Weighted forest carbon averages using FIA county data and shapefiles

Discussion created by calpoly2008 on Nov 24, 2011
Latest reply on Aug 7, 2013 by Dcamrine
Hello all,
I have a question about getting weighted averages ...

I have an excel file from FIA that has the following information for the US:
1. percent of forest land per county
2. tons of forest biomass carbon per hectare per county
3. total tons of forest biomass carbon per county

I have a shapefile of agroecological zones for the US and shapefile of US counties from divagis

What I need is the average of forest carbon for the US in each agroecological zone. There are about agroecological zones but they are not contiguous, there are about 200 different polygons and they fall into one of the 8 agroecological zones. So, one US County may overlap with one or all 8 agroecological zones.

I tried this:
I joined this table to a county level file from DIVA-GIS and converted to raster
I converted the agroecological zone shapefile to a raster
Combined the two rasters and exported the table in excel and calculated the area weighted averages based on the rasters

I was told that the estimates I got were too low, so I asked someone else about this and they did the following:
Joined the US counties shapefile with the FIA carbon table
Intersected the agroecological shapefile and the US counties shapefile
Note: He also reprojected the file so it was in a projected coordinate system got the land area of each county and agroecological zone)
The final file had a line for each polygon and the percent of each county in an agroecological zone and I multiplied this by the percent of forest land cover and forest carbon, but I ended up getting similar results - which seem very low.

I'm on a bit of a time crunch so any thoughts on where you think there is an error or an alternative approach would be greatly appreciated.

Many thanks!!!