Compilation of the data sets was supported by the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey. The objectives of the NAWQA Program are to: (1) describe current water-quality conditions for a large part of the Nation's freshwater streams, rivers, and aquifers, (2) describe how water quality is changing over time, and (3) improve the understanding of the primary natural and anthropogenic factors that affect water-quality conditions. National analysis of data, based on aggregation of comparable information obtained from across the United States, is a major component of the NAWQA Program. The data sets included in this report were created for NAWQA national data analysis activities.
The file "POPDENGA.GZ" is an ASCII file representing the population density data set "popdeng" in a format produced by the ARC/INFO GRIDASCII command. The file has been compressed using the "gzip" program, and can be extracted using the gunzip command:
gunzip popdenga.gz
The ASCIIGRID format file is included so users that cannot read
the gridded data in other formats can write a simple computer program to
convert the data into a format they can use. The format of the file includes
a set of keywords and parameters describing the georeferencing of the data
set (the location of the grid as referenced to the earth), for example:
| Keyword | Parameter | Description |
| ncols | 4521 | Number of rows in the grid |
| nrows | 2898 | Number of columns in the grid |
| xllcorner | -2462303.66 | X-coordinate of lower left corner of grid |
| yllcorner | 176796.66 | Y-coordinate of lower left corner of grid |
| cellsize | 1055.35 | Grid cell size |
| NODATA_value | -9999 | Grid cells with no valid data value |
In the DLG-format version of these data sets, the attributes are coded by setting the DLG major code to zero and the minor code to an integer column used to relate geographic features to the provided attribute tables. The attribute tables are provided as comma-delimited ASCII files with file extension ".ATT". These files include the column names in the first record and all the attribute data in subsequent records. The first column corresponds to the minor codes used in the DLG file.
The following table shows the relationship between the DLG and DEM files
and the attribute files:
| File name | Features | Attribute file |
| COWU95.DLG | polygons | COWU95.ATT |
| EFTRI.DLG | points | EFTRI.ATT |
| METROPOP.DLG | polygons | METROPOP.ATT |
| POPDENGD.GZ | grid cell | (no attribute file) |
These files have been archived using the UNIX "tar.exe" command and then compressed using the "gzip" program. They can be extracted using the following command line on UNIX and NT systems:
gunzip -c filename.tgz | tar xvof -
Other compressed files (with a ".gz" extension) can be uncompressed using the gunzip command:
gunzip filename.gz
This command will uncompress the file and restore its original name. For example: "COWU95.DLG" or "COWU95.E00".
At the time of preparing this report, PC-compatible viewer programs that can read many public-domain formats, including DEM, DLG, and SDTS, were available online at:
ftp://ftpmcmc.er.usgs.gov/release/dlgview/data/usgsview.exe
ftp://ftpmcmc.er.usgs.gov/release/viewers/
| Projection parameters: | |
| Spheroid | Clarke Ellipsoid 1866 |
| Datum | North American Datum 1927 |
| First standard parallel | 29 30' 00" North |
| Second standard parallel | 45 30' 00" North |
| Central meridian | 96 00' 00" West |
| Latitude of projection origin | 23 00' 00" North |
| Coordinate system parameters | |
| False easting | 0.0 |
| False northing | 0.0 |
| Planimetric units of measure | Meters |
Environmental Systems Research Institute, Inc. (ESRI), 1997, ARC/INFO 7.1.1 Help: Redlands, Calif., ESRI, [on-line documentation].
Federal Geographic Data Committee, 1994, Content standards for digital geospatial metadata (June 8): Washington, D.C., Federal Geographic Data Committee, 78 p., accessed July 21, 1998, at URL http://www.fgdc.gov/Metadata/ContStan.html
Federal Geographic Data Committee, 1998, Content standards for digital geospatial metadata, standard FGDC-STD-001-98: Washington, D.C., Federal Geographic Data Committee, 78 p., accessed December 27, 1998, at URL http://www.fgdc.gov/metadata
Lanfear, K.J., 1984, Digital map of counties and county equivalents in the conterminous United States: U.S. Geological Survey data available on the World Wide Web, accessed July 21, 1998, at URL http://water.usgs.gov/lookup/getspatial?county2m
Snyder, J.P., 1987, Map projections--A working manual: U.S. Geological Survey Professional Paper 1395, 383 p.
U.S. Bureau of the Census, 1995, County and city data book: 1994, CD-CCDB-94, Washington, D.C., [cd-rom disk].
U.S. Environmental Protection Agency, 1997, 1987-1995 Toxics Release Inventory, EPA 749-C-97-003, Washington, D.C., [cd-rom disk].
U.S. Environmental Protection Agency, 1998, Envirofacts Warehouse Homepage: U.S. Environmental Protection Agency, accessed February 20, 1998, at URL http://www.epa.gov/enviro/
U.S. Geological Survey, 1990, Digital line graphs from 1:2,000,000-scale maps, data users guide 3: U.S. Geological Survey National Mapping Program Technical Instructions, 70 p., accessed July 21, 1998, at URL ftp://mapping.usgs.gov/pub/ti/DLG/2mdlgguide/
______, 1993, Digital elevation models, data users guide 5: U.S. Geological Survey National Mapping Program Technical Instructions, 38 p., accessed July 21, 1998, at URL ftp://mapping.usgs.gov/pub/ti/DEM/demguide/
______, 1998a, SDTS Home Page: U.S. Geological Survey, accessed July 21, 1998, at URL http://mcmcweb.er.usgs.gov/sdts/
______, 1998b, Water use in the United States: U.S. Geological Survey, accessed July 21, 1998, at URL http://water.usgs.gov/watuse/
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