Abstract:
The U.S. Geological Survey developed this dataset as part of the Colorado Front Range Infrastructure Resources Project (FRIRP). One goal of the FRIRP was to provide information on the availability of those hydrogeologic resources that are either critical to maintaining infrastructure along the northern Front Range or that may become less available because of urban expansion in the northern Front Range. This dataset extends from the Boulder-Jefferson County line on the south, to the middle of Larimer and Weld Counties on the North. On the west, this dataset is bounded by the approximate mountain front of the Front Range of the Rocky Mountains; on the east, by an arbitrary north-south line extending through a point about 6.5 kilometers east of Greeley. This digital geospatial dataset consists of depth-to-water (unsaturated-thickness) contours that were generated from hydrogeologic data with Geographic Information System (GIS) software.
Purpose:
This dataset was created by the U.S. Geological Survey (USGS) in the development of the USGS Front Range Infrastructure Resources Project. This dataset was used in the creation of 1:50,000-scale hydrogeologic contour maps.
Supplemental_Information:
Using Arc/INFO software, USGS 7.5-minute Digital Elevation Models (DEMs) that encompass the study area were converted to raster surface formats (grids, in Arc/INFO terminology). They were mosaicked into a single continuous surface with the Arc/INFO GRID function, MERGE, and projected into an appropriate coordinate system with the command, PROJECT. To prevent the final depth-to-water contours from appearing too convoluted and implying more detail than is supportable by the water-level-elevation dataset, the resulting land-surface-elevation grid was then smoothed using a circular averaging filter with a radius of 9 grid cells (180 meters), using the grid command, FOCALMEAN.
A grid of water-level elevations was generated from a contour dataset of water-level elevation, bedrock-surface outcrop outlines (where water-level elevations are by definition 0) and supplemental water-level elevation points provided by the project hydrologist, using the command TOPOGRID, set to run for 45 iterations with contour data as the primary z-value, and no drainage enforcement. The generated grid was of the same cell size as the DEM-derived data (approximate 30-meter spacing between data points). Because of the interpolation process used in the TOPOGRID session, some areas in the water-level-elevation grid exceeded elevations in the land-surface-elevation grid. Those areas were replaced with corresponding areas in the land-surface-elevation grid.
A grid of unconsolidated-sediment thickness was generated from a contour dataset of unconsolidated-sediment thickness, bedrock-surface outcrop outlines (where unconsolidated-sediment thicknesses are by definition 0) and supplemental unconsolidated-sediment-thickness points provided by the project hydrologist, using the command TOPOGRID, set to run for 45 iterations with contour data as the primary z-value, and no drainage enforcement. The generated grid was of the same cell size as the DEM-derived data (approximate 30-meter spacing between data points). Because of the interpolation process used in the TOPOGRID session, some areas in the unconsolidated-sediment-thickness grid exceeded 0 where bedrock-surface outcrop existed. Those areas were set to 0.
The water-level-elevation grid was subtracted from the land-surface-elevation grid to produce the grid of unsaturated-zone thickness. Some areas in the unsaturated-zone-thickness grid exceeded the thickness of the entire unconsolidated sediment. Those areas were replaced with values from corresponding areas in the unconsolidated-sediment-thickness grid. Using the unsaturated-zone-thickness grid, the software generated the contour coverage at an interval of 20 feet.
The contours were clipped such that the remaining contours lay in an area where water-level-elevation contours were well controlled by data and hence drawn at an interval of 20 feet. In an editing session, the jagged appearance of the contours was addressed by smoothing them: the arc vertex grain tolerance (the minimum allowable distance between vertices in a line) was set to 90 meters, all the arcs were generalized with the ArcEdit commmand, SPLINE, and the grain tolerance was reset to 30 meters before splining the arcs again.
Manual editing corrected contours that were not correctly interpolated, as determined by consultation with the project hydrologist: contours that formed closed polygons with perimeters less than 1,000 meters were removed; contours left unconnected after the clipping procedure that were less than 100 meters were removed; contours interpolated in areas internal to the clipping area, yet still where water-level-elevation contours were poorly controlled also were removed; finally, contours with an associated unsaturated-zone thickness of 0 were removed.
The final coverage is intended to be used as one thematic layer in maps to be published at a scale of 1:50,000.
Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Although this Federal Geographic Data Committee-compliant metadata file is intended to document the dataset in nonproprietary form, as well as in ArcInfo format, this metadata file may include some ArcInfo-specific terminology.