NDVI Custom Classification/Reclassification

I have a 4-Band Satellite Image (RGBI:B1, B2, B3, B4) and when I want to extract the NDVI I used the predefined formula in GM v17.1 which is NDVI (RGBI)..
Thru this method, I really had the NDVI data file. My concern is, I want to reclassify some areas that are in between -1 to 0 or from 0 to 1 (in decimal units) or in other words, to create an area feature by ranging the reflectance difference or "height" of the NDVI layer.

I assumed that this "Height" value found in the lower left of the GM Window is the reflectance difference. Please correct me if my assumption is wrong. Since the value is changing as I navigate the entire window.

Please see attached images for your reference.

Thank you,
Sidoy

Best Answers

  • KatrinaKatrina Posts: 25
    Answer ✓
    The 'height' value in the lower left indicates the pixel value where the cursor is located. When you are looking at an NDVI result, this 'height' value is the NDVI value. NDVI is a vegetation index ranging from -1 to 1, with numbers above zero indicating increasing greenness/ vegetation. 
    Here is more information about NDVI from NASA: http://earthobservatory.nasa.gov/Features/MeasuringVegetation/measuring_vegetation_2.php ;

    To turn the NDVI into an area feature, you can open the overlay control center (Alt +C). Right click on the NDVI layer and choose the option 'Create Area Features from Equal Values in Selected Layers'. This tool allows you to vectorize the data based on the pixel values. 

    In this case the 'height' or 'Z' value isn't really a physical elevation, but an index value. You could use a 'Maximum Match Distance' of 1 to create separate areas of everything above and below zero. 
  • KatrinaKatrina Posts: 25
    Answer ✓
    When you calculate NDVI using a predefined formula, the shader color ramp for that layer will automatically be set to use our predefined NDVI shader, which is a blue to white to green color ramp. 

    The shader used for elevation layers is controlled from the shader toolbar drop-down menu (this is next to the hillshade and 3D view buttons).  At the bottom of the shader drop-down menu  you can choose to 'Add a custom shader...' and then define your own color values/ bins, and choose whether or not to blend the colors, etc. Here is more information on shader options and creating custom shaders: http://www.bluemarblegeo.com/knowledgebase/global-mapper/Content/Shader_Options.htm

    To apply your custom defined shader to the NDVI layer, open the overlay control center (Alt+C) and go to Options for the NDVI layer. 
    Near the bottom of the display tab you can set the shader to your custom shader, or choose 'Default Shared Shader' to give control of the layer display back to the main toolbar drop-down. 



Answers

  • henniehennie Global Mapper User Posts: 55Trusted User
  • SidoySidoy Posts: 16
    Thank you, Katrina and Hennie for that information.

    Just a follow up question:

    Is there an option in GM for Band Rendering? For example, for NDVI data which is a Single-band image, do I have an option to select what Render type like "Singleband Pseudocolor"; therefore, I can add more bins in my Legend?

    Again, Thank you.
    Sidoy
  • SidoySidoy Posts: 16
    Katrina said:
    When you calculate NDVI using a predefined formula, the shader color ramp for that layer will automatically be set to use our predefined NDVI shader, which is a blue to white to green color ramp. 

    The shader used for elevation layers is controlled from the shader toolbar drop-down menu (this is next to the hillshade and 3D view buttons).  At the bottom of the shader drop-down menu  you can choose to 'Add a custom shader...' and then define your own color values/ bins, and choose whether or not to blend the colors, etc. Here is more information on shader options and creating custom shaders: http://www.bluemarblegeo.com/knowledgebase/global-mapper/Content/Shader_Options.htm

    To apply your custom defined shader to the NDVI layer, open the overlay control center (Alt+C) and go to Options for the NDVI layer. 
    Near the bottom of the display tab you can set the shader to your custom shader, or choose 'Default Shared Shader' to give control of the layer display back to the main toolbar drop-down. 



    Thank you very much for the support.

    Sidoy
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