I am trying to sum small polygons (land disturbance) that are within larger ones (counties). Some polygons are completely within a county; others cross county boundaries. I would like to say, at the end of the day, "County X has 2,000 acres of disturbed land while County Y has 5 acres." (This is somewhat similar to this, except I am attempting to sum thousands of polygons within dozens of larger ones ).
Here is what I have done:
-Add county layer to ArcGIS
-Add land disturbance layer
-Spatially join them (Analysis Tools > Overlay > Spatial Join)
Target features: counties
Join features: land disturbance layer
Join operation: one to many
Join type: Intersects
I get a spreadsheet with a list of all of the polygons and their areas for each county. Some counties have 0, which is accurate. Other counties have several thousand entries, which is also accurate. The problem: The total area I come up with for land disturbance (if I sum everything by county) is greater than the total area of land disturbance in the original land-disturbance .dbf file.
If "intersect" over-counts, I figured using "contains" for the join type might be accurate. However, this undercounts the amount of total land disturbance. How do I get the right amount of land disturbance per county?
I would appreciate any help or advice. Thank you so much!
Richard, thank you so much for your help. I've just followed your directions, and unfortunately, I get a very similar result to the spatial join process.
Spatial join with intersect: 542,635 acres
Merge, then union: 542,543 acres
Original dbf: 412,435 acres
Here is what I did:
-Used Intersect tool to merge two feature classes together
-Used Union tool:
Input features: intersect1
Output Feature Class: intersect_union
-I think you may be right about overlap in the land disturbance layer. I used the Dissolve tool, and then used the Union of the counties with the dissolved land disturbance shape. That gave me the correct area output for the *total* disturbed land - not the amount of land disturbance per county. I keep playing around with this, because I think you're onto something, but I can't make it work yet.
Unfortunately, I still can't get it to work. I must be doing something wrong! Here's what I'm doing:
1. Load Tigerline county file (Census) and land disturbance file. Check they are in same projection.
2. Dissolve land_disturbance
Dissolve fields: Type
input files: land_disturbance_Dissolve, Tigerline
input files: land_disturbance_Dissolve_Union
dissolve fields: Area, Type, GEOID10, NAME10
5. Check results: 508,492 acres
I also tried this:
input: land_disturbance, Tigerline
dissolve field: Type, GEOID10, NAME10
statistics: Area (sum)
3. Check results: 542,543 acres