Table of Contents - The Atmospheric Studies Group at TRC

Section 2: Geophysical Data Processors
2.1 TERREL Terrain Preprocessor
TERREL is a preprocessing program that extracts and reformats Digital Elevation Model (DEM) terrain
data according to the options selected by the user (domain, resolution, etc.). Eleven DEM formats are
currently supported. TERREL has the ability to produce Cartesian gridded fields of terrain elevations or a
polar grid of terrain elevations. For the Cartesian gridded field option, TERREL averages all of the
terrain data points which fall within the grid cell to obtain the elevation assigned to the center of the userspecified
grid cell. When using the polar grid option, TERREL uses the maximum terrain elevation in the
area either from the current ring out to the next ring (user input switch - SCREEN) or halfway between
adjacent rings (user input switch - NORMAL) and halfway between the adjacent radials. TERREL can
produce terrain data files in the formats compatible with the following models: CALMET, MESOPAC,
NUATMOS, and ISC3. TERREL requires at least one input file and produces four output files.
TERREL can first be run without any data files and the program will indicate for the user the latitude and
longitude of the four corners of the area required to cover the user-specified domain. A message indicates
how many terrain data files of each type are required based on the domain parameters supplied by the
user. This is helpful, for example, when only UTM coordinates are known, but not the latitude and
longitude of the corners of the modeling domain. Once the appropriate data files are obtained, the
TERREL input file must be modified to reflect the names and types of the data files and TERREL must
be run again to process the terrain data. This could be done in one run or as an iterative process, where
intermediate results are stored in a binary file (e.g., TERREL.SAV) and incorporated into the next
TERREL run using the next set of digital terrain input data. The .SAV file option is helpful if the user
doesn't have the available disk space to store all of the raw terrain files at once.
TERREL has an input (ITHRES) which is used for quality assurance purposes. ITHRES is a whole
number (%) identifying the acceptable threshold of variance from the average number of data points
('hits') per cell. If a particular grid cell had less than ITHRES percent of the average number of data 'hits'
per cell, a warning message is written to alert the user to check the results. If using a mix of 1-degree
DEM data and 30 meter DEM data, the grid cells using the 30 meter data will have many more 'hits' than
the 1-degree DEM grid cells. The user might want to adjust the value of ITHRES to reduce the number
of warning messages written.
TERREL has the option (variable PMAP) to define the gridded output fields for a number of map
projections. Note that CALMET currently supports Universal Transverse Mercator (UTM) grid,
Lambert Conformal Projection (LCC) and Tangential Transverse Mercator (TTM). Lambert Conformal
Projection should be used when the modeling domain is large, (> 300 km), because a Lambert Conformal
grid accounts for the earth's curvature. If the LCC option is specified, TERREL uses the user-specified
standard parallels (latitudes) and reference longitude to calculate a "cone constant" and the east-west
distance from the reference longitude. The reference longitude is the longitude at which true north and
map north are defined to be the same. It also defines where x=0 in the Lambert Conformal grid. The
reference latitude defines where y=0 in the Lambert Conformal grid.
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