Monday, July 06, 2009

ASTER For Custom Scenery

A few days ago, the ASTER GDEM was released. Basically ASTER GDEM is a new elevation set with even greater coverage than the SRTM. Basically both SRTM and ASTER (I'll drop the GDEM - in fact ASTER prodcues more than just elevation data, but the elevation data is what gets flight simmers excited right now) are space-based automated measurement of the earth's height. But since ASTER is on a satellite (as opposed to an orbiting space shuttle) it can reach latitudes closer to the poles.

So what does this mean for scenery? What does it mean for the global scenery? A few thoughts:
  • ASTER data is not yet very easy to get. You can sign up with the USGS distribution website but you're limited to 100 tiles at a time, with some latency between when you ask and when you get an FTP site. Compare this to SRTM, which can be downloaded automatically in its entirety, or ordered on DVD. ASTER may reach this level of availability, but it's not there yet.

  • ASTER is, well, lumpy. (Nasa says "research grade", but you and I can say "lumpy".) Jonathan de Ferranti describes ASTER and its limitations in quite some detail. Of particular note is that while the file resolution is 30m, the effective resolution of useful data will be less.

    SRTM has its defects, too, but ASTER is very new, so the GIS community hasn't had a chance to produce "cleaned up" ASTER. And clean-up matters; it only takes one really nice big spike in a flat flood plane to make a "bug" in global scenery. I grabbed the ASTER DEMs for the Grand Canyon. Coverage was quite good, despite the steep terrain angle (steep terrain is problematic by design for SRTM) but there were still drop-out areas that were filled with SRTM3 DEMs, and the filled-in area was noticeable.

  • By the numbers, ASTER is not as good as NED; I imagine that other country-specific national elevation datasets are also both more accurate and more precise than ASTER.

  • The licensing terms are, well, unclear. The agreements I've seen imply a limited set of research uses for the data. The copyright terms are not well specified.

So at this point I think ASTER is a great new resource for custom scenery, where an author can grab an ASTER DEM in a reasonable amount of time, check it carefully, and thus have access to high quality data for remote parts of the Earth, particularly areas where locally grown data is not available or not high quality.

In the long term, ASTER is a huge addition to the set of data available because of its wide-scale coverage of remote areas, and because it can fill holes in SRTM. (ASTER and SRTM suffer from different causes for drop-outs, so it is imaginable that there won't be a 1:1 correlation in drop-outs.)

But in the short term, I don't think ASTER is a SRTM replacement for global scenery; void-filled SRTM is a mature product, reasonably free of weirdness (and sometimes useful data). ASTER is very new, and exciting, but not ready for use in global scenery.


krz said...

google earths height data is by far the best thing available...and by available i mean through sketchup. and that basically means having to zoom in all the way and import the meshes one by cover a huge area would take endless but its worth it for scenery builders who want to cover only a smaller area. i was looking for something more automated but i havent found anything yet. i guess this would also be against googles terms of fair use i guess?

Benjamin Supnik said...

It would be a complete violation of Google's terms of use. Google's data is _not_ available for free third party use, that's why we don't consider it as a GIS data source.

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