NATURAL RESOURCES GEOGRAPHIC INFORMATION SYSTEM

                            COVERAGE DOCUMENTATION

COVERAGE NAME:  BRTOPO                  DOCUMENTATION REVISION 2/22/93

DESCRIPTIVE NAME:  Bedrock Topography of Iowa

STATUS:  In progress

DESCRIPTION:  The arcs represent the elevation of the bedrock surface,
  regardless of age or rock type, as it is either exposed or buried
  beneath Quaternary drift deposits to various depths.  The lines were
  interpreted by staff geologists from a compilation of all well-log data,
  outcrop records, soil survey analysis, and topographic maps.  The
  coverage was developed by digitizing 93 hand drawn manuscript maps
  as overlays on the 1:100,000 county topographic maps.  Extensive editing
  and reinterpretation was done after the initial digital compilation was
  accomplished.  This was an unprecedented compilation conducted by the
  Geological Survey Bureau.  The contour interval is 50 feet.

GEOGRAPHIC EXTENT:  Ninety-three counties in Iowa

COVERAGE TYPE(S):  Arcs and polygons

COVERAGE SIZE:
  Arcs:  6575
  Polygons:  2785
  Labels:  0
  Tics:  167
  Annotations:  0
  File size:  4430 Kb

MAP PROJECTION:  Universal Transverse Mercator (UTM), Zone 15

MAP UNITS:  Meters

DATUM: NAD27

SOFTWARE/VERSION:  PC ARC/INFO 3.4D

COVERAGE CREATED:

COVERAGE DEVELOPER:  Bernard Hoyer

COVERAGE MODIFICATIONS:
  Bernard Hoyer: all arcs labeled properly; 12/91; Mary Howes:  polygons label-
    ed and elevations added using the lowest contour value for arcs enclosing
    each polygon + 1/2 the contour interval (25 ft.); 2/93.  

DNR CONTACT:  Raymond Anderson, Iowa Department of Natural Resources,
  Geological Survey Bureau, 123 N. Capitol Street, Iowa City, IA
  52242, 319-335-1575

ORIGINAL SOURCE INFORMATION:
  Media:  93 mylar overlays of paper, county maps
  Title:
  Author/Agency:  Raymond Anderson, Robert Rosenberg, Brian Witzke, Robert
    McKay, Gregory Ludvigson
  Published Date:
  Compiled Date:  1987-1990
  Scale:  1:100,000
  Projection:  Universal Transverse Mercator
  Geographic Control:  County corners of paper maps
  Data conversion:
  References:

ACCURACY ASSESSMENT:  Contour lines are interpreted from various data
  types with various accuracy and varying data densities; may vary from
  +/-100 meters to +/-3200 meters.

ASSOCIATED COVERAGES:
  NEBRTOPO:  Contains the six northeast counties of bedrock topography not
    contained in BRTOPO93

ASSOCIATED FILES:  Contour.lut (can use as 'join file' to draw contours as
  different colors)

COMMENTS:  Contours match adjoining contours in the 6 counties not contained
  on this coverage in NEBRTOPO.  Elevation assigned to polygons should be
  considered an average for the polygon.

ARC ATTRIBUTE FILE ITEMS (AAT):
  LENGTH:  Length of arc in meters
  BRTOPO93_:  Internal arc number
  BRTOPO93_I:  Sequential identification number
  CONTOUR:  Elevation in feet represented by the contour line

POLYGON ATTRIBUTE FILE ITEMS (PAT):  
  AREA:  area of polygon in square meters.
  PERIMETER:  length of arc(s) defining polygon in meters.
  BRTOPO_:  internal polygon identifier assigned by software.
  BRTOPO_ID:  user polygon identifer.
  ELEVATION:  elevation of bedrock surface, value of CONTOUR for lowest boundi
    ing arc + 25 ft.

POINT ATTRIBUTE FILE ITEMS (PAT):  None

ANNOTATION:  None

TICS:  167 tics located at county corners

****NOTICE****
This digital, geographically referenced data set was developed by the
Iowa Department of Natural Resources to carry out agency responsi-
bilities related to management, protection, and development of Iowa's
natural resources.  It is being developed by a DNR Bureau at this time,
as a part of the Department's Natural Resource Geographic Information
System.  Although efforts have been made to make the information accurate
and useful to the Department, the Department assumes no responsibility
for errors in the information.  Similarly the Department assumes no
responsibility for the consequences of inappropriate uses or interpre-
tations of the data made by anyone to whom this data has been made
available. The Department bears no responsibility to inform users of any
changes made to this data.  Anyone using this data is advised that
precision implied by the coverage may far exceed actual precision.
Comments on this data are invited and the Department would appreciate
that documented errors be brought to staff attention.