T. D. REID, R. L. H. ESSERY
Figure 5.
v and p calculated for 50 hemiphotos from Abisko, numbered
according to their sky view fraction. Solid lines show the results
with image thresholding only (TO), and dotted lines are results after
applying the branch-joining algorithm (BJA).
and +0.05 in . As might be expected, the differences are
larger when
P
v is smaller, because such hemiphotos contain
more vegetation to be processed by the BJA. A change in
v
of 3% may seem a small amount, but could have a significant
effect on calculations of surface radiation balance, especially if
conclusions were applied across large areas of forest.
It should be acknowledged that the BJA is rather ad-hoc in
the way it accounts for canopy geometry. A more sophisticated
BJA was trialled by preferentially attaching individual blobs to
others that lay in the direction of the major axis of the original
blob. This resulted in some twigs being drawn in strange places,
because there are many small blobs of 5 pixels or less in which
the major axis is not obvious. Overall, the simpler approach is
usefully non-specific; further levels of sophistication might make
the algorithm less generalizable to other canopies.
Conclusion
The techniques described in this Note address some of the
difficulties encountered on analyzing hemiphotos of leafless
Arctic canopies, and could provide useful additions to existing
hemiphoto software. The manipulation of individual RGB col-
our panes has proven very useful for correctly classifying bright
areas of canopy that would otherwise be missed out, and the
BJA fills many of the gaps left by the thresholding process.
Both techniques remain to be tested for different leafless forest
types, but for at least the case of boreal birch, they help to
standardize analysis across thresholded images.
The process of producing a binary image from a hemiphoto
will always be subjective to some degree, but these new meth-
ods go some way to reducing that subjectivity. The colour pane
manipulation provides a quick and easy way to correctly clas-
sify sunlit areas of bark, encouraging all users to pay special
attention to those areas; meanwhile the repairs made by the
BJA work to produce similar final results from binary images
that may have had different levels of branch fragmentation
resulting from thresholding by different users. The subjectivity
could be further reduced in future by adapting existing auto-
matic thresholding methods Nobis 2005, Ishida 2004 to work
for leafless canopies.
Most usefully, these methods make it possible to use hemi-
photos taken quickly under automatic camera settings and non-
ideal light conditions—often the only possible way to conduct
fieldwork in the Arctic winter. To improve on this study with-
out increased effort in the field, images should be saved in raw
format to avoid loss of detail on conversion to JPEG and allow
more sophisticated processing Lang 2010. Future work could
compare hemiphoto-based radiation transfer modelling to sub-
canopy radiometer measurements Link 2004 or terrestrial laser
scanning Antonarakis 2009, Cote 2009 to assess the benefits of
different image processing methods. Such independent verifica-
tion would be highly beneficial for forest studies on the whole,
because hemiphotos remain the fastest, cheapest and easiest
way to record forest structure in the field.
Acknowledgements
This work was funded by the UK’s Natural Environment Re-
search Council grant number NE/H008187/1. The authors
would like to thank all the support staff at Abisko Scientific
Research Station (ANS), as well as Nick Rutter, Maya King,
Cécile Ménard, Steve Hancock, Rob Holden, Michael Spencer
and Megan Reid for assistance in the field, Mark Richardson
and Melody Sandells for measuring and providing Abisko re-
flectivity spectra, and Casey Ryan for advice on processing
hemiphotos.
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