Obtaining .3tx files from almost any elevation source using VTBuilder

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 Long


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:

That's all!

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):

Again, that's all!

You can now import your terrain data in INM!