Find majority county coverage in model grid cells

Question

How do I figure out which grid cells (x, y) contain any part of the Chicago ozone nonattainment area?

Approach

Use a GIS to find the counties that intersect the model grid cells.

Details

Create a shapefile of the 12US2 modeling grid (396x246)

Use the Spatial Allocator raster tools to create the shapefile. See generateGridShapefile.csh script as an example.

  setenv GRID_PROJ "+proj=lcc +a=6370000.0 +b=6370000.0 +lat_1=33 +lat_2=45 +lat_0=40 +lon_0=-97"
  setenv GRID_ROWS 246
  setenv GRID_COLUMNS 396
  setenv GRID_XMIN  -2412000.0
  setenv GRID_YMIN  -1620000.0
  setenv GRID_XCELLSIZE 12000
  setenv GRID_YCELLSIZE 12000
  setenv GRID_SHAPEFILE_NAME ../output/conus_12US2.shp
  ../bin/64bits/create_gridPolygon.exe 

Add X Y columns to the shapefile

1. Load the CONUS12 shapefile into QGIS.
2. Open the attribute table, put the file in edit mode, and add two short integer columns: X and Y
3. Apply the following field calculator formulas, in this order (calculate Y first), to calculate X and Y; note that 396 = NCOLS

  Y = (floor(("GRIDID"-1)/396)+1)
  X = "GRIDID"-("Y"-1)*396

Intersect the county and grid shapefiles using GIS

Use the QGIS Intersect function to find all counties that intersect each grid cell.

1. Load a county shapefile into QGIS

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2. Load the grid shapefile and put the county shapefile and model grid shapefile on the same coordinate system

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3. Use Vector->Geoprocessing Tools->Intersection to intersect the two features. The grid shapefile is the Input Layer; the county shapefile is the Intersect Layer

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