Posted by
Michael Schmid on
Jun 02, 2009; 6:47pm
URL: http://imagej.273.s1.nabble.com/NDVI-analysis-tp3691320p3691324.html
Hi Christian,
a few remarks:
(1) For subtracting, you need a linear transfer function. Most 3*8-
bit color images created by digital cameras, e.g., JPG, are
nonlinear. You need additional software to convert your camera's RAW
format (if it has one) to some format (that is readable by ImageJ)
with a linear transfer function.
(2) Even worse, there is usually some color enhancement, e.g. the
saturation is increased (Canon cameras have a 'neutral color' mode
where this is not so much of a problem).
(3) The IR blocking filter will also slightly attenuate visible colors.
(4) ImageJ brightness units are pixel values as they are in the file,
unless you use Analyze>Calibrate to change this.
Michael
________________________________________________________________
On 2 Jun 2009, at 20:26, Christian Rixen wrote:
> Dear ImageJ community,
>
> I have a question about NDVI analysis of photos of vegetation plots.
> I would like to measure the NDVI as a measure for greenup of alpine
> vegetation during the summer.
> I have two digital cameras where the IR filter was removed
> (CanonPowershotG10, IR Filter removed by the company LDP LLC,
> www.maxmax.com). Then I have two filters, one for the visible light
> (XNiteCC1-58) and one for the IR range (850 nm). I should be able
> to get a reliable NDVI measurement by taking 4 pictures (visual
> plot, IR plot, visual sky, IR sky) and calculating the NDVI
> according the the equation NDVI= (NIR-VIS)/(NIR+VIS) (see method
> description on
http://www.iac.ethz.ch/staff/stockli/ndvimeasurement/
> ndvimeasurement.html).
> I would of course use a manual exposure, I would not move the
> camera between 2 shots and I would use a frame.
>
> Now, here is my ImageJ question:
> I want to get the red channel from my visible-light photo. Can I
> just analyze the red channel of my picture with
> Image -> color -> RGB split
> and then measure the mean brightness of my red picture? And then I
> could measure the brightness of my IR picture to obtain the
> brightness values that I need for my NDVI equation?
> I am a bit uncertain because I am not very familiar with image
> analysis. What is the unit of the brightness value in ImageJ? Is
> the value between 0 and 255 an official brightness value that I can
> use? If yes, I would be set and could do my NDVI calculations.
>
> Thanks to the community for your input.
> Christian