Stearns County Geologic Atlas, Part B

DESCRIPTION OF EACH LAYER

1. A TIN was made from a simplified potentiometric surface of the water table line coverage (see metadata for 'psfwt'), using the ARC command, 'arctin,' with the 'line' option. 'Simplified,' means that contours that ended 'in mid air,' i.e., those whose continuation could not be determined, had been deleted.
2. A lattice was made from this TIN with the ARC command, 'tinlattice' using the item, 'elev (elevation of the potentiometric surface)' with defaults.
3. This lattice was subtracted from the surface elevation DEM using GRID to create a depth-to-water-table grid.
4. The resulting depth-to-water-table grid was filtered with a 'low' pass with 5 iterations, then contoured at 10 ft. intervals, using the ARC command, 'latticecontour,' with default settings.
5. The contour lines were enclosed within the county boundary to create a polygon coverage. This was done by using the ARCEDIT command, 'get' to attach the county boundary coverage to the contour coverage, with 'intersectarcs' set to 'all.' Contour bits and pieces that dangled outside the county border were deleted, and the coverage was built with the 'polygon' option.
6. Values from the depth to water grid were attached to polygon labels of this coverage using the ARC command 'latticespot.' The resulting depth to water value which is attached to each polygon label was used to calculate depth range values. i.e., If a polygon label's depth to water value is '10.8,' then that label is given a range value of '10 to 20' ft. This works because all of the polygons in this coverage are bounded by equipotential lines at 10 ft. intervals. The value of a point ANYWHERE within a given polygon is NECESSARILY between two 10 ft. intervals. A polygon label with a depth to water value of .69 would be given a range value of '0 to 10,' meaning that the polygon defines an area within Stearns County where the estimated depth to water is at least 0, and at most 10 ft.
7. The resulting depth to water coverage was combined with a simplified version of NWI polygons, using the ARC command, 'union' to create a coverage that contains both DNR Waters-created depth to water polygons and NWI polygons. The NWI data was simplified by deleting all polygons less than 20,000 sq. meters, and dissolving the result on the item 'system,' leaving outlines of wetland types rather than outlines of very single wetland.
8. This coverage was, in turn, combined with the polygons from Stearns County Soil Survey Data Set polygon coverage, where depth to the water table was less than 6 ft to create the final water table near the surface polygon coverage. (Note: this data set was compiled at a larger scale than that of all of the other data, and the greater detail of its polygons might artificially enhance the appearance of precision of the pollution sensitivity coverage. The pollution sensitivity coverage is no more accurate than the 1:100,000 scale manuscripts that comprise most of the layers that went into it! Contact Stearns County Environmental Services at (320) 656-3613 for more information about The Stearns County Soil Survey Data Set) The item 'nearsurf' was added and given a value of 'Y' when one of the following conditions were met:
   • Depth to the water table is less than 6 ft. based on Stearns County Soil Survey Data Set (Step 8).
   • All wetlands (Step 7)
   • Estimated depth to the water table is less than 20 ft. (Steps 1 to 6)

A 1:100,000 scale manuscript showing selected CWI points colored according to 'score' value ranges was prepared in ARCVIEW and plotted on paper. 'Score' values are numbers indicating the estimated thickness of the subsurface impermeable layer. These cumulative scores were calculated by Hua Zhang from CWI Well Log data of about 4100 wells in Stearns County.

Each well log was examined by a computer program written by Hua Zhang that assigned a permeability unit score based on the thickness and types of geologic materials in the upper 50 feet. The unit scores range from 0 to 1, with 0 indicating highest permeability (i.e, gravel or sand)., and 1 lowest (i.e., clay or shale).

Then, a cumulative score was calculated by summation of the unit scores of all individual layers within 50 feet of the land surface. The calculated cumulative score ranges from 0 to 50. A well with clay material in the whole upper 50 feet, for example has a cumulative score of 50. A well with gravel or sand in the whole upper 50 feet has a cumulative score of 0.

The cumulative scores of all wells were plotted on paper at a scale of 1:100,000. Hua Zhang drew contour lines around groups of points having:

• a preponderance of cumulative score values greater than 40 (relatively low subsurface permeability)
• a preponderance of cumulative score values between 20 and 40 (relatively high subsurface permeability).

• digitally generated contour lines that had been made from a GRID of cumulative score values
•  MGS' surficial geology polygon coverage from the Stearns County Geologic Atlas, Part A, Plate 3.

Outline of till areas

This coverage was made in order to create a geographic area of till so that peat over till could be differentiated from peat that is NOT over till. It was derived from the MGS surficial geology coverage. Map units 'dc' 'P' 'st' 'stp' 'stt' 'dsc' 'dst' 'drt' 'dlt' 'thd' 'dt' 'wt' (till areas) were selected and put (ARCEDIT command 'put') into a new coverage. A dissolve was performed to eliminate all interior lines, resulting in a coverage that shows an outline of till areas, rather than each till polygon.

This process creates polygons within which many peat areas fall when a union that includes MGS surficial geology is performed. These peat areas can then be delineated by selecting those which fall "within" till areas and those which do not. However, many peat polygons are on "edges" of till areas, and would therefore not have ended up "inside" the till area outline created by this processes. In fact, it is difficult to know whether or not a given peat polygon on the "edge" of a till area should be considered 'peat over till,' or 'peat not over till.' Hua Zhang selected several "edge" peat polygons which were subsequently added to the till outline coverage, in effect broadening the geographic scope of this (till) coverage so that it subsumes those polygons after the union is performed. Other "edge" peat polygons were considered 'peat NOT over till, along with the peat polygons that are completely outside of till areas.

Surficial Geology from the Minnesota Geological Survey (SGPG)

This polygon coverage was created by the MGS, and is displayed on Plate 3 of The Stearns County Geologic Atlas, Part A. Relative permeability of different geologic types was estimated by Hua Zhang, and used in assessing pollution sensitivity values. See Ratings Matrix on Stearns County Geologic Atlas, Part B, Plate 10 for more detailed information.

COMBINING, DISSOLVING, SIMPLIFYING, AND ADDING ITEMS

The coverage was dissolved on the item 'senrating, and polygons 20,000 sq meters or smaller were removed with the ARC command 'eliminate.' Finally, the item 'sensymbol' was added and calculated as shown in 'Entity and Attribute Information' (below).
 

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Users may wish to verify critical information; sources include both the references here and information on file in the offices of the Minnesota Geological Survey and the Department of Natural Resources. Every effort has been made to ensure the interpretation shown conforms to sound geologic and cartographic principles. This map should not be used to establish legal title, boundaries, or locations of improvements.