Geographic information in the NGS Database

Mineral Resources On-Line Spatial Data

Geographic Information in the NGS Database

The NGS database contains three primary geographic data fields: LAT=latitude, LONG=longitude, and DATUM=map datum.

NGS Samples: For samples collected by the NGS (SW-ALASKA and State datasets) all coordinates were measured by handheld global positioning systems (GPS). Data were recorded in a variety of formats (decimal degrees, degrees plus decimal minutes, degrees-minutes-seconds, and UTM coordinates), and all were translated into decimal degrees.
NURE samples: The coordinates of NURE samples (in all NURE datasets) , measured in the pre-GPS era, were digitized from field maps; a discussion of these methods and what datum was used may be found here.
Other samples: The coordinates of all other samples (PLUTO and RASS datasets) were determined by unknown methods, unless specifically discussed in any primary publications on these datasets. Because most were collected in the pre-GPS era, most were probably digitized or otherwise read from field maps used during collection. It is likely that the NAD27 datum and Clarke 1866 ellipsoid were used on most such maps, but this is not known with confidence.

A number of fields were derived from the primary geographic data discussed above. The ArcView 3.2 program by ESRI was used to make all calculations. In each case, a "point-in-polygon" operation was performed to join the sample location points to an appropriate polygon (map) coverage, and thus to determine which polygon each point was in. The following table describes what was done for each of these derived fields:

QUAD250CODE This field contains a 5-digit code for the 1:250,000 scale quadrangle in which the sample plots. The 5-digit codes refer to the latitude and longitude of the SW corner of the quadrangle. Thus "40108" has its SW corner at 40° N, 108° W. The base map coverage used for this was derived by merging three separate files for the lower 48 states, Alaska, and Hawai`i (created manually from USGS index maps). When points plotted on the border between two quadrangles, assignment was made by the computer unless there was additional information indicating which quadrangle was preferable. For example, certain NURE samples contain a code for the quadrangle as part of the field name, which could be used to resolve uncertainties.
A table showing all codes and the quadrangle names to which they refer may be downloaded here.
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QUAD250NAME This field provides the name of the 1:250,000 quadrangle with the code in the QUAD2500CODE field.

QUAD100CODE This field contains a 7-character code for the 1:100,000 scale quadrangle in which the sample plots. The first 5 characters of this code are identical to the code for the 1:250K scale quadrangle that contains the 1:100k scale map. In most cases, there are four 1:100K maps in each 1:250K quadrangle; characters 6-7 are two letters indicating whether the 1:100k quad is in the NW, NE, SW, or SE corner of the 1:250K quad. A few 1:250K quadrangles have different sizes and orientations than most of the others; characters 6-7 of the 1:100k quads in these are chosen to reflect their positions as well, e.g. SS=the southern half of the southern half of a quad, SN=the northern half of the southern half of a quad. The 1:100K quadrangle coverage is available here.
1:100K map boundaries are not defined in Alaska. However, a series of 1:63500 quadrangles does exist. Therefore, the database field QUAD100CODE contains codes for these 1:63K boundaries for database records in this state. The code is constructed exactly as described above for 1:100K quads (the basemap used can be downloaded here), except that characters 6-7 contain the letter-number cell designation used for 1:63K maps in Alaska.
A table showing all codes and the quadrangle names to which they refer may be downloaded here.

QUAD24CODE 1:24000 scale (7.5 minute) quadrangle codes are defined by further subdividing the 1:100K quadrangles discussed above, completing a 3-level heirarchy. Each 1:100K quad contains 32 1:24K quads arranged in a 4-row, 8-column grid. These were assigned letter-number grid-cell names, beginning with A1 in the northwest corner, and ending with H4 in the southeast corner. The QUAD24CODE is then derived by concatenating the 7-character 1:100K quad code with this 2-letter cell code. For example, a value of "41110SEC2" indicates that a point is in 1:250K quad number 41110, the 1:100K quad in the southeast corner of this 1:250K quad, and the 1:24K quad in the 3rd row down from the north, 2nd column from the west. The base coverage used to assign 1:24K quad codes is available for download here
A table showing all codes and the quadrangle names to which they refer may be downloaded here.

FIPS_INT This field contains the 5-digit "FIPS" (Federal Information Processing Standards) code for the state and county in which the sample was collected. The base coverage used to derive this field is available for download here, and a discussion is available from NIST. When points plotted on the border between two quadrangles, assignment was made by the computer unless there was additional information indicating which county was preferable. For example, certain NURE samples contain a code for the state and county as part of the field name, which could be used to resolve uncertainties. A small number of NURE samples actually plot in a county that is different from the county indicated in the field name, but very close to the border with the indicated one. The FIPS code for these samples was changed from the one indicated by the geographic coordinates to the one used by the sample-collection team when it assigned field names.
A table showing all FIPS and HUC codes and their associated names may be downloaded here.

HUC_8 This field contains the 8-digit hydrologic unit code (HUC) as defined by the USGS. Note that the precision of sample coordinates measured by GPS is sure to be higher than the precision of the watershed coverage used to assign HUC's (nominally, 1:2M scale). Therefore, the assignments listed in the HUC_8 field of the NGS are fairly crude and should not be used for analytical purposes at large scales.
A table showing all FIPS and HUC codes and their associated names may be downloaded here.

U.S. Department of the Interior, U.S. Geological Survey
This page is part of U.S. Geological Survey Open-File Report 2004-1001
URL: http://mrdata.usgs.gov/geochem/doc/geography.htm
Maintained by Jeff Grossman
Last modified: 09:05:01 Wed 14 Mar 2012
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QUAD250CODE	This field contains a 5-digit code for the 1:250,000 scale quadrangle in which the sample plots. The 5-digit codes refer to the latitude and longitude of the SW corner of the quadrangle. Thus "40108" has its SW corner at 40° N, 108° W. The base map coverage used for this was derived by merging three separate files for the lower 48 states, Alaska, and Hawai`i (created manually from USGS index maps). When points plotted on the border between two quadrangles, assignment was made by the computer unless there was additional information indicating which quadrangle was preferable. For example, certain NURE samples contain a code for the quadrangle as part of the field name, which could be used to resolve uncertainties. A table showing all codes and the quadrangle names to which they refer may be downloaded here. .
QUAD250NAME	This field provides the name of the 1:250,000 quadrangle with the code in the QUAD2500CODE field.
QUAD100CODE	This field contains a 7-character code for the 1:100,000 scale quadrangle in which the sample plots. The first 5 characters of this code are identical to the code for the 1:250K scale quadrangle that contains the 1:100k scale map. In most cases, there are four 1:100K maps in each 1:250K quadrangle; characters 6-7 are two letters indicating whether the 1:100k quad is in the NW, NE, SW, or SE corner of the 1:250K quad. A few 1:250K quadrangles have different sizes and orientations than most of the others; characters 6-7 of the 1:100k quads in these are chosen to reflect their positions as well, e.g. SS=the southern half of the southern half of a quad, SN=the northern half of the southern half of a quad. The 1:100K quadrangle coverage is available here. 1:100K map boundaries are not defined in Alaska. However, a series of 1:63500 quadrangles does exist. Therefore, the database field QUAD100CODE contains codes for these 1:63K boundaries for database records in this state. The code is constructed exactly as described above for 1:100K quads (the basemap used can be downloaded here), except that characters 6-7 contain the letter-number cell designation used for 1:63K maps in Alaska. A table showing all codes and the quadrangle names to which they refer may be downloaded here.
QUAD24CODE	1:24000 scale (7.5 minute) quadrangle codes are defined by further subdividing the 1:100K quadrangles discussed above, completing a 3-level heirarchy. Each 1:100K quad contains 32 1:24K quads arranged in a 4-row, 8-column grid. These were assigned letter-number grid-cell names, beginning with A1 in the northwest corner, and ending with H4 in the southeast corner. The QUAD24CODE is then derived by concatenating the 7-character 1:100K quad code with this 2-letter cell code. For example, a value of "41110SEC2" indicates that a point is in 1:250K quad number 41110, the 1:100K quad in the southeast corner of this 1:250K quad, and the 1:24K quad in the 3rd row down from the north, 2nd column from the west. The base coverage used to assign 1:24K quad codes is available for download here A table showing all codes and the quadrangle names to which they refer may be downloaded here.
FIPS_INT	This field contains the 5-digit "FIPS" (Federal Information Processing Standards) code for the state and county in which the sample was collected. The base coverage used to derive this field is available for download here, and a discussion is available from NIST. When points plotted on the border between two quadrangles, assignment was made by the computer unless there was additional information indicating which county was preferable. For example, certain NURE samples contain a code for the state and county as part of the field name, which could be used to resolve uncertainties. A small number of NURE samples actually plot in a county that is different from the county indicated in the field name, but very close to the border with the indicated one. The FIPS code for these samples was changed from the one indicated by the geographic coordinates to the one used by the sample-collection team when it assigned field names. A table showing all FIPS and HUC codes and their associated names may be downloaded here.
HUC_8	This field contains the 8-digit hydrologic unit code (HUC) as defined by the USGS. Note that the precision of sample coordinates measured by GPS is sure to be higher than the precision of the watershed coverage used to assign HUC's (nominally, 1:2M scale). Therefore, the assignments listed in the HUC_8 field of the NGS are fairly crude and should not be used for analytical purposes at large scales. A table showing all FIPS and HUC codes and their associated names may be downloaded here.