Within country accuracy of tree crown transparency estimates using the image analysis system CROCO: a case study from Switzerland

We evaluated the accuracy of tree crown transparency estimates made with the image analysis system CROCO compared to visual estimates by field observers in Switzerland. Data from Swiss field training courses in 1998 and 1999 each comprising four sessions were used and the median values of the different observers’ scores were assumed as the true values. First, Swiss reference photographs alone were used to obtain the calibration curves needed to predict crown transparency from the image analysis measure DSO. In this case, the CROCO estimates were biased for many species. Second, data from the first three field training sessions were added to the reference photos to derive the calibration curves. In this second case, CROCO provided unbiased estimates of both years, while the estimates by some of the field teams were significantly different from the true values. The result suggests that CROCO can be used as a more reliable reference to detect observer bias within a country than control observers. We conclude that few reference photographs alone are not sufficient, so additional field data should also be used to estimate crown transparency more accurately using CROCO.