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Comment by Stefan Fleck on February 2, 2012 at 12:05

Dear Liisa and dear colleagues,

thank you for the comprehensive draft on planar photography. My impression is that planar/wide angle photography is principally a method to estimate canopy cover, but the derivation of LAI from these photos is difficult to be standardized.

The general problems do also occur in hemispherical photography: The necessary regression model to calculate LAI from estimated canopy cover will be dependent on the angle used to derive canopy cover (which is fixed to 180 degrees in hemispherical images, employing a cosine correction). So this angle needs to be given in the draft and preferably a cosine correction should be performed for larger angles. Also, the regression model may depend on the used lens, since the projection of lenses deviates in terms of distortion. This is solved in hemispherical photography by explicitely considering the projection type (polar projection). I propose that the regression model mentioned in the draft should explicitely be described, but I wonder how all the different types of cameras and lenses should be standardized to be able to use one regression model.

A possible solution would be that only camera/lens combinations are allowed to be used that have been validated to litter trap data or other safe LAI-measurements with an r²-value above 0.8 and a mean bias below 0.5. Would this be achievable?

Best regards

Stefan

Comment by Patrick Schleppi on February 2, 2012 at 10:14

Dear Lissa, dear colleagues,

The distinction between canopy cover and canopy closure is important and it is a very good point to have this described here.

I have a question about the term "planar photography". Since you use quite a wide angle, it is not really appropriate to use the word "planar". You really have a planar reproduction at a near-infinite focal length or by using skewing corrections based on distances, like for an ortho-photograph. I'd say that "wide-angle photography" would much better describe what is done here.

I doubt further if the method with the auto-exposure lock is a good one. If you mean holding the button pressed all the time, then this is barely practicable. And if you have cameras that can remember the exposure, why not, even if i never heard of such a function before. Then, however, the majority of cameras don't have this function, but you can use the manual mode. I would therefore write the text primarily about the manual mode.

The next question is about the need to describe the exposure at all. We have another chapter about hemispherical photography that poses exactly the same questions about exposure. Maybe one should just refer to it here...

Best regards.

Patrick