I'm attempting to calculate acreage of the various "prime farmland" classifications in the GSSURGO dataset for the Omaha Reservation in Nebraska using QGIS but when I sum the total it's about 86 times more acreage than the total area of the reservation. The steps I followed to get to this calculation was:

  1. Load the GSSURGO MUPOLYGON layer into QGIS and reproject to EPSG 5070.
  2. Join with MAPUNIT Table (that contains prime farmland classifications for each soil unit in MUPOLYGON) using shared MUSYM field and export as joined shapefile.
  3. Clip joined GSSURGO data to include data within exterior boundaries of Omaha Reservation.
  4. Add new Decimal String field and Calculate acres for each polygon within the clipped layer using the Calculate Geometry polygon.
  5. Generate Summary Statistics using Vector>Analysis Tools>Basic Statistic for Fields.

The result it gives me is 17,0120,077 acres. However, the area of Omaha Reservation (calculated using boundary file used in clip operation in step 3) is only 198,395 acres. I've double checked I was calculating acres instead of square meters or square feet).

Why are these totals so wildly off?

  • The unit for epsg 5070 is meters maybe that is causing some issue – GreyHippo Jan 9 '19 at 18:03
  • GreyHippo, I made sure to convert from meters to acres. – David Bartecchi Jan 9 '19 at 20:02

I figure it out what I was doing wrong. I was joining the polygon data and the tabular data by the MUSYM field when I should have been joining by the MUKEY field. I found this solution in this user guide for the GSSURGO data https://www.nrcs.usda.gov/wps/PA_NRCSConsumption/download?cid=nrcs142p2_051847&ext=pdf

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