HAGL data files

František Horínek edited this page Jan 4, 2018 · 7 revisions

For HAGL (Height above ground leve) estimation we are using data from SRTM mission, 3-arc-second, void-filled version. The resolution is ~90m. We are taking 4 data points and then interpolate them to get desired height at gps position.

To use this function you need:

  1. You need to have firmware at least 3329
  2. Download data files from our data repository.
  3. Unzip files in your computer
  4. Create AGL directory in your skydrop
  5. Copy the *.HGT files to your SkyDrop into AGL directory.
Please note that usb interface in SkyDrop is limited to ~250KBps so if you want to copy large amount of data, you need to be patient or disassemble the drop and copy the files directly to the sd card using your pc

The hgt files contains raw array 1201x1201 of int16_t values stored in big endian format. Filename convention is [latitude][longitude].HGT. For example N48E017.HGT file start as latitude N48° and longitude E17°.

30 meter resolution

The files offered at the skybean data repository have a resolution of 90m (1200 data points for each degree and 1 data point overlap). However, you can also use higher resolution files with a resolution of 3601x3601 data points which gives a 30m resolution. These files have a size of 25MB compared to a size of 2.8 MB for standard resolution.

You can get these higher resolution files for free from https://earthexplorer.usgs.gov (Guide). To download them, you have to register first, which is also free of charge and open to anyone. Unfortunately earthexplorer does not offer HGT files natively, so you have to download them in DT2 file format and convert them into HGT. You can use gdal_translate or some other software for the conversion. Here is how you would use gdal_translate:

gdal_translate -of SRTMHGT N48E017.DT2 N48E017.HGT

After the conversion put these higher resolution files into the same data directory as the standard files. You can also mix the two resolutions. Skydrop will use the right resolution for each individual file (by looking at the file size).

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