The nominal scale is 1:62,500. As of 2003, the data are typically distributed in geographic coordinates (longitude and latitude), decimal degrees, and the North American Datum (NAD) of 1983, and this is the default spatial reference of the ArcSDE feature dataset in which the data are stored. The data were originally developed, however, in a custom Lambert Conformal Conic projection and were distributed in that coordinate system for several years.
The data were digitized in the late 1960s and in 1984-85 from 7.5- and 15-minute USGS topographic quadrangles. Errors in the location of a given feature are dependent on the accuracy of the original maps and on the accuracy of digitizing. Estimates are that features have an average locational error of at least plus/minus 100 feet.
For more information on the origin of the data set, refer to the metadata for the IL_Public_Land_Survey_System feature dataset. Also refer to ISGS Circular 451 (ILLIMAP - A Computer-Based Mapping System for Illinois, Swann, DuMontelle, Mast & Van Dyke, 1970). That publication discusses the original digitizing work, coordinate conversions, and the accuracy of the results.
The database was maintained by the ISGS Computer Services Unit under Van Dyke's direction until 1985 at which time the data were converted to an Arc/Info coverage. The conversion was accomplished by Environmental Systems Research Institute (ESRI) of Redlands, California, working under contract with the ISGS via the Illinois Department of Energy and Natural Resources and its Lands Unsuitable for Mining Program. The remaining one third of the state was digitized from 7.5-minute quadrangles at this time (1984-85.)
In 2003 the data set was transitioned from the ESRI ArcInfo coverage data model to the ESRI ArcSDE enterprise geodatabase data model. The data are stored in an Oracle relational database management system (RDBMS) and maintained using ArcGIS software.
Recent updates to the data:
In March, 1998 the data set as it stood was designated Edition 1.0 for metadata and version management purposes.
Edition 1.0 of these data were reviewed for internal logical consistency in March, 1998. As a result, several unneeded .AAT and .PAT items were dropped, tics were replaced, three dangling arcs were corrected and COVER-ID values were recalculated to be unique. See logical consistency report and process steps for March, 1998 for more details. The updated data set was designated edition 1.1.
In June, 2003 the data set was transitioned to storage in an Oracle RDBMS using the ArcSDE geodatabase model. At that time this metadata document was significantly updated. The result (and most current edition) is designated Edition 2.0.
Links to these are provided in the Cross References section.
Individuals or entities may make fair use of copyrighted ISGS material, such as reproducing a single figure or table, or using a brief text quotation, without obtaining formal permission, but in all cases the Illinois State Geological Survey must be credited as the source of the material. To reproduce ISGS information beyond the fair use standard, permission must be obtained from the ISGS Information Office, 615 East Peabody Drive, Champaign, Illinois 61820, 217-333-4747, firstname.lastname@example.org. License fees and a license agreement may be required, depending on the proposed usage.
Map information is to be used at a scientifically and cartographically appropriate scale, that is, at a scale no greater than indicated on the map or as described in the documentation of the map or map data. Map information is not appropriate for, and is not to be used as, a geodetic, legal, or engineering base. Map information has no legal basis in the definition of boundaries or property lines and is not intended as a substitute for surveyed locations such as can be determined by a registered Public Land Surveyor.
The data do not replace the need for detailed site-specific studies.
There is no quantitative assessment of attribute accuracy available, however, this coverage has been used extensively by ISGS and DNR personnel since 1984, and it can be assumed most attribute errors have been detected and corrected. There are no know arc or polygon attribute values outside the appropriate value domains.
A logical consistency review was performed in March, 1998. At that time the data were maintained in ArcInfo coverage format.
The following checks were made:
The data set was DESCRIBED and coverage files listed to determine the feature types present. In this data set, the following feature types were found and checked: polygons, arcs, labels, nodes, and annotation. The presence of spatial indices was also checked.
For PAT and AAT files, each item was checked for the following: domain of allowable values, adherence of values to the domain, definition of attribute items, requirement of all unique values for an item, logic of items redefined over other items, and length of item name as regards conversion to shapefile format.
The topology status of the data set was checked, as were LABELERRORS, NODEERRORS (if appropriate), and the PROJECTION definition.
The presence of a BND file was verified.
The presence and location of TICS were checked.
Polygon and chain-node topology present. There are no dangling arcs. There is one polygon label per polygon, except for universe polygon. Boundary values appropriate for map projection and map units. "Fuzzy" tolerance is suitable for nominal scale. Several unneeded .AAT and .PAT items were dropped, tics were replaced, three dangling arcs were corrected, and COVER-ID values were recalculated to be unique. See process steps for March, 1998. There are no attribute values outside the appropriate domains.
In 1985 the data were converted to an Arc/Info coverage. The conversion was accomplished by Environmental Systems Research Institute (ESRI) of Redlands, California, working under contract with the ISGS via the Illinois Department of Energy and Natural Resources and its Lands Unsuitable for Mining Program. The remaining one third of the state was digitized from 7.5-minute quadrangles at this time (1984-85.) Details of the digitizing process are unavailable. It is reasonable to assume the positional accuracy was improved for newly digitized areas. However, it is also likely there was some degradation to the positional accuracy of the previously existing data due to automated coverage processing, for example, the use of the ArcEdit CLEAN command.
Subsequently, the horizontal positional accuracy was spot verified by manual overlay of source and hard-copy plots. Arcs within line-width of source maps were deemed acceptable. The size of the sample set is unknown. Errors in the location of a given feature are dependent on the accuracy of the original maps and on the accuracy of digitizing. Current estimates are that features in the data set have an average locational error of at least plus/minus 100 feet.
The source maps are subject to USGS mapping accuracy standards.
An internal logical consistency check was performed in March, 1998. Refer to the Logical Consistency report. As a result, the following changes/updates were made to the data:
TICS - The data set had eight tics along the northern border of the state. For better tic distribution, these were replaced with ten tics from quad100 from the ISGS GISDB archive, evenly distributed around the state. However, users needing registration tics for use with these data (or a subset of these data) should expect to create a task-specific tic set, usually based on section corners.
REMOVAL OF ITEMS - The following empty .AAT items were dropped: STATE2C.OLD#, STATE2C.OLD-ID, ORIG-ID, SYMBOL, SYMB.
UNIQUE COVER-ID VALUES - COVER-ID values in the .AAT and .PAT were calculated to equal (COVER# - 1) in order to make them unique.
At this time it was decided to maintain a record of GIS versions of this data set. The data set prior to review was (arbitrarily) designated Edition 1.0. Upon completion of the review, the data set was designated as Edition 1.1.
The various coverage feature classes were imported using ArcCatalog 8.2. Some fields that are redundant with the IL_Public_Land_Survey_System feature dataset were dropped.
Prior to import, the spatial reference of the data set was changed to Geographic, decimal degrees, NAD83 in double precision. The previous spatial reference was the customary ISGS Lambert Conformal Conic projection.
Also at this time, this metadata file was considerably rewritten and updated.
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Illinois Natural Resources Geospatial Data ClearinghouseGenerated by mp version 2.8.25 on Thu Apr 02 09:53:59 2009