I have downloaded some files from census website and did some cleaning to get ZCTA map for new york, but I noticed that there are overlapped regions (some regions are inside others). For some statistical purposes, It needs to be isolated. I looked online for some answers but I couldn't find a convenient answer to this.
I ended up producing file similar to this one: http://catalog.opendata.city/dataset/nyc-zip-code-tabulation-areas/resource/eec14f19-9794-46ef-9bf4-7f6835ffbf4c
If search "Manhattan"; you will see these small squares inside big ones.
Can anyone who has experience with GIS explain why ? Is it problem related to drawing polygons or it's really overlapped.