On Mon, Nov 24, 2008 at 12:25 PM, sylvain letuffe <[EMAIL PROTECTED]> wrote:
> Looks like someone of the opencyclemap (is that you andy ?) have included some > sort of interpolation or any other means to replace those old "white zones" > by "at least something" Sylvain wins the prize for noticing! Yep, it's known in the trade as "void-filling", if you want to read back through some of the mailing list archives on the subject. It's not specifically snow-covered areas, it's areas with no data from the SRTM mission - often found in high mountains (since the shuttle can't see especially steep north slopes, for example) but generally in areas with no radar return (including marshland). > See here what I'm talking about : > http://www.opencyclemap.org/?zoom=16&lat=45.3086&lon=5.85346&layers=B000 > > The POI "dent de crolles" is the highest point and you see the contours > being "very" incorect The void-filling is currently a pattern-based interpolation, so the areas of voids are even more fictional than the rest of them! > As I am on the process of constructing an hiking map, I am searching for > solutions... which might not even exist. > > I first suppose that there are not "free data" for contours at all (exept free > for personnal use SRTM data) am I right ? > > Has anyone tried other sources ? what about SRTM v3 ? is there a wiki page > that gather those research ? There's no SRTM v3. v1 - contours including noise from oceans and other water bodies. Nobody uses it. v2 - contours 'clipped' to water bodies SRTM1 - high res, USA only SRTM3 - medium res, world wide So I use SRTM3 v2, and it's the same for the piste map and so on. Both SRTM1 and SRTM3 have the same voids, since they come from the same ultimate data source. As for void filling, that's a huge project with lots of tradeoffs. It basically comes down to synthesising data, usually with a sprinkling of other data sources. So the algorithm used to void-fill the cycle map could have added constraints to it like making that mountain the right spot height and guessing the rest of the gaps. The most sophisticated models also do things like ensuring rivers run downwards (hydrological models). But it's a very hard thing to do on a global scale without using many man hours per void. > Has anyone worked on using GPS tracks elevation information to improve SRTM ? GPS elevation tracks aren't better than SRTM in general, due to their poor vertical resolution. Whilst they could be used for void-filling, it's generally only the position of the summits that could help - most voids are in inaccessible areas and the chances of getting sufficient coverage are remote. Better solutions involve other data source, e.g. other maps with compatible licenses where the contours could be digitised. I've been asked a few times as to how to "crowd source" void filling, and the best answers I have are summit heights and river courses. Beyond those two constraints for the model, better sources are required that generally are centrally gathered. Cheers, Andy _______________________________________________ dev mailing list [email protected] http://lists.openstreetmap.org/listinfo/dev
