polygon has associated “geometry modification” and “attribute modification” dates so that it is clearly understood exactly which polygons were modified when, and the date the geometry was last changed or the associated tabular data were last changed. Without this information, it is difficult to ingest updates or understand the geoprocessing history of the dataset.
- Obtain available white papers describing the methods and technologies used to digitize the data. Land corner coordinates should be collected multiple times and spatially averaged, an ability not found in off-the-shelf GIS software.
- Verify attributes for consistency. Have they been quality controlled for consistency? Can you automatically dissolve on a base layer to derive any subordinate layer? For example can you dissolve by Meridian+Township+Range to get township boundaries? Can you dissolve on Meridian+Township+Range+Section to obtain the section lines? Can you dissolve on Block Name and Block Number to create the block boundaries in Texas? All of these should be possible in a properly structured and well maintained GIS dataset.
- Verify delivery ability in Esri GeoDatabase with the desired coordinate system. Surprisingly, it is not straightforward to convert primitive shape files to a GeoDatabase, nor is it incredibly fast or reliable. A fair amount of expertise is required to make sure GIS data works right (in the appropriate coordinate system) not only the first time you load it, but subsequent times months down the line.