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Contour Lines difference

andresfpcandresfpc Global Mapper UserPosts: 8 ✭✭
edited September 2009 in Elevation Data
Hi there!! :(:(:(
Can anybody help me?
Why is that I have a difference between the contour lines generated from the SRTM file downloaded from Global Mapper (Download Online Data icon) from the ones created from the SRTM file downloaded from Google Earth. Ones appear to be moved from the others, wich one is the most accurate?? I don't know which one is best. And Why does it happen?
The projection is ok and datum also ok.

Regards
Andrés

Comments

  • global_mapperglobal_mapper Administrator Posts: 17,238BMG Staff
    edited July 2009
    Andres,

    If the SRTM data is natively in a different projection or aligned slightly differently the contours generated could be shifted slightly between them. The shift should be within the expected error of the source data though.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • andresfpcandresfpc Global Mapper User Posts: 8 ✭✭
    edited July 2009
    They have the same projection.
    Here is what I do both ways.
    If the SRTM comes from Global Mapper.
    I click on "Download Data Online" and select SRTM... on an open file with its own prijection. Then the SRTM file is downloaded and set into position. After that I generate contour lines.
    If the SRTM file comes from GEarth. I open it on my Global Mapper file with the default projection. Then I open "Control Center", select SRTM already opened and modify the selected projection into the one I'm using with my file. This makes the SRTM "move" a little. After that I generate the contour lines.

    Both contour lines differ in their position.
    What am I doing worng?
    Thanks again.
  • global_mapperglobal_mapper Administrator Posts: 17,238BMG Staff
    edited July 2009
    You don't want to modify the projection for a layer from the Control Center, you typically want to modify it on the Projection tab of the Configuration dialog to reproject. The Control Center projection change is for correcting an incorrect projection from load time.

    I'm just curious, how did you get SRTM data from Google Earth?

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • redbeardredbeard Global Mapper User Posts: 279Trusted User ✭✭
    edited July 2009
    I was wondering that too...
  • weoliweoli Global Mapper User Posts: 2 ✭✭
    edited July 2009
    Hi Andrés,

    where did you download the SRTM data from? ARC ASCII or GeoTiffs from CGIAR?

    As dpluigi described in this thread on this forum and this thread on condor soaring forum height data from CGIAR (ARC ASCII or GeoTiff) have 1/2 pixel shift compared to those downloaded as hgt files from NASA. So also contour line created from this CGIAR data have a shift. I also remarked that shift and wrote an email to Dr. Jarvis from CGIAR. But I didn't get an answer until now.

    So may be the contour lines you can see in GEarth don't have this 1/2 pixel shift compared to those created from CGIAR SRTM data.

    greetings
  • andresfpcandresfpc Global Mapper User Posts: 8 ✭✭
    edited July 2009
    Hi! I give you the web links to download a kmz file. Open it and click on "elevation". You'll see a grid where you can select the SRTM data area, zoom in and click the selected area to download the file.
    Hole-filled SRTM 90m Digital Elevation Data (V4.1) :Geodata :King's College London
    (they have an better explanation :D)
    CGIAR-CSI SRTM 90m DEM Digital Elevation Database
    (this is another site. I didn't have luck in downloading from here, the server is always jammed)
  • andresfpcandresfpc Global Mapper User Posts: 8 ✭✭
    edited July 2009
    Hi Mike!
    So you mean that the differences between each contour lines, the ones generated from SRTM global mapper and the others from SRTM google earth are supposed to happen? I mean this is what to expect or consider if I'm working with them?
    Is there any solution or I've to learn to live with it :D:D:D
    Which one is more accurate? I guess that the ones that come from Global Mapper beacuse I often compare them with a map zone that I use as a reference.
    Thanks
    Andrés
    P.S. Is there any tutorial refered to this subject??
  • global_mapperglobal_mapper Administrator Posts: 17,238BMG Staff
    edited July 2009
    Andrés,

    There can always be differences within the stated error of the data. I'm not sure which data source is more accurate as the one you say are "from Google Earth" are not actually from Google but just provided by a 3rd-party via a Google Earth interface. So long as your contours are within 90m or so horizontally of each other then they could basically be considered equivalent as the source data doesn't have any more horizontal resolution than that anyway.

    Let me know if I can be of further assistance.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • andresfpcandresfpc Global Mapper User Posts: 8 ✭✭
    edited July 2009
    Ok.
    Another question.
    There are different types of SRTM files. The 90 m which is free and others with better resolution which aren't free. If I use a better resolution SRTM means that my contour lines will be better??? due to the low interpolation of the unknown values?
    Where can I get those better quality SRTM's??
    Thanks again.
  • global_mapperglobal_mapper Administrator Posts: 17,238BMG Staff
    edited July 2009
    If you can get higher resolution data your results should be more accurate as the source data is more accurate. There is a 30m resolution SRTM data set in existence, but is really only available to a select few people except for the data covering the US. You can get the higher-resolution US data from <i>The National Map</i> Seamless Server.

    There is also a new ASTER GDEM data set with better coverage and resolution than SRTM that you should be able to download and use. Just do a Google search on it to find it.

    Thanks,

    Mike
    Global Mapper Support
    support@globalmapper.com
  • TeeGeeTeeGee Global Mapper User Posts: 57Trusted User ✭✭
    edited August 2009
    Hi Andrés,

    If you are looking for better accuracy than the 90 m resolution SRTM, I suggest a visit to this website : http://www.ersdac.or.jp/GDEM/E/index.html where you will find a 30 m resolution DEM for the world. There are some restrictions for the use of this DEM, but it's free.

    But anyway, you can't really trust contour generated lines based on these models, except if you need a map with a "great" scale (1/200 000). At "smaller" scale (1/50 000), you will find some significant differences with the real topography (Just try to superpose contour generated lines with à topographic map scanned and correctly georeferenced, especially in a mountainous region). The only DEM you can trust are those obtained from topographic measurements.

    Cheers,

    TeeGee
  • EDGAREDGAR Global Mapper User Posts: 77Trusted User ✭✭
    edited September 2009
    andresfpc wrote: »
    Hi! I give you the web links to download a kmz file. Open it and click on "elevation". You'll see a grid where you can select the SRTM data area, zoom in and click the selected area to download the file.
    Hole-filled SRTM 90m Digital Elevation Data (V4.1) :Geodata :King's College London
    (they have an better explanation :D)
    CGIAR-CSI SRTM 90m DEM Digital Elevation Database
    (this is another site. I didn't have luck in downloading from here, the server is always jammed)

    There is also another way to get Google Earth SRTM - KMZ files

    One can run the link: http://srtm.csi.cgiar.org/

    Below the page there is as follows:

    Google Link: http://www.ambiotek.com/topoview Download SRTM KML file click here Download Google-Earth click here
    At the moment I've no luck to enter, but maybe at some other time..

    Regards
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