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Title
Proceedings of the Symposium on Global and Environmental Monitoring

measurements has been used in the form of double
sampling or two-phase sampling. The principles
of two-phase sampling were introduced by Neyman
(1938) and applied to forest inventory in con
nection with remote sensing by Bickford (1932).
Thereafter, designs have been described and app
lications illustrated by many authors, e.g.
Frayer (1979), LaBau and Winterberger (1988),
Mattila (1985), Peng (1987), Poso (1972, 1988).
Two-phase sampling is a sensible method if there
is access to inexpensive auxiliary data. The
first phase data does not necessarily have to be
remote sensing data, but should be well corre
lated to the ground truth data. If several aux
iliary data sources, possibly from many diffe
rent dates, Landsat TM, SPOT, NOAA, digital ter
rain model, old forest resource inventory data,
and general map data, the stratification and the
whole system tend to become very complicated.
The technique suggested here is based on the
idea that each auxiliary data source is used
either separately or in a simple combination,
e.g. the combination of Landsat TM and a gene
ral map, for stratification. This results in se
veral different strata for each first phase
sample unit. The principles of the technique
have been described in specific applications
used in Finland (e.g. Poso et al. 1984, Peng
1987, Harma 1988). Accordingly, the procedure
of using several different stratifications pro
duces several sets of inventory data for each
first phase sample unit.
The methodology is illustrated in Figure 1. In
the first phase, units are defined on the basis
of a homogeneous grid and stratified on the ba
sis of Landsat TM imagery to 5 strata. In the
second phase, each first phase plot belonging
to stratum 1 receives the same ground variable
estimates from matrix Mi based on ground mea
surements. Correspondingly, plots belonging to
stratum 2 receive the ground variable estimates
from matrix M2. The procedure is carried out for
each stratum and results in formally complete
ground variable estimates for each first-phase
plot.
STEP 1:
Each plot is supplied
with first phase data
2
2
3 4
2
STEP 2:
2
3
4 2
4
Each plot is stratified
into homogeneous strata on
3
3
4 2
5
the basis of first phase
data, xi
3
4
4 5
1
3
5
1 1
1
STEP 3:
Y ll>
Y 12’ •'
... Yip
Measurement of ground truth
M h =
Y21»
Y 22 » ••
... Y 2p
data for each stratum, h,
Ymhl >
Ymh2 »•
resulting in a ground truth
Ymh >p
matrix Mf, (h=l,2,..5)
Y 2
y 2
Y 3 Y 4
y 2
STEP 4:
Each plot is supplied with
Y2
Y3
Y4 y 2
Y4
ground truth values from
the relevant matrix Mh to
Y3
Y3
y 4 y 2
Y5
be used as the final
estimates, Yj-,
Y3
Y 4
Y 4 Y 5
Yl
(h=index for a stratum)
Y 3
y 5
-<
-<
Yl
Figure 1. Steps in two-phase
sampling approach