Horizon visibility and accuracy of stocking determination on circular sample plots using automated remote measurement techniques

• We elaborated the theoretical model of horizon visibility in the forest.
• We validated the theoretical model of horizon visibility using field data.
• The visibility has little impact on the accuracy of the stocking determination.
• The tree visibility is affected by the spatial distribution of trees.
• To ensure small errors, the radius of the plot should not exceed 15 meters.

When using such methods as terrestrial laser scanning (TLS), Bitterlich relascope and methods utilizing short-range terrestrial photogrammetry, one of the major factors influencing the accuracy of stand characteristics determination is the visibility of trees on a sample plot, which is often obscured by the shadow effect caused by trees located closer to the plot center. In the presented study the equation for assessing the visibility of the horizon in relationship to the sample plot radius and such characteristics of stands as stocking and quadratic mean diameter (QMD) was presented. The verification of the formulae using data from 30 sample plots with simple structure confirmed the practical applicability of the derived equation for the determination of horizon visibility on circular sample plots with radii above 15 m. It has also been verified, that spatial arrangement of tress influences the derived relationship in the investigated stands with simple structures. The relationship between the horizon visibility and the accuracy of stocking and basal area has also been examined. It was concluded, that this relationship is relatively weak and significant only for close distances to the plot center. At the same time, the percentage of the visible trees and basal area also depends on the distance from the plot center. It has been concluded, that for the analyzed plots the error of stocking determination does not exceed 5% for a plot radius up to 15 m. The accuracy of tree identification is also particularly influenced by other trees located close to the plot center, which cause relatively large, significant obstacles in horizon visibility.