Global Mapper v24.0

How can I compare raster pixel colour difference between multiple plant research plots

I have taken drone imagery of a trial plot (raster) and need to calculate the difference between each plot using RGB or Multispectral the orthomosaic with polygons around each plot.

Answers

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User

    To compare the color of Lidar you would need to do something like:

    • Create a color (RGB or RGBI) grid from each colorized point cloud using the Analysis -> Create Elevation Grid menu command. You can choose to grid color rather than elevation if you have Global Mapper Pro (or the Lidar module for pre-v23)
    • Use Analysis -> Raster Calculator to create a new raster layer where you subtract the color bands of each layer from each other, if that is how you want to compare them
  • Hi Mike, thanks for your help. I've tried to upload more information for you but I keep getting error 403.

    This image has loaded and it shows an example of the sort of trial that I am trying to evaluate. There are multiple plots and the trial is flown with a drone using a multispectral and RGB camera. At this point in time I run the images through Pix4D Mapper and import the plot areas (shape file) created through Global Mapper. Each plot is then displayed with the percentage of pixels in 5 colour bands.

    Using this output I can statistically compare all of the plot to see if I have a response to my treatments.

    What I hope to be able to do is to bring the processed image into Global Mapper, put polygons around each plot and use the raster calculator to give me the values. This would often be done just using the RGB imagery from the drone. In theory greener areas would have more green pixels than poorer growing areas.

    I feel the Global Mapper should be capable of doing this analysis.

    Please let me know if you have any knowledge of this process.


    Cheers,

    Marcus

    (Australia)

  • bmg_mike
    bmg_mike Global Mapper Guru Moderator, Trusted User

    Marcus,

    You should be able to attach a .zip file here, or you can email the data to me directly at mikec@bluemarblegeo.com .

    If I understand you correct, your input is multi-spectral imagery (acquired via drone), correct? For some reason I thought you had colorized Lidar point clouds.

    You should be able to come up with a formula to apply to the image to create a new layer that assigns each pixel value in the input drone image to a bin based on the band values (are you sure a NDVI calculation wouldn't be better, assuming you have the NIR band from the multi-spectral sensor?). You would use the Analysis -> Raster Formula command to do this. Before you open the dialog, select all of the polygon areas, then on the Tiling tab of the Apply Formula dialog select to tiled based on selected areas so you get a separate image for each plot polygons.

    Once you have that, you can open the Metadata dialog for each layer and go to the Histogram tab to see a breakdown of the values. You can also select the plot areas with the Digitizer Tool, then use the right-click Analysis -> Calculate Color Statistics option to add some statistics about the colors to each polygon directly.

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