Influence of levelling technique on the retrieval of canopy structural parameters from digital hemispherical photography

- We compare hand and tripod levelling approaches for hemispherical photography.
- Both methods agree within 2% at the plot scale for PAIe.
- Both methods agree within 1% at the plot scale for P0.
- We suggest that hemispherical photography guidelines are updated to include hand levelling.

Digital hemispherical photography is a simple, non-destructive method for estimating canopy biophysical parameters for ecological applications and validation of remote sensing products. Determination of optimum and repeatable acquisition procedures is well documented in the literature but so far this has not focused on evaluating the levelling procedure used to align the camera. In this paper, the standard recommendation that tripod levelling is a necessity is tested by comparing it with a hand-levelled procedure. The results show that the average difference between the two procedures is < 2% for effective plant area index and < 1% for gap fraction at the VALERI plot scale, which generally falls within the variance. Users implementing the hand-levelled technique can expect large reductions in data acquisition time, allowing many more samples to be collected without compromising the overall quality of the data retrieved.