I'm trying to cut census block data to a study area. I have a study area polygon (made from selecting whole counties of interest) and clipped the census data by this. When I visited the new Census file I found this process has created "slivers" from the original census dataset that are very small (area is less than a mile) but its population is in the thousands; the entire population of the original block has been cramped into this tiny area. I had thought clipping the data by the study are would've been successful since block groups, at least to my knowledge, do not cross counties. Is there any way to easily remove these extraneous slivers or a better way to cut the census data to a study area?

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    Which software are you using QGIS or ArcGIS? which version ?
    – MKJ
    Commented Mar 17, 2015 at 16:50
  • That would depend on whether your county data is coincident with the block data. Are they from the same source? The groups may not be supposed to cross a county boundary, but if the two boundary lines aren't drawn the same you get slivers. Also, without special care/processes (see the term apportion), when you divide up a polygon its attributes are not affected in any way - all pieces/parts/remainders retain the same attributes as the parent shape. A select and export may serve better than a clip in this case.
    – Chris W
    Commented Mar 17, 2015 at 19:53
  • Do you have 2 questions? I'm picking up a 2nd one about whether (or how) the population values or other attributes should be changed to reflect area changes.
    – mkennedy
    Commented Mar 17, 2015 at 20:01

1 Answer 1


The problem is probably that your County boundaries and your Census blocks are not identical and don't line up right leaving you with slivers. Instead of clipping, You could just query out the Counties you would like to use, within the study area, using the "COUNTYCU" field in the attribute table. This field gives a unique identifier to each County. Using this identifier would ensure that you are using blocks that are within your study area only.

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