Sampling survey to the total area of the forest of Heshun county in Shanxi province in year 2000
- Calculate the total optimal sample size n according to the known accuracy requirement ;
- Calculate the sample size nh of each layer h according to the proportion distribution /optimal distribution design of the sample size;
；
- Extract nh samples from layer h respectively according to the simple random sampling method;
- Estimate the total forest area (unit m2), and estimate the sampling accuracy.
Sample size according to the optimal distribution design:
-
Total optimal sample size:
- Sample size of each layer:
- The total forest area is estimated according to 81 sampling sites:
The total optimal sample size is 72 in fact
The sampling size after extending 10% is 81
If the user knows correlation coefficient r of each knowledge level,then in the formula 
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.
Sampling accuracy:
of each layer
;
All the image pixel size
N
in the research areas ;
Each layer variance 
；
Each layer cost 
；
User variance
V;
Layer mean 
；
Layer weight 
；Population mean 
；
Variance
.