We have downloaded the population estimate data for France (frap05ag.asc). When we use this file to calculate population for a given polygon, it does not match the response returned by the Population Estimation Service provided. We were wondering what is your criteria for including a grid cell in the calculation? Are you giving weight to a cell based on polygon intersection area? Also, wanted to clarify the xll, yll corner values - do the values -6, 40 represent the center of the bottom left cell or do they represent bottom left corner of the bottom left cell?

The Population Estimation Service uses a boolean rule rather than a weight for including cells: if the cell is more than 50% covered by a polygon it is included in the analysis. For cases where more than one polygon intersects a grid cell, the polygon that covers the center of the grid cell is assigned to that grid cell.

The results from the population estimation service should be similar to a zonal sum operation completed in GIS.

The ASCII header refers to the x and y coordinates of the corner of the lower-left cell.

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