The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences. Vol. XXXVII. Part B7. Beijing 2008 
data provided by the operating department subordinated to the 
government. In order to obtain the data of crops acreage every 
year in good time, RSAC has to consider two factors as the 
operating department, which are time and money. Since the 
spatial range monitored is very large and the kinds of crops are 
so many, it is impossible to investigate the overall fields either 
by RS or by ground survey using GPS. Therefore, RSAC 
chooses sampling method using RS and GPS. 
2. OTHER WAYS OF GETTING CROP AREA DATA 
2.1 In European Union (EU) 
Monitoring Agriculture with Remote Sensing (MARS) is a 
project facing European in order to obtain crop yield 
information constituted by European Union Committee (Liu, 
1999). It is a kind of three-stage sampling based on 
unsupervised classification (Duda, 2002) using multitemporal 
RS data (Panigrahy, 1997). The first-stage sampling unit 
composed of 60 sites is designed square with the side length of 
40 kilometres. There are 16 component parts in each site and 
each part has 40 sampling points. Because unsupervised 
classification method is used with RS data to cover the third- 
stage sampling points 5-6 times every year, all crops in 
sampling units are recognized and then the acreage and yield 
will be worked out after statistical calculation from the third- 
stage sampling points to the first-stage sampling units. 
2.2 In America 
In America, the prediction of total crop yield is acquired from 
crop acreage and crop yield per unit. The crop acreage data had 
been gotten by June Agricultural Survey (JAS) (Hu, 2002). 
Two different sampling units used by JAS were area frame 
covering America and name list frame. The name list was 
composed of registered farmers. Every year about 2400 
investigators contacted more than 120 thousand farmers in the 
first two weeks of June in order to get crops acreage data. 
2.3 In China 
The operating prediction of crops acreage is mainly provided by 
RSAC. RSAC adopts two methods to obtain the acreage of 
main crops such as wheat, com, cotton, soybean, rice, etc. One 
method is stratified sampling using RS and the other is ground 
random sampling using GPS. Stratified Sampling Method (SSM) 
using RS is the major one that works as illustrated by the flow 
chart below (Chen, etc., 2000). 
To use SSM, the first step is to select the sampling units 
according to the surveyed crop and to order appropriate RS 
images covering all sampling units in the region. The second 
step is to discriminate the crop with RS data and get the area 
data of the crop distributed in the sampling units. The last step 
is calculating the result with SSM. 
Figure 1. Flow chart of getting crop area by SSM 
3. SAMPLING METHODS 
RSAC selects the sampling methods that include stratified 
sampling in spatial regions with RS and Ground Random 
Sampling (GRS) with GPS. 
3.1 Stratified sampling using RS and GIS 
The aim of sampling is to estimate the total quantity of the 
object such as crop area since the total quantity is too large to 
count and it is of no necessity to survey entirely. Stratified 
sampling is a kind of methods like random sampling, multi 
stage sampling, etc. It is mainly applied to such case as that the 
target individuals are very different from one another but have 
quantitative attributes 
Selecting sampling unit: Sampling unit has two key points: 
quantity and accuracy. Meanwhile, it should be convenient for 
operating and calculating reasons. RSAC selects two kinds of 
sampling units: one is the polygon of administrative counties, 
and the other is the quadrangle frame of relief map on which the 
scale used is 1:50,000 or 1:25,000, and longitude difference one 
quarter degree and latitude difference one sixth degree (Jiao, 
2002). The difference between the two sampling units is that the 
former is easier for statistics analysis but the quantity is too 
small to satisfy the demand of sampling some crops because the 
areas of sampling units are apparently different among the 
polygons of administrative counties, the latter is just opposite. 
3.1.1 Layers and layer amount: In stratified sampling 
survey, layer amount has influence on the effect of sampling. 
The proper amount of layers is related with the characters of 
number of sampling units. The appropriate amount of layers 
will minimize the population variance of total layers and the 
sampling cost. During a certain threshold interval, adding layer 
number could commonly lower population variance, but 
increase the workload. In an attempt to balance the effect of 
sampling and the expense of survey, the number of six is 
confirmed the maximum layer amount in sampling survey of 
crops area according to many tests done by RSAC. 
The so-called layer is a kind of data set based on sampling units. 
There are obvious differences in sampling size among the layers. 
In survey of crop samplings, the sampling units that belong to a 
certain layer are generally distributed relatively centralized. The 
map below shows the distribution of six layers of Chinese late 
rice of 2007, on which sampling units is the quadrangle frame 
of relief map with the scale 1: 50000. There are totally 5340 
sampling units covering 15 provinces of China in this map. It is