DHPT 1.0: New software for automatic analysis of canopy closure from under-exposed and over-exposed digital hemispherical photographs

Canopy closure (CC) is an important parameter in many fields. It can be obtained from digital hemispherical photographs (DHPs) by converting the pixels to black or white according to their nature: canopy or not. DHP exposure is a key factor that controls accuracy in CC determination. Many accurate pre- and post-treatments exist for enhancing the exposure, but these methods often require specific knowledge. To be more “user-friendly” compared to the raw equations provided in the literature, we have proposed a new software, DHPT 1.0, that calculates CC from DHPs using hue-enhancing methods before an automatic thresholding calculation. DHPT 1.0 does not require manual settings, thus avoiding errors caused by operator misjudgement. The first step of this study was to calibrate the enhancement methods to make DHPT 1.0 able to accurately calculate CC regardless of the photograph exposure. We have therefore used 210 digital photographs that regroup seven locations along a visual gradient of CC and of exposures. After calibration, the accuracy of DHPT 1.0 was compared with the accuracy of three software packages that are widely used by experts: Canopon2, GLA 2.0, and WINPHOT 5.0. A batch of two sets of 10 DHPs divided into one set of over-exposed DHPs and one set of under-exposed DHPs from 10 locations along a gradient of CC was used for the comparison. DHPT 1.0 provides the most accurate results, with an average error of 8% between the same under-saturated or over-saturated DHP.