2. DATA AND METHODS
A WV-2 image from March 21* was used in this study (figure
1). The image was pre-processed and atmospherically
corrected. Field reflectance measurements in the study area
coincident with the satellite image acquisition, contributed to
the validation of the atmospheric correction.
Figure 1. True color combination of WV2 image for the study
area, and description of the three main vegetation covers
present.
2.1 Spectral libraries
Two sets of spectra were collected for different species.
2.1.1 From pure image pixels: The first field dataset was
a compilation of GPS locations where species were identified.
The WV2 spectral signatures from the purest pixels were used
later as endmembers in MESMA: the species registered were
M. faya (12), L. novocanariensis (6) and Pinus radiata (15),
and spectral signatures for herbaceous species and fern (19) in
degraded areas were also collected.
2.1.2 From field measurements: The second set was
obtained from ASD FieldSpec 3 JR reflectance measurements
in stands of M. faya, L. novocanariensis and E. arborea. A
total of 69 spectral signatures were collected during a 4-day
field campaign, including the day of the WV2 pass (March
14*, 15". 20^ and 21%).
International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XXXIX-B8, 2012
XXII ISPRS Congress, 25 August — 01 September 2012, Melbourne, Australia
ASD field WV2 pure Empirical
Wavelength | signature for | pixels signature calibration
(nm) M. faya for M. faya ASD > WV2
427 0.0111 0.0037 -0.0074
478 0.0137 0.0157 0.0019
546 0.0302 0.0299 -0.0003
608 0.0267 0.0296 0.0029
659 0.0214 0.0243 0.0029
724 0.1630 0.1368 -0.0262
831 0.3153 0.2573 -0.0580
908 0.3454 0.2669 -0.0786
Table 1. Empirical calibration parameters resulting from the
comparison between M. faya field and WV2 image samples.
—ADiield TESAIMpled ese
spectral signatures
(E. arborea)
Reflectance
427 478 546 608 659 724 831 908
Wavelength (nm)
eee E. arborea flowered = = E. arborea flowered (average)
-" E arborea green E. arborea green (average)
0.4 o WM 2-callibrated té tr I ta ERE
spectral signatures
0.3 -
Reflectance
427 478 546 608 659 724 831 908
Wavelength (nm)
E. arborea flowered == == [ arborea flowered (average)
E. arborea green
E. arborea green (average)
Figure 2. Empirical calibration of field spectra from Erica
arborea measurements for endmember extraction in WV2
image. Two patterns noticeable between different phenological
stages.
2.1.3 From re-calibrated field measurements: Given its
less defined crowns compared to other species, it was difficult
to identify pure image pixels for E. arborea, only with ASD
measurements available. This was solved empirically by
comparing ASD and WV2 spectra in other species where both
data sets were available, and then obtaining a conversion
coefficient to each band (table 1). These coefficients were
added to E. arborea ASD spectra as a calibration method to
obtain WV2-adjusted spectra (figure 2).
2.2 Forest mapping
First, a NDVI threshold (Rouse et al., 1973)) was used to mask
out all non-vegetation areas in the study area. Only pixels with
NDVI > 0.5 were included in MESMA. Two unmixing cycles
were applied. In the first cycle (mesmal), three endmember
libraries were used: M. faya + L. novocanariensis -because of
their spectral and ecological similarity they were fused
together-, E. arborea and herbaceous/fern.
For the second MESMA (mesma2) cycle only the pixels with a
sub-pixel cover fraction estimate above 80% of M. faya + L.
novocanariensis were analyzed. This second cycle allowed us
to differentiate between the main dominant tree species in the
area: M. faya, L. novocanariensis and P. radiata.
Inte
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Howe
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