ANALYSIS OF RANDOM AND SYSTEMATIC-SAMPLING METHODS FOR RESIDUE COVER MEASUREMENT

A procedure to compare the precision of random point and systematic gr id point sampling methods for corn and soybean residue measurement was presented. Both sampling methods were performed on images for a wide range of residue coverage and sampling points. Residue coverage of eac h image was measured using an automatic thresholding algorithm. A comp uter program carried out all the calculations for thresholding, sampli ng point generation, and point counting. Repeated estimations of resid ue cover in an image were normally distributed, and both sampling meth ods were unbiased estimators of residue cover. Systematic sampling was more precise than random sampling for both corn and soybean residue i n most cases. Four coefficient of variation (CV) regression equations were developed. The CV equations of random sampling corresponded well to theoretical results. The CV equations of systematic sampling could be easily used to find the number of sampling points needed to achieve predefined precision. An example shows how to design a sampling patte rn so that the least number of sampling points is used to make sure th at there is 90% confidence that the estimated residue cover of an imag e is between 95 and 105% of real residue cover. This analysis procedur e can be readily used to investigate other sampling methods and other types of residue.