A data search by state was conducted to ascertain where digital, geospatial mine feature information from 7.5 and 15-minute topographic maps were available. Geospatial mine feature data, available for Nevada and California at 1;24,000-scale, were acquired and used as a starting point for those states. For remaining states, historical 7.5- and 15-minute topographic maps were acquired from the HTMC website (https://geonames.usgs.gov/pls/topomaps/
) as geoPDF files. In the 7.5-minute series, about 60 percent of the quadrangles had multiple versions. In the 15-minute, 1:48,000-scale series, about 10 percent of the maps had multiple versions, and in the 15-minute, 1:62,500-scale series, about 20 percent of the maps had multiple versions. For quadrangles with two map versions at the same scale, both maps were acquired. For quadrangles with more than two versions at the same scale, the oldest map, and versions at approximately 10-year intervals up to the most recent were acquired. Using Global Mapper software, acquired maps were converted from geoPDF to geoTIF format to make them accessible in ArcGIS.
Capture of mine features was accomplished at each scale by visually inspecting the maps for mine symbols and digitizing points and/or polygons into an ArcGIS file geodatabase. No reconciliation between the data from different map scales was done. For example, if a mine feature occurred on a 7.5-minute map and a similar feature occurred on a 15-minute map, the feature was digitized twice. Information describing the feature type (prospect pits, mines, adits, dumps, tailings, etc.) and the map date, name, and GDA and Scan identification number was compiled in the corresponding attribute table. Where mine symbols occurred on multiple versions (dates) of a map of the same scale, the first occurrence of the symbol was digitized. If subsequent versions of the map showed the same symbol within a permissible tolerance of the original location, the symbol was not re-digitized. The permissible tolerance for point symbols was roughly the area of a “box” which completely enclosed the map symbol. In the case of polygonal features, if the shape of the feature changed between map versions, the entire polygon was re-digitized. Slight shifts between versions of a map were common and reflected map scanning and georegistration variance.
When mine symbols were located within a topographic depression but no explicit pit outline was shown on the map, the depression was assumed to be a pit related to mining activity and the outermost depression contour was digitized as a polygon. The polygon was then attributed according to the mine symbol and/or the label contained on the map. When a map label name was surrounded by numerous symbols, the closest major symbol received the name. For example, if a name was surrounded by prospect pit and adit symbols on the map, the name would be applied to the closest adit symbol.
Pre-existing, 1:24,000-scale mine feature data for the states of Nevada and California were checked for locational accuracy; attribute table schema were modified and updated for consistency; and mine features (point and polygon) from older versions of maps were captured.
Data inspection and checks were performed by the individual digitizing the maps (first-level review), by team members reviewing a random selection of maps in a state (second-level review), by a project lead who reviewed data compiled at the state level (third-level review), and a final spot check of the merged data for the western US by USGS colleagues (fourth-level review). In each case, reviews addressed the accuracy and completeness of mine feature capture, completeness and consistency of attributes, adherence to established project schema, and representation of mine features through time.