LIBRARY:        ELEVATION DATA (LATTICE)
LAYER NAME:     PRJ1_LAT
LOCATION:       GRID1

=DESCRIPTION:

DEMs were imported from 1:24,000 Digital Elevation Models (DEMs) developed by the US Geological Survey (USGS).  Each DEM represents 7.5' x 7.5' on the ground, and contains elevation postings every 30 meters.

Data were obtained in units of 7 1/2 minute quadrangles (see Quad Boundaries), and then merged into a single lattice for the entire study area.  Data were also run through the hillshade function in ArcView to produce the version seen in the project.

VITAL STATISTICS:
  
  Horizontal  Datum:       NAD27 (spheroid: Clarke 1866)
  Vertical Datum:          NGVD 29
  Projection:              UTM zone 10
  Units:Meters
  1st Std. Parallel:       34 00 00
  2nd Std. Parallel:       40 30 00
  Longitude of Origin:     -120 00 00
  Latitude of Origin:      00 00 00
Source:                    U.S Geological Survey
  Source Media:            CD ROM
  Data Scale:              1:24,000
  Data Resolution:         30 meters
  Source Ground Unit:      Meters
  Source Z Unit:           Meters
  Capture Method:          Various (scan techniques)
  Source Accuracy:         Because elevation postings model a surface and do not represent specific features, horizontal accuracy is difficult to measure and is not stated by the USGS.  7 meters is the desired vertical
  
  Conversion Software:    ARC/INFO Rev 7.0.4
  Data Structure:         Grid Cell-based
  ARC/INFO Coverage Type: Lattice
  ARC/INFO Precision:     Double
  ARC/INFO Tolerances:    Not applicable
  Number of Features:     Approximately 171,000 per quad (Approximately 380 columns by 450 rows)
  Layer Size:             Average 3 MB per quad
  Processing platform:    Sun Sparc 10 workstation
  Data Completed:         March 1997
  
  NAMING CONVENTION: 

Quad names begin with a "d" and are based on the latitude and longitude of the southeast corner of the quad.  For example, the southeast corner of quad "d36121a1" is located at latitude 36 00 00, longitude -121 00 00.

lat  MM SS	long MM SS
a  = 00 00	1 = 00 00
b  = 07 30	2 = 07 30
c  = 15 00	3 = 15 00
d  = 22 30	4 = 22 30
e  = 30 00	5 = 30 00
f  = 37 30	6 = 37 30
g  = 45 00	7 = 45 00
h  = 52 30	8 = 52 30

ADDITIONAL PROCESSING NOTES:

Although the USGS DEM specification indicates that each posting is a floating point number, in most cases these have been rounded to the nearest integer.  Therefore these lattices are stored as integers rather than floats, reducing storage requirements significantly.  The DEMs were imported with the INT option.

 The elevation values for the majority of the DEMs were meters.  Those stored as feet were converted to meters by multiplying the values by 3.2808, adding .5, and converting to integer: outdem = int((indem / 3.2808) + .5)

Other than converting from feet to meters and from NAD83 to NAD27 for some quads, and merging nonstandard quads, these DEMs have not been modified.  They have not been filtered, or corrected where values are obviously wrong, nor have they been edge-matched.  Therefore when you use them you should check for missing or incorrect data, and consider filtering to meet your needs.

Because changing the projection of a grid requires resampling that can cause edge-matching problems, the DEMs have been left in their original projection, which is either UTM zone 10 (west of longitude -120) or 11 (east of longitude -120).  The best procedure for changing the projection is to merge adjacent grids first, then re-project.  The most appropriate re-sampling methods are either bilinear interpolation or cubic convolution.  Don't use the default nearest neighbor.


  DATA QUALITY ASSESSMENT:
  
  The following are subjective comments regarding this data.

Some DEMs were produced using a profiling technique that yielded a stronger autocorrelation between points along the profile than between profiles.  The result of this is clearly visible striping parallel to the profiles.  Some success has been found by filtering with the "LOW" option, or using a 1x9 weighted filtering window.


  Additional information is available from the USGS document
  "Digital Elevation Models, Data Users Guide".  This can be
  ordered from:
  Earth Science Information Center
  U.S. Geological Survey
  507 National Center
  Reston, Virginia  22092
  (703) 648-6045
  Or it is available online (Word Perfect format) at:
  ftp://www-nmd.usgs.gov/pub/ti/DEM/demguide/dugdem.wp5
  

  DATA CONTACT:
  
     Contact Name:       Andy Richardson
     Contact's Phone:    916-263-3649

  DOCUMENTATION DATES:

     6/29/98         29 7.5 minute quads merged to fit project study area

     Contact Name:   P. J. Trichilo
                     Spatial Information Systems Institute
                     Humboldt State University
                     (707) 826-5417