Full text: XVIIIth Congress (Part B7)

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2. MATERIAL AND METHODS 
The test site was located in Hyrynsalmi, referred H, in 
Eastern Finland (centre Long. 28°30’ E, Lat. 64°30’N). The 
forest in the study area is typical Boreal Forest dominated by 
coniferous species (Pinus sylvestris and Picea abies). An 
operational forest stand map based on aerial photo 
interpretation was combined with multitemporal images, and 
these forest stands were applied as observation units. Only 
mineral soil stands were used in this work. The untreated 
observations totalling 390 stands were used in calibration and 
the change analyses was based on all observations totalling 
496 stands (Table 1). The mean area of a stand was 6.0 ha. 
Table 1. Number of observations in Hyrynsalmi data. 
Name Obs. Description 
  
Untreated (Unt) 390 only normal growth exist 
Uncommercial thin- 13 clearing or thinning of 
ning (Unc. Thinn.) young stand 
Commercial thinning 15 about 30% of basal area is 
(C.thinn) removed 
Preparatory cut (Prep 26 thinning of mature stand 
cut) 
Regeneration cut 13 only seed trees remain 
(Reg.cut) 
Clear cut 28 all trees removed 
Soil preparation (Soil 11 harrowing for regeneration 
prep.) 
  
There were three Landsat TM images available for the study 
area. In addition two acquisition from Varjo’s (1996) study 
were used (Table 2). The details of the Nurmes data, referred 
N, used for comparing image pairs form different locations 
are presented by Varjo (1996). 
Table 2. Landsat TM images used, H=hyrynsalmi data, 
N=nurmes data. 
  
  
Date Track Row 
H 21.06.1990 188 15 
H 10.06.1992 188 15 
H 31.07.1993 188 15 
N 08.06.1988* 187 15 
N 23.06.1990* 186 15 
  
*From Varjo’s (1996) study 
Robust regression was used to make an earlier image 
radiometrically comparable with a later one within one image 
pair. Regression coefficients between stand means were 
estimated using unmanaged forest stands. For Landsat TM 
channels 3,4 and 6 multiple regression was used (Equations 
1,2 and 3) and for other channels simple regression was 
selected (Equation 4). For the image pair 187,186/15 band to 
band regression, such as in equation 4, was used for all the 
TM channels (Varjo 1996). 
v n n7 i-n n^ i-n 
yas UD 2 ft * B1 xoa t Boxers +E (1) 
-- n n ^ í-n n^ -n 
Yona (n) = Bg t Bi xaa * B5xaa *€ (2) 
"t —i —t- 
Yos) D. a B xw kB xat, D 
- n n -n 
Yay) V0) = A" + Bi XchG) + € (4) 
B, = parameters, pe {0,1,2} 
y (i) = mean intensity of a stand on channel 1 at the 
moment t 
X ch(i) = mean intensity of a stand on channel ie {1,2,5,7} at 
the moment t-n 
n = interval between image acquisitions ne {1,2,3} 
E — error term. 
The inverse of within stand variance on the independent 
channel of the earlier image was used as weight in parameter 
estimation. The calibration parameters were estimated twice. 
After the first estimation the outliers and leverage points were 
detected and excluded from calibration and the final 
parameters were estimated without those observations (Varjo 
1996). An observation was considered to be an outlier if the 
difference of the residual from zero was statistically 
significant according T-test at 5% risk. Similarly, an 
observation was considered to be a leverage point if the 
Cook's distance was greater than 1 (Rousseew & Leroy 1986, 
Varjo 1996). 
After calibration, the differences of stand means (Equation 5) 
and standard deviations (Equation 6) were formed for each 
channel between the calibrated earlier image and the original 
later image. 
At-n 
i. ai 
AM s 7 Yi - Yi (5) 
^ t—n 
t 
ASD = SDy; — SD y; (6) 
AM,,, = difference of stand intensity means expressed in dn 
on channel i 
ASd, = difference of within stand standard deviation 
expressed in dn on channel 1 
y; = original stand mean intensity on channel i at the 
moment t 
An 
y i = stand mean intensity on channel i at the moment 
t-n calibrated to the intensity level of moment t 
SDy; = standard deviation of stand intensities on channel i 
at the moment t 
^ t-n 
SD y; = standard deviation of stand intensities on 
channel i at the moment t-n calibrated to the 
intensity level of moment t. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996 
 
	        
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