Use of OpenAthena requires loading a GeoTIFF Digital Elevation Model (DEM) file, stored as a GeoTIFF .tif file.
GeoTIFF files store terrain elevation (height) for an area of the Earth. OpenAthena performs a ray-cast from a drone camera's position and orientation towards terrain (as represented by the DEM). This may be used to precisely locate any selected point within a given picture.
OpenAthena for iOS only supports GeoTIFF file generated with Method 1
There are two methods supported for obtaining DEM GeoTIFF files. Method 1 is recommended for users on Windows computers, Method 2 is recommended for users on Mac or Linux.
The website OpenTopography.org (which is not affiliated with OpenAthena) has a free browser-based tool for downloading a customized portion of the SRTM 30m Global terrain elevation dataset.
Open the tool here:
https://portal.opentopography.org/raster?opentopoID=OTSRTM.082015.4326.1
Ensure that you are using the correct dataset. The title of the page should appear just like the image below:
Next, zoom into the area for which you wish to download a Digital Elevation Model for use with OpenAthena.
When you're ready, click the blue square button which reads "SELECT A REGION". Click and drag your mouse cursor until the desired area is covered by a rectangle, as demonstrated in the image below:
Enter in your job title, description, and email address in the appropriate fields. When ready, click the blue rectangular button which reads "Submit", located at the bottom of the page:
This will submit a raster job to the opentopography website's server. You will be taken to a new webpage while your job is pending.
Once your job is completed, a link will become available to download your GeoTIFF DEM file, contained in a compressed .tar.gz tarball file:
Click on the link to download the .tar.gz tarball file (which contains your desired GeoTIFF DEM).
Once it is downloaded, you must extract your desired DEM from the compressed file. Open up a PowerShell window (a program included with Windows 10). If you are using a system older than Windows 10 or PowerShell is not available to you, you may use the program 7-zip instead for this step.
By default, you may type in the command:
cd ~/Downloads
...to make your PowerShell window target your Downloads directory.
Next, type in the command:
dir
to obtain a list of files in the current directory. Observe that your downloaded tarball file should appear in this list. When ready, type the following but do not yet press the "RETURN" key:
tar -xzvf
type in a space charachter , then type rasters and press the TAB key, located on your keyboard above CAPS LOCK. PowerShell will automatically complete the rest of the filename for your downloaded tarball file.
If the filename appears correctly, press the "RETURN" key to run the command in PowerShell:
If you have followed these steps correctly, a file ending in .tif should appear in your Downloads directory. This is your desired GeoTIFF DEM file, which is compatible with OpenAthena:
Finally, copy this file to your Android or PC device for use with OpenAthena.
Method 2 requires the use of Python3, gdal, and the elevation package for Python.
Python3 (and the included pip package manager) must be installed first
Ensure your version of pip is up to date:
python3 -m pip install --upgrade pipNote: "python3" may just be called "python" depending on the configuration of your system
Clipping GeoTIFF files requires the gdal program, installable with your system's package manager. MacOS does not come with a package manager by default, so users must first install brew, then run the command:
brew install gdal
(if Linux use apt or yum or pacman, etc.)
...to install this prerequisite program. For Linux or Unix computers, gdal is installable with the system's default package manager. A gdal and pygdal program exist in the pip package manager for Python, however they are incompatible with elevation and must not be used.
The Python elevation package is capable of downloading and joining tiles from the SRTM 30m dataset. Use of this tool requires installation of gdal as a pre-requisite. It is not compatible with Windows computers (except on WSL2). elevation is installable with pip, and can be invoked with the eio clip command. It also requires the gdal program, available from your system's package manager.
elevation can be installed on most systems with the command:
python3 -m pip install elevation
or alternatively:
python -m pip install elevation
To test your installation, run the following command:
eio selfcheckIf your installation is valid and operable, you will see output like the following:
Your system is ready.
The eio clip command may take a while to complete. This is normal.
You may now identify the geographic bounds of the area of interest and fetch the DEM with the eio command. For example: to clip a GeoTIFF DEM from the SRTM 30m dataset for the city of Rome, Italy (around 41.9N 12.5E) to the filname Rome-30m-DEM.tif use:
eio clip -o Rome-30m-DEM.tif --bounds 12.35 41.8 12.65 42
In the example above, 12.35 is the western longitude bound, 41.8 is the southern latitude bound, 12.65 is the eastern most longitude bound, and 42 is the northernmost latitude bound of the rectangular area of the desired GeoTIFF file.
Note that the --bounds option accepts latitude and longitude coordinates (more precisely in geodetic coordinates in the WGS84 refernce system EPSG:4326 for those who care) given as left bottom right top similarly to the rio command form rasterio.
To clean up stale temporary files and fix the cache in the event of a connection error use:
eio clean
For more information, read the Python elevation package's documentation available here.
The downloaded elevation data is stored in the GeoTIFF file format, which performs efficient, lossless compression. Large areas can be downloaded for later use by OpenAthena offline, all while requiring very little disk space.
The excellent elevation data set is generated by a technique known as Synthetic Aperature Radar, which is becoming exceedingly effective and common as a military and GEOint data gathering tool. One advantage of this technique is that radar wavelengths are long enough to pass through some cloud cover before bouncing off of hard surfaces such as metal, buildings, and terrain.
The dataset for the elevation API backing both opentopography.org and the eio clip was generated by the (Space) Shuttle Radar Topography Mission conducted in the year 2000. In 2015 a high resolution global digital elevation model, derived from the SRTM dataset, covering most of the world was released to the public without restriction. This DEM derived from the SRTM mission has a high resolution of 1-arc second (approx 30 meters) between each datapoint. Studies have shown the vertical accuracy of the datapoints are very good, with error being observed to be within +/- 10 meters, despite the +/- 16 meters figure which was published in the SRTM data specification.
OpenAthena can accept DEM GeoTIFF files of higher resolution or accuracy than the SRTM dataset, if provided by the user. GeoTIFF files must be of similar format to that of the SRTM dataset to avoid undefined program behavior (Currently, OpenAthena is not capable of translating alternative coordinate systems to WGS84).
One challenge of the SRTM dataset (including improved derivations such as NASADEM, of which OpenAthena is compatible) is that it tends to include the height of trees and buildings above bare terrain. LIDAR datasets, where available, measuring bare earth terrain elevation will produce superior results. In Europe, the EU_DTM is available which models bare earth terrain elevation reconstructed using Machine Learning techniques, but it is not currently compatible with OpenAthena. A non-free dataset FABDEM "A 30 m global map of elevation with forests and buildings removed" is available globaly for paid use.