Terrain Data

Survex 1.2.18 adds support for displaying terrain data.

You'll hopefully find this useful, but it is still a work in progress - this page documents the remaining issues, and also where you can get terrain data from.

Obtaining Terrain Data

The following sources can be download free of charge, though the data may have restrictions on use and redistribution, so check the licence in each case to see what it allows you to do.

Sources with Near-Global Coverage

• ​Shuttle Radar Topography Mission (SRTM) - this is based on data collected by a NASA shuttle mission in 2000. It was originally released with a resolution of 1 arc-second (about 30m) for the USA and 3 arc-seconds (about 90m) for the rest of the world (for latitudes between 60 degrees North and 54 degrees South). Starting in 2014, 1 arc-second data is being released for more of the world and as of June 2015 this appears to be available for everywhere the 3 arc-second data is apart from an area covering Greece, Turkey, the Middle East, and some surrounding countries. Due to the way the radar mapping was done, there are some "voids" in this data in areas where the steep terrain shaded the ground from the radar beams. Even the "void filled" versions still seem to have some voids, and interesting caves are often in areas of steep terrain.

You can download SRTM data from various places. I used the ​USGS Earth Explorer which requires registration (which doesn't cost anything) - click on the "Data Sets" tab and look at the "Digital Elevation" section of the tree on the left. One of the icons in each entry allows you to see the coverage of each dataset (where it actually has data for).

• ​Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) - this is based on data collected since 2000 by a Japanese sensor on the Terra satellite. The data resolution is about 30m, and covers latitudes from 83 degrees North to 83 degrees South (so much better coverage of polar areas than SRTM), but the data seems to be lower quality than SRTM with various artifacts and other problems.

You can also download this from the ​USGS Earth Explorer.

• ​Viewfinder Panoramas has data compiled from various different sources to ​3 arc-second data globally and ​1 arc-second data for the Alps, Tatra, central Spain, and an area covering Scandanavia and part of adjacent countries. At least for the area around where CUCC visit in Austria this seems to be distinctly better than either SRTM or ASTER.

Sources Specific to a Country or Region

• For the Styria region of Austria, 10m resolution data licensed as CC-BY-3.0-AT can be downloaded from: ​http://data.steiermark.at/cms/beitrag/12014326/97428847/ (Currently you can't directly convert this for use by aven though).

Formats

Aven 1.2.18 and 1.2.19 expect the terrain data to be in a ZIP file (which is how you'll usually download it, so just don't try to unzip it yourself).

Inside the ZIP file, the following are currently supported:

• An STRM .hgt file (which is the format Viewfinder Panoramas provides).
• An ESRI .bil file and associated metadata files (which is one of the smaller options for downloading SRTM and ASTER data from Earth Explorer).

Aven 1.2.20 adds support for the same formats, but with the files directly on disk. You will probably find this useful on Windows, where the wxWidgets zip file reading code always seemd to fail to read data with "generic read error".

The terrain data currently needs to be in WGS84 lat/long.

If you have data in a different format, then ​GDAL may be useful for converting it.

Eventually we'll probably get aven to use GDAL to allow reading terrain data in more formats. If there are particular formats it would be useful to be able to read, please open a ticket.

Further Reading

• ​Comparison of free high-resolution digital elevation data sets (ASTER GDEM2, SRTM v2.1/v4.1) and validation against accurate heights from the Australian National Gravity Database - This 2014 study (appears to pre-date the updated 1-arc second SRTM data) compares the accuracy of several publicly SRTM/ASTER-derived datasets using Australian reference stations.

Known Issues

• In 1.2.18, aven quietly does nothing if pointed at a ZIP file which etiher it can't read, or which it doesn't find anything it recognises as terrain data in - this will be fixed in 1.2.19.
• #55 Fix terrain rendering to not show terrain through itself
• #56 Sort out handling of geoid
• #57 Smooth shading of terrain
• #53 Texture mapping onto terrain data
• #64 Terrain loading fails on Windows