Crop-residue cover measurements are needed for research, farm management, and farm program evaluation. We measured cover by use of still video imaging and video image analysis to study the influence of observation pointer size on accuracy ofsurface cover measurements. There are two alternative measurement procedures, the binomial "hit-miss" and the quantified proportional or "mixed" evaluations. An objective was to identify the "Critical Value" of pointer size above which the accuracy ofhit-miss evaluations will be unacceptable and mixed evaluations will be needed. Measurement error with the binomial method, was minimized by use of the smallest observation pointers of 0.2 mm. Error increased with pointer size; for example, error was upto 35 percentage points for small 2.2 mm size pointers. Tested residue physical dimensions were not correlated with unacceptably high measurement errors for pointers less than 2.2 mm in size. The proportional method was relatively insensitive to size ofobservation pointers and to dimensions of pieces of residues. The Critical Value limit for similar results from the binomial and proportional methods appeared to be in the range of pointer sizes from 0.2 to 0.3 mm. Therefore, if perfected, theproportional method should be preferred for residue cover measurements. These results apply to vision-based measuring methods, such as transect lines, and to evolving sensor-based methods which are expected to be used to examine residue cover withdiscrete sized pointers or equivalent fields of view.
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