Where are the attribute tables for the GPWv3/GRUMP data? I would eventually like to extract statistics (such as population count) using features in another layer, but am not sure that I'll be able to do so given that there are no accompanying attribute tables. Could you advise as to how I should proceed?
Raster files in ESRI can have attribute tables if they are integer (whole numbers). The GPW grids are typically floating-point. Even if we round them to integer, a Value Attribute Table (VAT) cannot be built because:
“A VAT is built by default for any integer grid that results from an expression if the range of values (maximum minus minimum) in the grid is less than 100,000 or if the number of unique values in the grid is less than 500. If the range of values is greater than 100,000 and the number of unique values greater than 500, then no VAT is built.”
Above quote from the ESRI help subject on building and optimizing VATs.
The GPW/GRUMP population data have too many unique values to build a raster attribute table in ArcView. Our population grids have a range of greater than 100,000 and more than 500 unique values (for the globe). Some countries may have fewer, if this is the case a user can create a VAT using the BUILDVAT command.
The best way to get a total is to use the Zonal Statistics function of Spatial Analyst to get the total population or land area from the population count or land area grids. If you want a total for the whole grid, simply create a boolean raster at the same resolution as the land area raster, using a simple conditional statement such as:
braster = con(gluareag gt 0, 1)
To obtain population totals or land area totals, you must set the resolution equal to the grids (for GRUMP this is 0.00833333 decimal degrees). Otherwise, the zonal statistics function will underestimate or overestimate the values.