Calibration of dpi for photos
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There is a calibration mechanism in CooRecorder.
This mechanism may be used to calibrate a photo of a tree ring sample when the distance between two points on the photo is known.
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A warning on bad measurement accuracy from a photo taken with a camera
Even if you take a photo "at right angles" with your camera
placed centrally above your sample, only the central part of your photo will have an
approximately linear picture representation of your sample. This means that the dpi value
varies over your photo with e.g. 10% lower dpi values at the outer borders than
at the centre of the photo.
Accordingly your measurments with CooRecorder will then underestimate
the ring width thickness in these outer areas.
This probably does no matter if your data is intended
only for crossdating your sample - i.e. finding the cutting year of the tree.
Though if you intend to use your ring width data to study tree growth then such measurements
may be very misleading.
So be careful!
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Photo of a ruler
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Measurement points in CooRecorder
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The "ring width curve" made from the millimeter marks should be a straight line if dpi value were constant along the measurement line!
A high resolution flat bed scanner will not give you this type of problems!
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