Process narrative for construction of 30 m. WRB DEM
1. Obtaining data
As part of its Baseline97 collaborative project, the Oregon
State Service Center for GIS coordinated assembledge of 30 m., 7.5
min. digital elevation models into a statewide
database. The DEM's are organized in one degree blocks as described
in this
file. Data for the nine, one degree blocks contained within the green
line in this
image were transfered via ftp to the
Sun Sparc20 at the UO Data Services Laboratory.
2. Preparing the data
1. Uncompress one degree blocks (e.g.): unzip 43121.zip
The quads are seven directory layers below the current one after this step:
extracting: HOPPER/BASELINE/DIRECTOR/LATTICES/45122/H/8#/5122H8LG.ZIP
each of which must be individually decompressed in the next step.
2. Uncompress 7.5 min. quads:
find . -name \*.ZIP \-exec unzip {} \;
This creates 64 ArcInfo export files per one degree block, e.g:
Archive: ./HOPPER/BASELINE/DIRECTOR/LATTICES/45122/H/8#/5122H8LG.ZIP
exploding: 5122H8LG.E00
These are then moved to a convenient working directory in the next step.
3. Move 7.5 min quads to working directory:
find . -name \*.E00 \-exec mv {} /scratch/ise/ \:
4. The Arc 7.0.4 IMPORT command will not recognize export files whose
names contain upper case characters. The first step is to create a file
containing the names of these files:
ls *.E00 >> names
The second step is to process the export file names using an awk
script processed by the gnu awk interpreter:
gawk -f fnm.awk names
This changes a name like 5122C2LF.E00 to 5122c2lf.e00
5. Then perform the Arc imports using another gawk script in two steps:
The new set of import file names are, like the original ones, placed into a
file via the ls command:
ls *.e00 >> newnames
This file is used as input to the demimp awk script:
gawk -f demimp.awk newnames
3. Assembling the quads
1. Merge quads using LATTICEMERGE command:
This command has a limit of 49 input files per execution requiring the merge
to be done in multiple passes. This AML is an example of the method used.
4. Evaluating the results
These results were examined for defects some of which
are evident in this image of the 45122 one degree
block. The size of these gaps will change depening on which quads are selected
in a particular merge operation, but they are present both when processed
by the Arc latticmerge command and the GRID MERGE function. In this map
the red lines show the edge gaps and the green line is the basin boundary.
In this low-relief detail area north of Eugene,
the larger east-west gap is roughly 750 m. wide. A similar situation is
present in areas of high relief in which a less
conspicuous horizontal banding is also visible.
5. Solving gap problems
The lattices provided by SSCGIS have been projected
to a standard Lambert projection adopted for the state. Edge matching difficulties
in merging the grids arise because of the use of this non-rectangular projection.
Back- projection of the Lambert grids to UTM, for example, is not a potential
solution because the grids produced have differing cell center spacings.
The solution adopted here is to use the 'raw' ASCII
DEM'. Even when produced by the same agency, however, these files have
differing units, feet (1) and meters (2), and some, identified in the z_un_raw
field as 0 and 3, have values not described in the acompanying documentation.
DEMLATTICE treats the 0 as no unit, and places %2% in the Z UNITS field
of the projection file for an input Z unit code of 3.
For DEM's with elevation values in feet (1), the
z_min and z_max values provided in the SSCGIS documentation are expressed
in meters, whereas the actual values in the header records of these files
are in feet. This, and the 0 and 3 Z units problems means that all of the
DEM's must be checked and corrected before processing. Since multiple versions
exist for some of the quadrangles, we began the process by using the USGS
version in any case in which one was available, the BLM version next, and
USFS last. Other data quality issues included the presence of negative
elevation values in some grids, and anomalies
in slope data derived from the DEM.