[Geotiff] using DEM data encoded in GEOTIFF format

[Pascal Peuch]

hello,

>>>>>And is this height stored relative to some value ( like any value seen
in  ModelTiepointTag  .. )

If you don't only need to display the data, which is not really the aim of DEM data, you do need to take into account that data are "elevations" (which you call heights). Maybe you have to make calculations from your DEM : terrain analysis, line of sight, hypsometric representations, 3D display, ....
Elevations are relative to a vertical reference, usually mean sea level. Your geokeys unfortunately don't show any information on it.

If your application is not very demanding (accuracy, safety, ...) you can just don't mind or assume that the vertical reference is EGM96 (guessed from WGS84 used for geographic reference system).
Anyway if your application is critical, you have to check it from the producer of the data.

You also have to check how the elevation is encoded in the 16 bits sample. Are elevations meters of feet ? How are encoded the negative values (keep in mind that some regions of the world are below the sea level). Is there any "no data" convention?

Mind too that DEM are usually "PixelIsPoint". What is your GTRasterTypeGeoKey ?

Hope that helps

Pascal
France



  ----- Original Message ----- 
  From: Prajjval 
  To: geotiff at lists.maptools.org 
  Sent: Wednesday, September 06, 2006 9:26 PM
  Subject: [Geotiff] Re: Geotiff Digest, Vol 26, Issue 1


  Hello,

  I think that there is no such difference in reading the geotiff, either it is a DEM or anything else.
  So read the geotiff image data and use it as is. Means it does not matter that the data is DEM or pixel value. The DEM data which is in geotiff format means that the values stored in it are height. 

  Also the value stored in geotiff is always used as it is and it is not relative to anyother value.

  The ModelTiepointTag stores the top left location of your geotiff image. And ModelPixelScaleTag stores the pixel size of your geotiff image. 

  Vineet Bhargava
  India

  Firstly I am  new to the geotiff world . 


  I am trying to  read DEM data which is supplied in geotiff format. Its
  geotiff image with 181 rows x 181 cols
  I  wrote some code to read the basic tags. And they are as follows 

  SAMPLESPERPIXEL  1
  TIFFTAG_BITSPERSAMPLE  16
  TIFFTAG_PHOTOMETRIC  1     (1       /* min value is black */ )
  TIFFTAG_PLANARCONFIG  1   ( single plane image)

  Now my doubt how do I read the height data stored in this Geotiff Image 
  ?
  And is this height stored relative to some value ( like any value seen
  in  ModelTiepointTag  .. )

  Any API's in libgeotiff  / pointers on handling Digital Elevation data
  in geotiff will be appreciated . 



  FYI ..
  This is the Geotiff info for the input data

  Geotiff_Information:
     Version: 1
     Key_Revision: 1.0
     Tagged_Information:
        ModelTiepointTag (2,3):
           0                0                0 
           6.99861111       54.0013889       0
        ModelPixelScaleTag (1,3):
           0.00277778       0.00277778       0
        End_Of_Tags.
     Keyed_Information:
        GTModelTypeGeoKey (Short,1): ModelTypeGeographic 
        GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
        GeographicTypeGeoKey (Short,1): GCS_WGS_84
        GeogCitationGeoKey (Ascii,6): "wgs84"
        GeogAngularUnitsGeoKey (Short,1): Angular_Degree 
        End_Of_Keys.
     End_Of_Geotiff.


  Thanks in advance,
  --Chandrakant



    Today's Topics:

       1. Processing Elevation data in Geotiff
          (Chandrakant Mohire (RBIN/ECM2))


    ---------------------------------------------------------------------- 

    Message: 1
    Date: Wed, 6 Sep 2006 14:58:07 +0530
    From: "Chandrakant Mohire (RBIN/ECM2)" <Chandrakant.M at in.bosch.com>
    Subject: [Geotiff] Processing Elevation data in Geotiff 
    To: <geotiff at lists.maptools.org>
    Message-ID:
            <6222DD7606D1314C85F63D9CF40207E00304C87C at kormail02.in.bosch.com >
    Content-Type: text/plain; charset="us-ascii"

    Hello ,

    Firstly I am  new to the geotiff world .


    I am trying to  read DEM data which is supplied in geotiff format. Its
    geotiff image with 181 rows x 181 cols 
    I  wrote some code to read the basic tags. And they are as follows

    SAMPLESPERPIXEL  1
    TIFFTAG_BITSPERSAMPLE  16
    TIFFTAG_PHOTOMETRIC  1     (1       /* min value is black */ )
    TIFFTAG_PLANARCONFIG  1   ( single plane image) 

    Now my doubt how do I read the height data stored in this Geotiff Image
    ?
    And is this height stored relative to some value ( like any value seen
    in  ModelTiepointTag  .. )

    Any API's in libgeotiff  / pointers on handling Digital Elevation data 
    in geotiff will be appreciated .



    FYI ..
    This is the Geotiff info for the input data

    Geotiff_Information:
       Version: 1
       Key_Revision: 1.0
       Tagged_Information:
          ModelTiepointTag (2,3): 
             0                0                0
             6.99861111       54.0013889       0
          ModelPixelScaleTag (1,3):
             0.00277778       0.00277778       0
          End_Of_Tags.
       Keyed_Information: 
          GTModelTypeGeoKey (Short,1): ModelTypeGeographic
          GTRasterTypeGeoKey (Short,1): RasterPixelIsArea
          GeographicTypeGeoKey (Short,1): GCS_WGS_84
          GeogCitationGeoKey (Ascii,6): "wgs84" 
          GeogAngularUnitsGeoKey (Short,1): Angular_Degree
          End_Of_Keys.
       End_Of_Geotiff.


    Thanks in advance,
    --Chandrakant





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