This section of the report presents maps of several elements on a county-by-county basis in the
conterminous states. Included are elements of importance to environmental studies and major
elements. The mean and standard deviation of each element was calculated for each county.
This was done on the basis of gridded maps (see separate description), which
are interpolations of the original point data. The calculations were done using the "zonal statistics" function
of the Spatial Analyst extension to ESRI's ArcMap 9.2 program, which averaged the
concentration of each element at each pixel of the geochemical grid contained within each county.
Ideally, the statistic that should have been used here would be the median
instead of the mean. This was not possible with software available at the
time this report was prepared. However, a customized routine to calculate
more sophisticated statistics has now been developed, and the results will
appear in future releases of this report.
The results of the calculations are displayed as a series of index maps, where counties are shaded according to
the relative concentrations of the elements. Clicking on an index map will zoom to that area of the country.
Holding the mouse over any county in the zoomed map will reveal the mean concentration of the element in the county.
Clicking on the county will allow the actual data within the county to be
examined and downloaded.
[These are the only elements available at this time]
Download a file of all county-by-county averages (30 Sept 2008; 1.71 mb):
The calculation that was done does NOT simply average the concentrations of samples that plot within
a given county. Pixels in a gridded map are influenced by all sample-points in their vicinity,
without regard to county borders (i.e., the grid points are interpolated). For this reason, averages are given for
some counties that do not actually contain samples in the NGS. In addition, some averages given for large counties may be based
on data that only covers a fraction of the area and may not be representative of the entire county. It is left as an exercise
for the user to evaluate how well a particular county average represents the entire area by examining the actual point-data for
the county of interest and surrounding regions.
Geochemical trends are not influenced by county or other political
borders. They are mainly the product of natural processes, in some
places overprinted by anthropogenic contamination. Large counties
almost always span several heterogeneous geochemical provinces, and thus a
county-wide average for an element may not actually represent the
concentration of that element anywhere in the county. The user should
use caution, and, again, should always examine the actual point data to gain
a better understanding of local variability.
All sample media have been combined to make these calculations. In some
areas, there may be statistical differences between media for some
elements. Point data should be examined to evaluate this possibility
in any area of interest to the user.