The data of this dataset represent analyses of geologic material samples collected in support of various USGS programs. The historical geochemical databases of the Geologic Discipline in the USGS were created with the intent of storing data predominately for regional projects. The new combined National Geochemical Database is an agglomerate of data from projects that had differing analytical needs and that ranged in scale from studies of an outcrop to reconnaissance surveys of an entire state or country. Initially, the intended user of the data was the original submitter or associated project personnel, and the primary means of publicly releasing data were hardcopy USGS reports, as well as professional journals. Within this paradigm, some decisions were made which have affected the completeness and accuracy of the attributes within the database.
1) Coordinates: When samples were brought in for analysis, the submitter was required to include information about the sample for the database; descriptions, geocoding, and latitude-longitude coordinates. In the days before map digitizing boards and GPS units were common, the determination of coordinates from field sheets was a time consuming and error prone process. In order to facilitate the analysis of samples, a decision was made to allow submitters to enter the coordinates for the lower right (southeast) corner of the submitter's working field map on which the samples could be plotted, which was most commonly a 7.5' or 15' quadrangle map. In theory, the precise coordinates for these samples would be determined and added to the database at a later date. In practice, most of these precise coordinates were used in the USGS Open-File data releases but were never entered back into the PLUTO database. Therefore, the precision of coordinates in the PLUTO database varies from "good to the nearest second of latitude or longitude" to "good to the nearest 15 minutes of latitude or longitude".
2) Geocoding: The submission of sample descriptive information (geocoding) with samples was mandatory for some fields and optional for others. Therefore, the completeness of geocoding can vary. In addition, most geocodes were not checked for completeness, accuracy, or validity during data entry into the early databases. Therefore the database contains some incorrect and invalid codes.
3) Analytical Data: The samples in this data set were chemically analyzed by a variety of techniques over a period of time from the early 1960's to the present. The accuracy of the data varies with the analytical methodology and with the concentration of the element being analyzed.
4) Qualifiers: A qualifier such as "N" (less than the detection limit of the analytical method) or "G" (greater than the upper determination limit of the analytical method) accompanies some analytical data values. These qualifiers are defined as follows:
"L" = the element was detected by the technique but at a level below the lower limit of determination for the method. The value of the lower limit of determination is given in the adjacent data field.
"G" or ">" = the element was measured at a concentration greater than the upper determination limit for the method. The upper limit of determination is given in the adjacent data field.
"N" = the element was not detected at concentrations above the lower limit of determination for the method. The value of the lower limit of determination is given in the adjacent data field.
"<" = the element concentration was determined to be less than the lower determination limit for the method for this element. The value of the lower limit of determination is given in the adjacent data field.