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Global Mapper versus USGS

george_martin
george_martin Global Mapper UserTrusted User
edited November 2012 in Elevation Data
Hi,
I have very strange problems. First, when I download elevation data from USGS (http://demex.cr.usgs.gov/DEMEX/)
it doesn't matter if I chose ASTER 30 or STRM 90 or GTOPO30, The resolution and files size for my selected areas are always the same. But when I use global mapper built-in features for downloading elevation data, the SRTM 3-ARCSECOND data differ from ASTER 1.5 ARCSECOND data in terms of resolution and size.
So, my first question is :Am I doing something wrong on downloading elevation data from USGS?. Why's that the resolutions for all three sources are the same?
Another problem is that the resolution of elevation data, downloaded by GM is much lower from that is downloaded from the USGS.
It's what I've got from USGS
USGS.jpg
and it's from GM
gm.jpg
How's that possible? same source but different resolution?
The reason I'd choose the USGS over GB for downloading elevation data is that the USGS can cover more area, and it take less time to download data and transfer it to geotiff.
I would be appreciated if you could spare some light for me.
USGS.jpg 166.3K
gm.jpg 111.8K

Comments

  • global_mapper
    global_mapper Administrator
    edited November 2011
    The ASTER GDEM data offered in GM is only 1.5 arc second spacing while the native ASTER GDEM 30 is 1 arc second (about 30m resolution), so you might be able to get slightly higher resolution results from the USGS site for that source. Likewise you can get 1 arc second spacing SRTM data for the US from the USGS, but only 3 arc second (90 meter spacing) outside the US, and it is the later that GM supplies. I would expect some differences between ASTER and SRTM data though. What region of the world are you getting data for?

    Note that if you are in the US I would use the 1/3rd arc second (about 10m) NED data rather than either ASTER GDEM or SRTM.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • george_martin
    george_martin Global Mapper User Trusted User
    edited November 2011
    I'm living in Iran. Thanks Mike. I got the answer for my second question, but the answer for the first one is still remained. Why the resolutions for Aster and Strm and GTOPO30 are the same? I really need GTOPO30 elevation data for large area, and on occasions, it would be necessary that you use SRTM data layer under aster(because of some hole) and for the reason I've mentioned earlier, I cannot do it, so in your view is there anyway to get different resolutions with different sources? .
    Thanks again
  • global_mapper
    global_mapper Administrator
    edited November 2011
    The GTOPO30 data should only be 30-arc-second spacing (i.e. about 1km), so if you are seeing the same for that as for SRTM or ASTER GDEM something isn't coming down how you expect from the USGS site. Are you sure you are actually getting GTOPO30 data?

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • ANGLICO
    ANGLICO Global Mapper User Trusted User
    edited November 2011
    Please educate an old guy. Thirty years ago, if I contracted with a topographic mapping firm to panel, survey and then fly a remote area, and then provide a topographic map, the contract would at a minimum reference USGS mapping standards, and specify that 90% (?) of random points would be within +/- 1/2 contour interval.

    For USGS DEMs, what is the modern standard? Contour interval might not be the basis for a standard, or it could be, based on the 7 1/2' quad.
  • george_martin
    george_martin Global Mapper User Trusted User
    edited November 2011
    Yes, I'm sure. The resolutions and files size for both asters, and GTOPO30 is the same. I don't know what's going wrong, but if you let me, I can upload a video clip of what I'm doing when downloading elevation data, maybe it could help.
  • george_martin
    george_martin Global Mapper User Trusted User
    edited November 2011
    Sorry mark, I would be happy if you could look at this short video. It's the way I'm downloading elevation data from USGS.
    http://www.mediafire.com/?jgxeavzcfx6ykpf
    Maybe you can make something out.
    THANK YOU AGAIN.
  • global_mapper
    global_mapper Administrator
    edited November 2011
    George,

    In your video you are downloading the ASTER product each time. The checkboxes on the right just control what is shown on the preview window, but when you actually select to download there is a drop-box for the product. This is ASTER in both downloads. I think that is what you need to change to actually download from the different products.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • global_mapper
    global_mapper Administrator
    edited November 2011
    You can find information about the USGS DEM standards at http://nationalmap.gov/standards/demstds.html. Standards for other products (like SRTM, ASTER GDEM, etc.) will vary significantly, with premium services like Intermap DEMs (5m horizontal spacing) or Lidar products offering much more resolution and quality than free offerings (at much greater price of course).

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • george_martin
    george_martin Global Mapper User Trusted User
    edited November 2011
    I got it. Thanks again for inspiration and guidance.
  • ANGLICO
    ANGLICO Global Mapper User Trusted User
    edited November 2011
    2.3.3 Level 3
    Level 3 DEM's are derived from DLG data by using selected elements from both hypsography (contours and spot elevations) and hydrography (lakes, shorelines, and drainage). If necessary, ridge lines and hypsographic effects of major transportation features are also included in the derivation. An RMSE of one-third of the contour interval is the maximum permitted, with no errors greater than two- thirds contour interval. The DEM record C contains the accuracy statistics acquired during quality control.

    This 7 1/2' DEM standard is even better than the one-half contour interval standard long ago. But not all DEMs are created equal. Today it's more complicated.

    Thanks for the link.
  • wsoyer
    wsoyer Global Mapper User
    edited November 2012
    Hi Mike,

    I have 2 questions related to this:

    - do you have plans to update to (or add) the 1 arc sec ASTER V2 DEM for direct download, that's typically downloaded on pages like the global data explorer or earth explorer? That would help us in our geophysical work when the ASTER is credible.

    - For SRTM, there's a subtle shift between otherwise identical grids obtained via Global Mapper and the above mentioned online sources. With other words: when downloading internally from GM, the displayed grid is slightly shifted (0.5-1 grid cells?) with respect to the GM displays of SRTM tif files downloaded from e.g. USGS sources. Suspicion is that the GM downloaded one is off and the other display more correct, but could not confirm this. Can you confirm and indicate which one is correct?

    Many thanks,

    Wolf
  • global_mapper
    global_mapper Administrator
    edited November 2012
    Wolf,

    Unfortunately distributing the ASTER v2 DEM full resolution data is not something that is allowed. However in our investigations we have found that you can request the entire data set for your own uses. We just can't make the full resolution data available ourselves. What is in there now is from a NASA/JPL server.

    The shift you are seeing could be simply from the reprojection done to host it on the server if it is a sub-pixel shift. The data is hosted in Web Mercator while the native data is in lat/lon/WGS84, so the new samples won't coincide perfectly with the original SRTM samples, so there will be slight differences. They should be less than the inherent error of the data set though.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
  • wsoyer
    wsoyer Global Mapper User
    edited November 2012
    Mike,

    many thanks.
  • wsoyer
    wsoyer Global Mapper User
    edited November 2012
    Mike,

    (hit the return too soon before)

    Thanks for your answers.

    Clear re. ASTER availability, appreciate that.

    On the positioning - I had a closer look, see attached PDF. Shift is about half a sample in this case. Also downloaded two more grids, this time from the Global Data Explorer (http://gdex.cr.usgs.gov/gdex/), one in Lat/Long the other in UTM. In the end, ALL 4 grids show these type of shifts between them, none of them plot precisely on top of another. Somewhat disturbing... maybe some of this is a display question only? Possibly download the XYZ data points of the SRTM set directly from the server - this is an option - and then grid in GM may be the most precise?

    Cheers,

    Wolf
  • global_mapper
    global_mapper Administrator
    edited November 2012
    Wolf,

    If the data is in different projections some type of sub-pixel shift is pretty much always going to be there as the cell sample locations don't coincide, so you will get shifts due to how the resampling differs. These should always be sub-pixel shifts and well under the actual accuracy of the data anyway, so while they may be visually disturbing they are quantitatively significant compared to the error that is already in the data.

    Thanks,

    Mike
    Global Mapper Guru
    gmsupport@bluemarblegeo.com
    http://www.globalmapper.com
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