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Title
Proceedings of the Symposium on Global and Environmental Monitoring

2. MATERIALS AND METHODS
2.1 Ground data collection
The studied area is 40 km by 30 km. It is
situated close to Umea in northern Sweden, with
the center at 64°05' N and 20°30'E.
Representatives for all owners of forest land in
the area were contacted and asked for thinned
forest stands and the dates for thinning. All
known stands that were thinned once between
august 1984 and july 1989 and were larger than 2.5
ha and more than 100 m wide were selected. After
excluding 2 extremely heterogeneous stands did 44
thinnings remain. The field survey of the stands
were carried out in October 1989.
The tree species were Scots Pine (Pinus
Silvestris), Norway Spruce (Picea Abies) and
Birch (Betula spp.). Only 1 % of the stems were
other deciduous species than Birch.
In each stand nominally 10 sample plots were
placed in a regular grid. The distance between the
field plots were 40-80 m depending on stand size.
A 50 m grid was chosen in the majority of the
stands. No plot was placed less than 25 m from
the stand border.
Stem and stump diameters were measured in
circular sample plots of 154 m 2 . About 20 more
variables were also recorded. In 20 % of the
stands, surveyed in the beginning of the study,
the stump diameters were measured for 128 m 2
plots. The basal area in these stands were
measured by relascope and a smaller number of
field variables were recorded.
The stump diameters were transformed to diameter
at breast height by regression functions from the
NFI. The basal area is defined as total stem
cross-section area at breast hight in m 2 per
hectare (ha). The basal area for the remaining
trees at the year of thinning were calculated
from the measured basal area 1989, using functions
from the NFI. Thus, all basal areas refer to the
year when the individual stand were thinned. The
thinning grade is then defined as basal area cut
divided with total basal area before thinning.
To illustrate the spectral changes for stronger
cuttings, five seed tree stands were also
included in the analysis. These stands were
subjectively chosen and field surveyed with the
same methods as the thinnings.
2.2 Satellite data used
The Swedish Space Corporation (SSC) provided the
project with digital Landsat TM data from the
summers 1984 - 1989, (table 1). SSC-Satellitbild
did also make the geometrical precision correction
to the Swedish national grid, with 20 m pixel
size.
Table 1. Satellite acquisitions used.
Satellite Sensor Reg. date Scene
Landsat
5
TM
1
aug.
1984
194/15
Landsat
5
TM
26
june
1985
193/15
Landsat
5
TM
6
june
1986
192/15
Landsat
5
TM
12
aug.
1987
192/15
Landsat
5
TM
27
june
1988
192/15
Landsat
5
TM
21
june
1989
193/15
The satellite scenes were radiometrically
calibrated, using the calibration lamp onboard the
satellite. The digital numbers (DN) measured in
the satellite scene might be converted to physical
units for spectral radiance using the following
formula:
Lmax - Lmin
Spectral radiance = Lmin + DN *
255
The values of Lmin and Lmax used in the pre
processing of this data set are found in table 2.
Table 2. Calibration values for converting DN
values to physical units for spectral radiance,
expressed in W/ (n? *sr*pm) .
channel
colour spectral range(um)
Lmin
Lmax
TM 1
blue
0.45 - 0.52
-1.50
152.10
TM 2
green
0.52 - 0.60
-2.80
296.80
TM 3
red
0.63 - 0.69
-1.20
204.30
TM 4
near-IR
0.76 - 0.90
-1.50
206.20
TM 5
middle-IR
1.55 - 1.75
-0.37
27.19
TM 7
middle-IR
2.08 - 2.35
-0.15
14.38
Thus, a data set of 36 channels were available.
In the analysis phase, the TM data were handled as
12 blocks of 500 by 500 pixels, corresponding to
10 km by 10 km.
For each block the data from one year was adjusted
to 1:10 000 maps. The remaining scenes were
manually adjusted to the reference scene with best
integer pixel precision. For best fit compared to
1:10 000 maps, the data on an average had to be
moved 22 m pooled distance, with a standard
deviation of 22 m. One of the reasons for the
remaining geometrical errors is obviously height
parallaxes due to scenes from different paths, but
also the 16 lines sensor swaths were sometimes
displaced.
2.3 Extraction of standwise DN mean values
Map coordinates were assigned to each field
surveyed plot. For each plot the DN values for the
pixel containing plot center, and the eight
neighbour pixels, were extracted in all the 36 TM
channels. Pixels already assigned to a plot were
not used again.
The location of the plots were now for the first
time checked together with the satellite images.
If a stand for a given acquisition was covered by
clouds, cloud shadows or technical errors, no
data were extracted for that year. To reduce the
problem with mixel values at stand borders, all
boarder pixels that in any channel were above the
99.9 % level in a t-test were excluded for all
years and channels. This procedure eliminated
5.6 % of the stand pixels. On an average 60
pixels per stand remained, which corresponds to
2.4 ha.
2.4 Satellite data calibration functions
For the between-scene calibration functions, an
independent set of pixel values was systematically
sampled from the studied area. The values were
sampled under a forest mask obtained from
thresholding in TM 5, 1989. The mask covered