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Article # 0051
Terrain Data for Industrial Source Complex Air Dispersion Modeling
Air dispersion modeling is
often used to predict the off-property ground-level concentrations of
pollutants emitted from industrial facilities.
During the air permitting process, the Texas Commission on
Quality (TCEQ) compares the predicted concentrations from air
modeling to standard (TCEQ-created) screening levels to evaluate
emissions from a proposed facility will have an adverse effect on the
surrounding area. To
predicted concentrations at specific locations (or receptors), the
locations are entered into the modeling input file.
It is assumed that the reader is familiar with
procedures for generating the required 100-meter and 25-meter receptor
surrounding the facility.
For the purpose of air
dispersion modeling, the Texas Commission on Environmental Quality
defines flat terrain as terrain equal to the elevation of the stack
(emission points). When
the terrain to
be modeled is not flat, the TCEQ requires the receptor data used in the
modeling to include appropriate receptor elevations.
The purpose of these
instructions is to describe how to generate elevation data for
receptors to be
used in air dispersion modeling for facilities in the United States. Specific instructions will
be given for input
file formatting of receptor data to be used with EPA’s Industrial
Classification (ISC) Short-Term (ISCST3) model.
However, the processed elevation data could be used with
U.S. Geological Survey
(USGS) elevated terrain receptor data can be obtained through GeoComm
International Corporation (known as GeoCommunity) that has an agreement
the USGS to distribute digital elevation model DEM data. USGS DEM 7.5
tiled data in Spatial Data Transfer Standard (SDTS) for the appropriate
quadrangle should be used. The horizontally-referenced data obtained by
method are North American Datum (NAD) NAD83 in units of meters relative
National Geodetic Vertical Datum of 1929 (NGVD29). This data is
the USGS to produce a DEM with a 10-meter sampling interval. It is possible that the
interval available for a specific quadrangle may be 30-meter instead of
10-meter. The data
set with the smallest
sampling interval should be used for the most precise results.
Golden Software SURFER 8 can
then be used to convert the USGS DEM STDS data into the required ISCT3
file format. This mapping software uses standard algorithms to
terrain heights for the 100-meter and 25-meter grids selected from the
spaced set. The base height of each emission point should be set to its
terrain height in the model input file.
- Generate the receptor grid(s)
needed for modeling input files. The
coordinates used in the ISC input files, as well as the data below are
in UTM coordinates.
- Determine which USGS quadrangle
needed. The viewer
is interactive and can be clicked to zoom to the area needed. Alternatively, the link "Find Place" in the left pane can be used
to zoom to a specific county.
- To see the quadrangles, turn on
the appropriate layer in the right pane.
Check the box next to "USGS QUADRANGLES" under the
selection "Topographic Maps" in the right pane.
- Determine if elevation data is
required by reviewing the elevations on the USGS quadrangle.
- If elevation data is needed,
continue. If the
facility being modeled is near the boundaries of the quadrangle, it may
be necessary to download dataset(s) from the surrouning
the elevation data is extracted as explained below, it will be apparent
which receptors fall outside the quadrangle first selected.
- Go to http://data.geocomm.com/dem/demdownload.html
to download the data in the correct DEM format.
The website will require creation of a user account to
proceed. Create the
account and login before proceeding.
A map of the US will appear with download
instructions below it.
- Click on the state where the
facility is located (enable pop-ups first to allow the next window to
open), then click on the link for the appropriate county.
- Click the first link "Digital
Elevation Models (DEM) - 24K"
- Scroll down to the name of the
quadrangle you determined in step three.
- Click the green arrow icon on
the right side to download.
- If both 10 and 30-meter data is
shown, download the 10-meter data.
Otherwise, download the 30-meter data.
The format of the file name will be similar to
Be sure to download to a separate folder for each
quadrangle. This is
a zipped file and will create several extracted files.
- Download the accompanying text
file as well for information on resolution that will be needed later
during the modeling analysis. Pay
close attention to the resolution and the units used in the elevation
data. Some datasets
use units of meters, while some use feet.
- Unzip the data file using a
program like WinZip (not Windows folder decompression).
If WinZip is utilized, select the options in these
instructions prior to extraction.
Right click the file and select WinZip "Extract to
here". The dialog
box that pops up will ask "Should WinZip decompress it to a temporary
folder and open it?" Click "No". A
partially extracted file in the Winzip format will be created in the
the file to open WinZip. Turn
off the "TAR file smart CR/LF conversion" by unchecking this option in
the box in the "other" category under the "Miscellaneous" tab found in
the "Options/Configuration" area. This will ensure that all the files
are unzipped correctly.
- Open the SURFER 8 program. A blank plot document
should be the first screen. If
not, create a new one.
- From the "Map" menu select
"Contour Map" and then "New Contour Map".
Browse to the folder with the unzipped files. Select any file with the
extension "DDF" and click "Open".
- From the "Grid" menu, select
select any file with the extension "DDF" and click "Open". Another prompt will appear
asking for a name for the new grid file.
Name it appropriately and click "Save".
- Place the receptor coordinates
for which elevation data is desired into an Excel spreadsheet. UTM Easting coordinates
should appear in the first column (A), while Northing coordinates
should appear in the second column (B).
Place a zero in column three (C) for every row where
you have a pair of coordinate.
- Open the Excel file with the
receptor coordinates in SURFER 8.
Choose "Save As" and save as file type "Golden
Software Data (*.dat)". Let
it select defaults for saving. If
it gives a warning that formatting will be lost, disregard and continue
to save the file in the data format.
- Go back to any Plot window and
from the "Grid" menu select "Residuals".
Browse to the grid file you created in step 16 and
select it, then click "Open". Another
dialog box will open. Select
the data file you created in step 18 and click "Open".
Make sure the data is formatted correctly for
retrieval in the dialog box that opens and then click "OK". X data should be "Column
A", Y data should be "Column B", Z data should be "Column C" and
residuals should be selected to be stored in column D.
Click "OK" and the program will pull all elevation
data into column D. Elevation
data will be negative numbers at this point.
Save data file.
- If some of the elevation data is
blank, but some is filled in, it means that the receptor grid spans
more than one quadrangle. Go
back to step 2 and process the additional elevation data from the
needed additional quadrangle(s). Consolidate
all elevation data into one Excel file containing all receptors and all
- Copy data and paste into blank
Excel document or open the DAT file in Excel with appropriate
negative elevation numbers to positive numbers by multiplying by
negative one. Insert
a column A and fill down the receptor command for the ISC input file
"RE DISCART". Give
all three columns of data one decimal point and space the column widths
so that a conversion to text will leave one or two spaces between the
data in the columns.
- Save as "Formatted Text, Space
delimited (*.prn)". Open PRN receptor file in
Notepad, select receptors, copy and then paste into the receptor
section of the ISC input file. Be
sure to obtain the associated elevations for
the EPN SO command lines as well.
This procedure works
successfully as of June 28, 2010 with the current website
the software versions noted.
M. Bullard, P.E. is an Engineering Partner and the President of Bullard
Environmental Consulting, Inc. She has over 16
years experience in
environmental engineering, compliance, and permitting. She
worked for the
Texas Commission on Environmental Quality (TCEQ) for four years as an
Permit Specialist in the Coatings and Combustion Section, where she
thorough understanding of the governmental procedures and policies in Texas.
Karen has a Bachelors
of Science Degree in Chemical Engineering from the University of Texas at Austin.
M. Bullard, P.E. No. 88449
Article # 0051
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