How prevent Intersect from duplicating records?

[HerbKuehne]

I am using the Intersect geoprocessing tool on a small sample of individuals with lon/lat coordinates.   I'm intersecting them with blockgroups. 

When I do so, a number of records are duplicated, so that the original list of entries now is larger by a small fraction....2%.  But it shouldn't be larger at all, or at least, so it seems to me.

What am I doing wrong or what should I do in order to correct the duplication process?

[DanPatterson_Retired]

The picture in the link How Intersect works—Help | ArcGIS for Desktop  where a polygon overlaps two polygons, may illustrate what you are experiencing.  If this is the case, then you will notice that two pieces result from the overlapping polygon, combining with the results of the others.

The An overview of the Overlay toolset—Help | ArcGIS for Desktop  tools have visual examples which are more informative than the textual differences, perhaps one of those options is more useful for your desired output,  for instance 'update' or one of the others.

[HerbKuehne]

Thanks, Dan and Neil.

Neil's reply helped me the quickest, but I definitely need to learn more about those overlay tools.

As it turns out, the real issue was duplication in several "block-group" polygons that I got, I think, from the Census Bureau. After describing the issue and Neil's reply to a friend, she taught me to use the "Find Duplicates" and "Remove Duplicates" tools. Those helped me clean up the duplicates and now all is intersecting as would be expected.

I still wonder how I got the duplicates got into the "block-group" polygon layer.

Thanks again for your quick responses and help.

Herb Kuehne, Sioux City, IA

[NeilAyres]

If you are using Identity or Intersect or similar overlay tools with a point feature class and underlying polygons, then this will happen if the polygons overlap. Points lying in the portion of overlap will be duplicated in the output because they intersect with 2 (or more) polygon features.

[HerbKuehne]

Thanks, Dan and Neil.

Neil's reply helped me the quickest, but I definitely need to learn more about those overlay tools.

As it turns out, the real issue was duplication in several "block-group" polygons that I got, I think, from the Census Bureau. After describing the issue and Neil's reply to a friend, she taught me to use the "Find Duplicates" and "Remove Duplicates" tools. Those helped me clean up the duplicates and now all is intersecting as would be expected.

I still wonder how I got the duplicates got into the "block-group" polygon layer.

Thanks again for your quick responses and help.

Herb Kuehne, Sioux City, IA