by Davide Borelli, Davide.Borelli@ingegneria.studenti.unige.it, 2006
FAA's Integrated Noise Model (INM) uses the .3cd format to compute terrain data in its studies.
This binary format (.3cd) is sold by Micropath Corporation for U.S. regions; non U.S. users must manually write their .3tx ASCII text file, which INM software reads and converts in a .3cd binary one, where the name is given by the Lat/Long specified at the first line of the .3tx file.
As one can see in INM's manual, a .3tx file contains terrain elevations covering a one-by-one degree square area.
Here is a description of the .3tx ASCII text format:
Lat is the latitude of SW corner (integer degrees)
Long is the longitude of SW corner (integer degrees)
Elevation is terrain elevation MSL (integer meters)
The first line in the file defines the southwest corner of the one-by-one degree area.
North latitude and east longitude are positive integers; south latitude and west longitude are negative integers. The SW corner must be on integer degrees.
There must be exactly 1201*1201=1442401 lines following the first line. Each line contains one number - the elevation, in integer meters, of the terrain above mean sea level.
The elevation of the SW corner is the first integer, the elevation that is 3 arc-seconds to the north is the next integer, and so on until the top edge (integer degree latitude) is reached.
Then imagine to move 3 arc-seconds to the east, start at the bottom edge, put in another 1201 elevations while moving north, and so on until the file is finished.
Obtaining such file has been a big problem for some years for INM users from all over the World, until the function to import/export .3tx files has been added to the VTBuilder software!
The best dataset to use to obtain .3tx files using the export function of VTBuilder is, as far as I know, the SRTM (Shuttle Radar Topography Mission) version 2 data at ftp://e0srp01u.ecs.nasa.gov/srtm/version2/, because the file format is the same of the .3cd file format, that's what we want to have.
To convert SRTM file we just have to:
drag the required .hgt file in VTBuilder;
click on "Elevation"-->"Fill In Unknown Areas";
select from the "Elevation" menu "Export To..." and choose 3TX
If SRTM data is not available, it's possible to use almost any other kind of elevation data and follow these steps (for example I used the "Europe" data at http://www.ngdc.noaa.gov/seg/fliers/se-1104.shtml):
drag Europe.hdr in VTBuilder window;
click in "Area Tool"--> "Numeric Values" and fill "Origin And
Size" with, for example (12,00000000, 57,00000000), (1,00000000, 1,00000000), then click "Ok";
go to "Area Tool"-->"Merge & Resample Elevation", select "Output"-->"To File" and give as name N57E012 and desktop as destination folder; then in "Sampling"-->Grid Size" put 1201 in both fields and press again "Ok"; you'll find on the desktop two files: N57E012.bt and N57E012.prj;
open it again and drag inside the window N57E012.bt;
click on "Elevation"-->"Fill in Unknown Areas";
at last: "Elevation"-->Export To..."--> 3TX
Again, that's all!
You can now import your terrain data in INM!