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Sampling strata 
The sampling strata were distinguished based on the brightness 
(intensity) of the digital image scanned from a panchromatic 
negative of the aerial photograph. 
  
Fig 2. The sampling strata 
First we examined the intensity distribution of the image. On 
the histogram we identified the modals according to 2 and 4 
strata (Fig 2). We determined the area of the individual strata, 
and the average intensity inside the respective strata, which 
values were used for the weighted sampling plans. The area 
was about 50 ha large, and since Hungarian standards for the 
reclamation of salt-affected soils prescribe one sample each 5 
ha, our minimal sample size was 10. It was increased by 2, 4, 8 
times in order to increase the spatial representation. 
Table 1. The tested sampling plans 
In the case of random sampling, the sampling points were 
allocated independently from the information content of the 
image. 
In one series of sampling plans we intended to express the 
work/investment requirement of gypsuming, so the number of 
sampling points was weighted by the average intensity values 
of the sampling strata. Compared to the non-weighted sampling 
plan, now we had relatively more sampling points in the high- 
intensity strata in order to increase the precision of predicting 
gypsum requirement in these strata. The number of sampling 
points determined is shown in Table 1. 
stratified (2 strata) 
weighted stratified 
(4 strata) 
  
Fig 3. Comparison of original and interpolated images based 
on sample size of 80 
The sampling plans were tested by Monte-Carlo simulation. 
We generated completely random spatial distribution of 10, 20, 
40, 80 points, which are the realizations of any homogeneous 
spatial Poisson point process. 
  
  
Sampling plan 4-strata 2-strata 
Sampling size in stratum 
Sample size 1. 2. 3. 4. 1-2 3-4. 
Total sample 134 9 30 50 45 39 95 
Sampling plans: 
Weighted by intensity 
80 6 20 32 22 26 54 
40 3 10 16 11 13 27 
20 2 5 7 13 
10 1 2 4 3 3 7 
Non weighted 
80 3 14 33 30 17 63 
40 2 7 16 15 9 31 
20 1 3 8 8 4 16 
10 0 2 4 4 2 8 
  
We wanted to test the precision of the individual sampling 
plans. Therefore we determined the intensity of the image in 
the simulated sampling points, actually the average of the 
pixels found in its 50 cm radius neighborhood. Then this value 
was related to the given geometrical point and the intensity 
values were interpolated for the image with inverse-distance- 
Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 179