Working with SRTM data#

Introduction#

To make full use of the path attenuation calculations provided by pycraf we recommend to use NASA’s Shuttle Radar Topography Mission (SRTM) data for height-profile generation. pycraf can work with so-called .hgt files, a very simple binary format. Each .hgt file, a so-called tile, just contains 1201x1201 16-bit integers. From the file naming scheme, one can infer the associated coordinates. Tiles contain one square-degree of data and are named in a very simple manner, e.g., N50E006.hgt. The first character is either “N” (North) or “S” (South), indicating whether the following number is positive or negative latitude. Likewise, the 4th character, “W” (West) or “E” (East), means negative and positive longitude.

Unfortunately, we cannot provide SRTM data as part of the package, due to the large file sizes and legal reasons.

Obtaining SRTM data#

There are various versions of SRTM data. “Official” are the Versions 1 and 2.1 available on https://dds.cr.usgs.gov/srtm/. There is even a NASA version 3, but we couldn’t find a site that allows direct download. It may work with an EarthData Account on https://lpdaac.usgs.gov/data_access/data_pool.

Then, there is a version V4.1 by CGIAR and an unofficial version by viewfinderpanoramas.org.

Warning

Working with the SRTM data from the above sources may not be allowed for commercial use unless you get explicit permission to do so. For research or private use you may have to properly acknowledge the source of the data. Please check the web sites for more information!

Configuring pycraf to use SRTM data#

When you use the functionality from the pathprof sub-package, pycraf will initally assume that .hgt files are in the local folder (or a sub-directory). If you have many different projects and want to store the tiles in a custom (system-wide) directory, there are two options, explained in the following.

Warning

Placing the .hgt files in the current work directory (where Python is started) and having pycraf look for them there seems handy, but it can be slow if many other sub-directories are present as pycraf will recursively search for .hgt files.

Using `SRTMDATA` environment variable#

If you have downloaded some (or even all) tiles into a directory, you can let pycraf know by defining an environment variable in the terminal where you run your program.

On windows:

set SRTMDATA=C:\[path-to-srtm]\

On Linux/MacOS (sh-like):

export SRTMDATA=[path-to-srtm]/

Of course, it is possible to make this permanent. On Linux/MacOS put it into your shell config file (e.g., .bashrc). On Windows 7+, you could click on the start menu, type “env” and open the dialog that allows to edit your user environment settings.

Changing the SRTM path during a running session#

It is also possible to change pycraf’s behavior during runtime. In the pathprof sub-package, the SrtmConf class is defined, which allows to change the SRTM directory, but also makes it possible to download missing tiles during run-time:

>>> from pycraf.pathprof import SrtmConf
>>> SrtmConf.set(srtm_dir='/path/to/srtmdir')  

Alternatively, if only a temporary change of the config is desired, one can use SrtmConf as a context manager:

>>> with SrtmConf.set(srtm_dir='/path/to/srtmdir'):
...     # do stuff
...     pass

Afterwards, the old settings will be re-established.

It is also possible to allow downloading of missing .hgt files:

>>> SrtmConf.set(download='missing')  

The default download server will be server='nasa_v2.1'. One could also use the (very old) data (server='nasa_v1.0') or inofficial tiles from viewfinderpanoramas.org (server='viewpano'):

>>> SrtmConf.set(server='viewpano')  

Note

Note: As of Spring 2021, NASA decided to put all SRTM data products behind a log-in page, such that automatic download ceases to work. For the time being, the default server (and only server) will thus be viewpano. If you prefer to use NASA tiles (over viewpano), please use their services, e.g., the Land Processes Distributed Active Archive Center.

Of course, one can set several of these options simultaneously:

with SrtmConf.set(
       srtm_dir='/path/to/srtmdir',
       download='missing',
       server='viewpano'
       ):
   # do stuff

Last, but not least, it is possible to use different interpolation methods. The default method uses bi-linear interpolation (interp='linear'). One can also have nearest-neighbor (interp='nearest') or spline (interp='spline') interpolation. The two former internally use RegularGridInterpolator, the latter employs RectBivariateSpline that also allows custom spline degrees (kx and ky) and smoothing factor (s). To change these use:

>>> SrtmConf.set(interp='spline', spline_opts=(3, 0))  

We refer to RectBivariateSpline description for further information.