SIZE OF POINTERS FOR RESIDUE COVER MEASUREMENT

Authors: Morrison Jr, John; McCool, Donald; Petersen, H

Submitted to: Transactions of the ASAE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/27/1997
Publication Date: N/A
Citation: N/A

Interpretive Summary: Conservation of agricultural cropland is helped by farmers keeping some or all of the residues from old crops on top of the soil. Farmers, researchers, and regulatory agencies need to be able to accurately measure the amount of residue cover which is on the soil to be able to estimate the degree of soil protection available that year. Current residue cover measurement methods are limited in accuracy by the requirement of visually evaluating very small points on the surface for the presence of residue, and repeating this process thousands of times. Instruments are now in various stages of development to objectively measure residue cover. Information was needed on the effect of the size of pointers used for measurement on accuracy. This study established that the pointers must be no larger than "needles" of approximately 0.2 to 0.3 mm diameter for reliable measurements with current methods. Instrumentation methods may need to use an alternative proportional method of measurement which involves averaging estimates of hundreds or thousands of individual observations, if larger pointers are required.

Technical Abstract: Measurement of crop residue cover on the soil surface was simulated by use of still video imaging and video image analysis. This was done to investigate the influence of the size of pointer used in the measurements on accuracy. 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 individual observation pointer size above which the accuracy of hit-miss evaluations would be unacceptable and mixed evaluations will need to be used. With the binomial method, measurement error was minimized by use of the smallest observation pointers of 0.2 mm. Although the binomial method was affected at different levels of significance by the tested residue physical dimensions, larger size pointers increased error; for example, error was up to 35 percentage points for small 2.2 mm size pointers. The proportional method was relatively insensitive to size of observation 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 0.2 to 0.3 mm. Therefore, if perfected, the proportional 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 with discrete sized pointers or equivalent sized field of view.