Global Mapper 15
Page 1 of 2 12 LastLast
Results 1 to 10 of 13
  1. #1
    Global Mapper User
    Join Date
    Jul 2009
    Posts
    8

    Post Contour Lines difference

    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

  2. #2
    Administrator
    Join Date
    Jun 2007
    Posts
    15,867

    Default

    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

  3. #3
    Global Mapper User
    Join Date
    Jul 2009
    Posts
    8

    Default

    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.

  4. #4
    Administrator
    Join Date
    Jun 2007
    Posts
    15,867

    Default

    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

  5. #5
    Global Mapper User
    Join Date
    Apr 2009
    Posts
    279

    Default

    I was wondering that too...

  6. #6
    Global Mapper User
    Join Date
    Apr 2009
    Posts
    2

    Default

    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

  7. #7
    Global Mapper User
    Join Date
    Jul 2009
    Posts
    8

    Default

    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 )
    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)

  8. #8
    Global Mapper User
    Join Date
    Jul 2009
    Posts
    8

    Default

    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
    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??

  9. #9
    Administrator
    Join Date
    Jun 2007
    Posts
    15,867

    Default

    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

  10. #10
    Global Mapper User
    Join Date
    Jul 2009
    Posts
    8

    Default

    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.

 

 
Page 1 of 2 12 LastLast

Tags for this Thread

Bookmarks

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  
Powered by vBulletin® Version 4.2.1
Copyright © 2014 vBulletin Solutions, Inc. All rights reserved.
Content Relevant URLs by vBSEO 3.6.1
Copyright 2008-2012 Globalmapperforum.com
All times are GMT. The time now is 05:13 PM.
vBulletin 4.0 skin by CompleteVB