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 saturated-thickness polygons that were generated with a Geographic Information System (GIS).
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 saturated-zone-thickness polygons 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 outcropped. The unconsolidated-sediment thicknesses in those areas were set to 0.
A grid of unsaturated-zone thickness was generated by subtracting the water-level-elevation grid from the land-surface-elevation grid. Some areas in the unsaturated-zone-thickness grid exceeded the thickness of the entire unconsolidated sediment. The unsaturated-zone thicknesses in those areas were therefore replaced with unconsolidated-sediment thicknessses.
A grid of bedrock-surface elevations was generated by subtracting the unconsolidated-sediment-thickness grid from the land-surface-elevation grid.
The bedrock-surface-elevation grid was subtracted from the water-level-elevation grid to produce the grid of saturated-zone thickness. Some areas in the saturated-zone-thickness grid were negative. Those areas were set to 0. Other areas of the saturated-zone-thickness grid exceeded the thickness of the unconsolidated sediment. Those areas were replaced with values resulting from the subtraction of the unsaturated-zone-thickness grid from the unconsolidated-sediment-thickness grid. The values of the corrected saturated-zone-thickness grid were then reclassified to make a saturated-zone-thickness grid consisting of specific ranges of values.
Using the reclassified saturated-zone-thickness grid, the software generated a polygon coverage of saturated-zone thicknesses in 20-foot ranges. Polygon attribute "SYMBOL" was added to the polygon attribute table of the coverage and populated based on values in polygon attribute "GRID-CODE." Polygons with perimeters less than 1,000 meters were removed from the coverage. Adjacent polygons with the same GRID-CODE attributes were merged together. 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 240 meters, all the arcs were generalized with the ArcEdit command, SPLINE, and the grain tolerance was reset to 30 before splining the arcs again. Polygons representing saturated-zone thicknesses between 0 and -20 were removed.
A polygon dataset representing the area where water-level-elevation contours were well defined (where contours were drawn at 20-foot intervals) was added to the reclassified saturated-zone-thickness coverage so that saturated-zone-thickness polygons outside of the area could be removed. New polygons with perimeters less than 1,000 meters were formed by the addition of the linework. If those polygons were within the extent of well-defined water-level-elevation contours, they were merged into larger adjacent saturated-zone-thickness polygons. New polygons with negative saturated-zone-thicknesses that also were formed by the addition of the linework 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.