In this paper

we selected a test area about 5km by 5km in Taigu of Shanxi Province to assess the accuracy of crop type proportion obtained with GVG instrument on transect line. Roads on the QuickBird imagery were digitized with length of 255km

and 16.4km of the roads were selected as transect line for field sampling.The sampling ratio was about 6.4%. Crop type proportion for each crops was obtained using GVG instrument on the transect line.323 pictures were taken with clear crop identification. The density of taking picture was about 1 picture for every 50 meter. At the same time

the intensive field works were taken to map the crop pattern in detail. QuickBird imagery was used to make parcel map with a unique identity code. Field survey recorded the crop type for each parcel. If more than one crop exist in a parcel

the proportions are assigned to different crops. The crop type proportion can be calcuated with detailed crop pattern map. By comparing the crop type proportion derived from sampling and mapping

we could draw conclusions; Crop type proportion from GVG instrument on transect line has a very high accuracy for main crops

98.7% for maize

98.98% for soybean and 95 12% for vegetables. The accuracy is acceptable for the operational use of China Crop Watch System. For other crop types with low proportion

the accuracy is low and cannot be used in China Crop Watch System. This means GVG instrument can only be used to obtain the main crop proportion. Crop type proportion from GVG instrument on transect line is somewhat larger than the true values

this means that the missing samples for some crop types with low proportion. In Taigu test area

sesame

cottage and some medicinal vegetables are not sampled on transect line with GVG instrument