Creating a Polygon Grid Shapefile with Rectangular Cells



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Creating a Polygon Grid Shapefile with Rectangular Cells
February 5, 2010

Bongghi Hong and Dennis Swaney


In the previous document “17_Using_ASCII_Format_Files_in_ArcGIS.doc”, a global N deposition map from Frank Dentener, which has a rectangular grid cell of 5 by 3.75 degrees (ftp://daac.ornl.gov/data/global_climate/global_N_deposition_maps/comp/global_N_deposition_maps_readme.pdf), was imported into ArcGIS. Below is a summary of the procedure applied:
(1) Import the ASCII file into ArcGIS as a raster grid with the cell size of 1 by 1 (unitless)

(2) Georeference the imported raster using the “Warp” tool of the ArcToolbox

(3) Convert the georeferenced raster into a polygon shapefile

(4) Calculate the area-weighted average of N deposition for the area of interest using the Polygon In Polygon Analysis tool of the Hawth’s Analysis Tools


The second step using the “Warp” tool involves resampling of the original data (with rectangular grid cells) to the ArcGIS GRID raster (with square grid cells). The above procedure works well when the data grid cell size is much smaller than the areas of interest. If not (as in this example), the resampling of original data could generate differences in the results that are hard to ignore. In this document, we will describe an alternative procedure that does not involve the resampling of original data and thus preserves the original data as much as possible:
(1) Create a polygon grid shapefile with rectangular cells

(2) Assign each rectangular cell with the N deposition values extracted from the ASCII file



(3) Calculate the area-weighted average of N deposition for the area of interest using the Polygon In Polygon Analysis tool of the Hawth’s Analysis Tools
Note that this method does not involve handling the ArcGIS GRID raster at all. When the data grid cell size is small, it would be impractical to create a polygon grid shapefile, and thus a variation of the earlier method can be a preferred option.
A polygon grid shapefile can be created using the Create Vector Grid tool of the Hawth’s Analysis Tools. In our example, the tool can be specified as shown on the right (refer to the earlier document for the information of map extent and cell size of the original data):

Hit “OK” and the polygon grid “grid_rect_cell” with rectangular grid cells will be created:



Now we should assign each polygon of this grid with the N deposition values extracted from the ASCII file. Opening and investigating the attribute table of the grid polygon, the unique identification number “FID” is arranged as shown in the below figure:

In this example, we will create a text table containing a column of unique identification numbers (from 0 to 3455) and columns of corresponding N deposition values, and join the table to the polygon grid shapefile (there may be other ways). First, open the ASCII files containing the N deposition values using Microsoft Excel (downloading the ASCII files is described in the earlier document):

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