GeoStor uses the AHTD County boundary as the clip layer for extracting all data requests clipped by county name.  That boundary has been in use on GeoStor for delivering GIS data because it is a more accurate representation of county boundaries.  Its source is from improved basemap data.

Users who are downloading Census blocks for Redistricting should be aware there are minor discrepancies between the AHTD County Boundary and the Census County Boundary.  The Census data is built with integrated topology and therefore all Census population geometry is nested within that county geometry and correspond to that boundary.

The result is that in most cases there are 6 foot slivers/gaps around county borders in the GeoStor clipped file.  Those small slivers carrying the all the attributes including the block count of the neighboring county.

If a user simply totals the pop 100 column they will get an over count or under count of their actual total population.

To overcome this issue, users may wish to download a geographic area that is larger than the subject county and then conduct attribute queries on the data using the COUNTYFP10 (FIPS) code for the subject county.  The selected set can then be exported and will only contain the blocks and population totals associated with the subject county.

Special thanks to David Britton from Van Buren County who brought this issue to our attention.