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    1. In the Urban extents data, how were the urban areas identified?

    2. GPWv4 releases data for five years: 2000, 2005, 2010, 2015, 2020. Is it advisable to compare data sets of different years in order to assess changes in population density over time? Are there constraints in using the GPWv4 target year data sets as a time series?

    3. What is the difference between the Population Count and the UNWPP-Adjusted Population Count data sets? And between the Population Density and the UNWPP-Adjusted Population Density data sets?

    4. Will SEDAC develop other similar regional collections?

    5. What does a zero in the population density grid mean? What does No Data mean?

    6. Which are the last updated and the next scheduled GPW collected data?

    7. In GPWv3, what exactly does "No Data" (-9999) mean?

    8. I want to visualize/mashup many of your Web Map Services in ArcGIS Online, or even ArcMap. But, I'm not having much luck with ArcGIS. Is there some guidance you can provide to help me here. I'm sure I'm not alone with this request and as ArcGIS Online is becoming more proliferate other will want to leverage your great services themselves in their own mashups.

    9. For which part of the earth's land surface is the human footprint data available?

    10. I am looking for projection information for your Mexican Municipios data. I need to reproject the data and I need to know what the original projection was for the coverage.

    11. Which ellipsoid / spheroid is the geographical data of the Fundamental GIS: Digital Chart of China based on?

    12. How is SEDAC’s Gridded Species data different than the IUCN Red List?

    13. I see you have data on mammals and amphibians, do you also have richness grids for bird species?

    14. What is the difference between "..ag"  and "...g" files?  For example, I've downloaded the population grid for the whole world for 1990, in "grid" format, and there are two sets of raster files.  When I open them in ArcGIS, they seem to show similar data, but the range of values for the raster is somewhat different: 0 to 1.24 million versus 1 to 1.17 million.

    15. I am very interested in the gridded population density data for Africa.The source for the maps you provide is cited as the UN, but having contacted them they are unsure of the data you have used to produce your maps.Is it possible to obtain these data from you, including population numbers for different regions of Africa as well as the density calculations that you have mapped?

    16. Where can I find land area data for 1990 census tracts? I am trying to determine the median square mileage of census tracts.

    17. In GPW data that is comprised of 2.5 arc min cells, are the cells georeferenced from the cell center, or right/left and upper/lower corner?  I understand that xllcorner and yllcorner give the starting lat/lon for overall grid position, but are the cell data themselves valued at each lower left corner?

    18. Do the RDS data sets (which are matched to these points) contain different information from the raster grids (GeoTiff?).

    19. I have a question regarding the data set Contours of China and the Surrounding Areas. What Surrounding Areas are included in the data? What is the difference between the contour.zip file and the contourp.zip?

    20. I plan to use the global population data. I would like to know what information is stored in the *.stx and *.blw files. Also is there a way to map the grids to (latitude, longitude) ?

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