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Thread: So what are the current sources for DEM now?

  1. #91
    Join Date
    Apr 2003
    Location
    Silicon Valley, CA, USA.
    Posts
    3,385

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    I still don't believe the data has 10m horizontal resolution (1/3 arc second).

    There were 3 SRTM missions. #1 used 3 arc second and 1 arc second mapping -- that's 90m and 30m DEM data. US data was 30m, the rest of the world was 90m. Mission number two used 1 arc second (30m) and was used to fill holes that the original data had. Mission 3 was also 1 arc second (30m). It's purpose was to collect data so the original 90m data could be replaced. All of the SRTM data has accuracy problems with very steeply sloped terrain (cliffs). Everywhere else vertical accuracy is around 10m.

    NONE of the SRTM flights used 1/3 arc second (10m) resolution.

    I am almost positive that none of the files Dale obtained are 10m horizontal resolution. The only place you can get that is via the current downloads from the USGS which is offering a replacement data set for the older SRTM data. Most of hte US is now 1/3 arc second (10m) resolution. A portion is 1/9 arc second (3m).

    IMO given the MSTS/OR grid size of 8m there may be little reason to download 1/9 arc second (3m) resolution.

    Everything i've written can be checked by googling strings like "what resolution was mission 3 srtm?" and "SRTM accuracy".
    Dave Nelson

    Seldom visiting, posting less often that that.

  2. #92
    Join Date
    Nov 1999
    Location
    Elkins Park, PA, United States.
    Posts
    383

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    I came across this site while searching for DEM data for a few tiles that I realized that I need to add to my current project near the Ohio/Indiana border. The site is www.webgis.com and they seem to have DEM data for the entire US that will work in DEMEX at this link. http://www.webgis.com/terr_us1deg.html Has anyone else tried using this site for DEM data to create routes? Hopefully this will be helpful to everyone.

  3. #93
    Join Date
    Aug 2002
    Location
    Poteau, OK, USA.
    Posts
    4,662

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    Quote Originally Posted by gxm149 View Post
    I came across this site while searching for DEM data for a few tiles that I realized that I need to add to my current project near the Ohio/Indiana border. The site is www.webgis.com and they seem to have DEM data for the entire US that will work in DEMEX at this link. http://www.webgis.com/terr_us1deg.html Has anyone else tried using this site for DEM data to create routes? Hopefully this will be helpful to everyone.
    As I read it, be advised that this link you shared: http://www.webgis.com/terr_us1deg.html

    ...takes you to 90 meter DEM. That means it was sampled at 90 meter intervals. (Not very accurate.)

    However, the same website also offers 7.5 min DEM that is 30 meter DEM. (NAD27 data) That is preferred to the 90 meter. Here is a direct link to that service:

    http://www.webgis.com/terr_us75m.html

    I downloaded one DEM from the above (Colorado's "Ouray" county), extracted the zip, and then opened DEMEX. Sure enough, DEMEX read it and displayed it within its terrain screen A-okay. I took one of my Colorado routes and used this sample DEM to create terrain, which DEMEX did successfully. Upon going into my route to take a look, the terrain was indeed successfully created, but as is typical when using SDTS type DEM to merge with NED Seamless, there were slight mismatches to the adjoining 10 meter NED Seamless terrain I had used on this route.

    Still, this DEM IS usable and it WILL work. IF this 30m SDTS-type DEM is used to add DEM to a route that originally used NED Seamless, there will some smoothing needed at the seams if it is a "foreground" merge.

    But it IS certainly usable and this is a great find.

    Thank you gxm149.

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