The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
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3.2.2 Getting the data of crops area: Using GPS to get 
spatial information attribute of polygons. The proportion of 
surveying crop such as rice based on sampling frames is easily 
calculated in GIS, which is used as sampling unit data. RSAC 
used two continuous years’ data of GRS to calculate the 
variation rate of crop area based on statistical rules at a certain 
surveying region such as administrative district. 
3.2.3 Sampling accuracy and samples amount: It is 
obvious that increasing samples amount can improve the 
sampling accuracy. However, increasing samples amount will 
definitely increase the cost of surveying. Thus it is necessary to 
balance the cost and the accuracy of surveying. The formula 
used to express the relation between samples amount and 
sampling accuracy is shown as below (Wu, 2004): 
(9) 
Where (7 = standard deviation 
n = samples amount 
A = sampling accuracy 
Z = the parameter of standard normal distribution 
1 — a = confidence level 
In this formula, the standard deviation can be replaced with the 
variable error of samples via testing in advance. When the 
sampling accuracy is fixed on a certain value such as 0.95, the 
least samples amount can be calculated. 
4. CONCLUSION 
Using RS and GIS, stratified sampling method has been 
successfully applied to main crop area monitoring for many 
years at national scale in China. RSAC uses two kinds of 
methods to stratify: one is called Frequency Accumulation 
Means (FAM) and the other is System Cluster Means (SCM). 
The two methods have different characters. With GPS, Ground 
Random Sampling (GRS) has also been adopted as complement 
to RS when estimating the variation rate of main crop’ area in 
China. 
ACKNOWLEDGEMENTS 
The data used for stratified sampling and calculating crops area 
is produced by RSAC. With deep gratitude we want to give our 
thanks to all the members of RSAC. 
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