What is accuracy?
Why bother with accuracy?
Are painted sensors more accurate?
What are the most accurate sensors?
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What is accuracy?
The accuracy is the extent to which a measurement agrees with the true
value and assumes that all known corrections have been applied (Anonymous
1983). In consistency with the definition of SWD, the true value was defined
as the duration of visible water on leaves. The mean absolute accuracy
is the absolute average difference between the observed duration of wetness
and the sensor.
Mean absolute error = absolute value (average( true SWD - sensor SWD))
Most sensors measure SWD indirectly. Sensors may have distinctly different physical properties than leaves. Thus there may be substantial differences between the surface wetness duration between a particular crop and the sensor.
Are painted sensors more accurate?
Painting greatly increases the accuracy of sensors (Gillespie and Kidd 1978). The mean absolute accuracy of painted Campbell Scientific Sensors was found to be 0.7 hours ((Potraz et al. (1994) during six dew events in a tomato field. The sensors tended to underestimate wetness by an average of 0.3 hrs per event. These results were obtained with a wet/dry threshold of 6999 kOhm; reducing the threshold to 900 kOhm reduced the mean absolute accuracy to 1.7 hours. In more recent studies, the same group found that unpainted sensors at 45o angle underestimated the onset of wetting by an average of 4.2 hrs, although for sensors with 2 coats of paint the onset of dew was only underestimated by 0.6 hrs. (Lau et al. 1997). In grape, Magarey et al. (unpublished # 2) found that three painted Campbell scientific sensors had a mean absolute accuracy of 1 hour, three unpainted sensors had an accuracy of 2.2 hours. Note the unpainted sensors where used at a 150 kOhm threshold.
What are the most accurate sensors?
In most instances there is little data to make accuracy comparisons between sensors of different types. One of the main reason for this is that there is no real standard for the comparison of sensors.
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