detector saturation limits) yield more distinct waveform am-
plitude profiles that are more easily interpreted with respect
to canopy height analysis (Wagner
et al.
, 2004; Mallet
et al.
,
2009). Low energy returns are more difficult to interpret, as
they are more susceptible to being masked by background
noise. The RH
100
canopy height retrieval method is more
susceptible to this issue than RH
ROS
, as it can often rely on
two locations (canopy top and ground) where the distinction
between signal and noise is critical. RH
ROS
relies on a single
location (canopy top) where this distinction is important, as
its ground location (where distinguishable from vegetation
returns) is identified at intensities greater than background
noise levels. As such, RH
100
is likely to produce less consistent
canopy heights with respect to RH
ROS
. Low laser transmission
energies yield systematically shorter canopy height retrievals
than those from high energy laser transmissions because of a
reduced signal/noise ratio, as observed in
ALS
data (Hopkin-
son, 2007); this is also observed in
GLAS
data (Figure 7)
.
Figure 8. Stratified control comparisons for the best GLAS height method and ALS pXX as a function of ALS data source: (a) all returns, (b)
first returns, and (c) raster; and each study site: i) Robson Creek, ii) Tumbarumba, and iii) Watts Creek. Linear models and associated 95
percent confidence intervals are also illustrated.
T
able
7. S
ummary
S
tatistics
for
C
ontrols when
F
iltered
A
ccording
to
S
tudy
S
ite
;
p
XX
is
the
M
ost
C
losely
R
elating
ALS H
eight
P
ercentile with
E
ach
GLAS
H
eight
D
erivation
; I
ndication
of
a
B
est
O
verall
C
omparisons
are
B
old
for
E
ach
ALS D
ata
S
ource
. N
ote
: U
nder
S
ite
, RC = R
obson
C
reek
, TA = T
umbarumba
,
and
WC = W
atts
C
reek
.
RH
100
RH
ROS
Data Site N
RMSE R
2
F
20
F
B
pXX
RMSE R
2
F
20
F
B
pXX
Raster
RC
47
1.16 15.30 0.10 0.79 0.15 p99 0.79 13.59 0.09 0.85 0.23 p90
TA
0.93 12.36 0.00 0.87 0.06 p90 0.80 13.44 0.03 0.85 0.21 p90
WC
1.04 16.85 0.13 0.94 0.06 p100
0.95 13.17 0.17 0.94 0.03 p90
All
RC
47
1.16 17.83 0.10 0.68 0.17 p100 1.01
14.6
0.01 0.81 0.07 p95
TA
1.03 11.04 0.19 0.89 0.02 p99 1.03
9.66
0.25 0.89 0.03 p95
WC
1.04 12.69 0.32 0.96 0.04 p100
1.05
7.95
0.57 0.98 0.06 p95
First
RC
47
1.16 17.83 0.10 0.68 0.17 p100 0.91 14.67 0.00 0.81 0.09 p99
TA
1.01 11.05 0.18 0.91 0.00 p99 0.99
9.66
0.24 0.89 0.02 p95
WC
1.04 12.69 0.32 0.96 0.04 p100
1.03
7.73
0.56 0.98 0.03 p95
360
May 2016
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING