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Article # 0051


Procedures for Generating Elevated Terrain Data for Industrial Source Complex Air Dispersion Modeling





Air dispersion modeling is often used to predict the off-property ground-level concentrations of air pollutants emitted from industrial facilities.  During the air permitting process, the Texas Commission on Environmental Quality (TCEQ) compares the predicted concentrations from air dispersion modeling to standard (TCEQ-created) screening levels to evaluate whether emissions from a proposed facility will have an adverse effect on the surrounding area.   To calculate the predicted concentrations at specific locations (or receptors), the coordinate 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 grids surrounding the facility.


For the purpose of air dispersion modeling, the Texas Commission on Environmental Quality (TCEQ) defines flat terrain as terrain equal to the elevation of the stack bases (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 EPAís Industrial Source Classification (ISC) Short-Term (ISCST3) model.  However, the processed elevation data could be used with other modeling programs.


Documentation of Methods


U.S. Geological Survey (USGS) elevated terrain receptor data can be obtained through GeoComm International Corporation (known as GeoCommunity) that has an agreement with the USGS to distribute digital elevation model DEM data. USGS DEM 7.5 minute tiled data in Spatial Data Transfer Standard (SDTS) for the appropriate quadrangle should be used. The horizontally-referenced data obtained by this method are North American Datum (NAD) NAD83 in units of meters relative to the National Geodetic Vertical Datum of 1929 (NGVD29). This data is processed by the USGS to produce a DEM with a 10-meter sampling interval.  It is possible that the minimum sampling 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 model input file format. This mapping software uses standard algorithms to interpolate terrain heights for the 100-meter and 25-meter grids selected from the 10-meter spaced set. The base height of each emission point should be set to its actual terrain height in the model input file. 




  1. 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.


  1. Determine which USGS quadrangle needed.  The viewer at http://nmviewogc.cr.usgs.gov/viewer.htm 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.


  1. 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.


  1. Determine if elevation data is required by reviewing the elevations on the USGS quadrangle.


  1. 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 surrounding quadrangle(s).  Once the elevation data is extracted as explained below, it will be apparent which receptors fall outside the quadrangle first selected. 


  1. 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.


  1. 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.


  1. Click the first link "Digital Elevation Models (DEM) - 24K"


  1. Scroll down to the name of the quadrangle you determined in step three.


  1. Click the green arrow icon on the right side to download. 


  1. 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 "3809112.DEM.SDTS.TAR.GZ".    Be sure to download to a separate folder for each quadrangle.  This is a zipped file and will create several extracted files.


  1. 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.


  1. 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 folder.  Double-click 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.


  1. Open the SURFER 8 program.  A blank plot document should be the first screen.  If not, create a new one.


  1. 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".


  1. From the "Grid" menu, select "Convert".  Again, 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".


  1. 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.


  1. 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.


  1. 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.


  1. 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 elevations.


  1. Copy data and paste into blank Excel document or open the DAT file in Excel with appropriate delimiters.  Convert 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.


  1. 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 configurations and the software versions noted.



Karen 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 Air Permit Specialist in the Coatings and Combustion Section, where she developed a 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.


Karen M. Bullard, P.E. No. 88449




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