A workflow used to generate county level shape file with mean(z) in each county from an xyz data table in ArcGIS

note this is based on @MLowry's suggestion, adding almost step-by-step instructions.

Open ArcCatalog##

1. create new personal geodatabase (e.g. foo.mdb)
2. file -> new -> personal geodatabase
3. add data

• right click on map.csv -> export to geodatabase (single) -> foo.mdb (note not clear if this needs to be exported to geodatabase, but this is how I made a table from the csv)
• right click on counties.lyr -> export to geodatabase (single) -> foo.mdb
• select map.csv file --> select output geodatabase --> ok

1. create featureclass (.shp files) from xy table

• open foo.mdb
• right click table
• input fields x = lon, y = lat, z = yield
• coordinate system of input coordinates import -> from counties.lyr (or, equivalently, -> geographic .. -> world -> wgs_1984 -> open -> ok)

1. import feature classes to geodatabase (multiple)

• select shp files created in prev. step -> add -> okay

Open ArcMap

1. Add data

• select foo.mdb -> CTL + select tables -> open

1. Spatial Join

• ArcToolbox -> Spatial Statistics -> Spatial Join -> Join counties.lyr to xyz table map
• Z value = yield; select 'mean' (or alternative statistic)