Paper: Use of digital webcam images to track spring green-up in a deciduous broadleaf forest


  1. On thing that caught my eye was the way that they used the color channels.  They calculated an index based on the green spectrum: (2*Green) – (Red+Blue).  In the figures they published it gave a really good measure of the green-up of the measured forest.  This could be used in the initial segmenting of the images.  We could use a model based on this color normalization to segment the green patches from the non-green patches :).
  2. In the context of measuring NDVI-like values with digital cameras, this paper does not try to use digital imagery for NDVI measurements.  It’s using the images to measure phenological happenings.
  3. It does compares the behavior of NDVI with digital camera measurements in light of known phenological happenings.  It states similarities and differences and possible explanations.
  4. The paper makes a good defense of detecting phenological happenings with digital cameras.  But notice that we are trying to go a bit further than just measuring the amount of green in a ROI.  We are trying to detect individuals in plots.
  1. May 12: (About comment 1).  I completely missed an additional way of combining the color channels.  The thing with the “2G-(R+B)” is that it is very sensitive to brightness, according to Richardson.  What was done to correct for brightness sensitivity was to use a channel percentage measure within the ROI.  Chanel%=sum(channel)/(sum(R)+sum(G)+sum(B)).  This, Richardson says “eliminated the variability associated with overall brightness”
  2. May 12: Another comment that caught my eye on the second read was the fact that Richardson did not see any new insight from a normalized 2G_RBi  measurement. He does not give an equation for the normalized 2G_RBi.

About joelgranados

I'm fascinated with how technology and science impact our reality and am drawn to leverage them in order to increase the potential of human activity.
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4 Responses to Paper: Use of digital webcam images to track spring green-up in a deciduous broadleaf forest

  1. Andreas Westergaard-Nielsen says:

    I have tested the three indeces 2G_RBi, NDGI and %G. The %G seem superior to the other indeces in terms of avoiding brightness-related issues. The results can be found here:

    • joelgranados says:

      Hey Andreas.
      WOW, the %G looks very stable. Thanks for sharing the results!
      I see 3 different sets of results. Are these results from ROI with different cameras
      Is the issue about different “greeness” values with different cameras still exists with G%?
      I guess the question still remains if there is a way to do “real” NDVI with digital imagery.

      • Andreas Westergaard says:

        The three different sets of results are from three different periods (each consisting of three days in a row with different weather and cloud cover). The greenness values between cameras are not at first more equal.

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