Vertical Height Errors in Digital Terrain Models Derived from Airborne Laser Scanner Data in a Boreal-Alpine Ecotone in Norway

Erik Næsset

doi:10.3390/rs70404702

Abstract

It has been suggested that airborne laser scanning (ALS) could be used for operational monitoring of vegetation changes in the alpine tree line caused by climate change. Because the vegetation is low in such tree-less areas close to the alpine zone, the accuracy of the digital terrain model (DTM) becomes crucial for early detection of, e.g., pioneer trees representing an ongoing tree migration given that the height of the vegetation may be on the same order of magnitude as the DTM uncertainty. The goal of this study was to assess and exemplify the vertical height errors of DTMs derived from ALS data under varying flying altitudes and pulse repetition frequencies (PRF). Important effects in the analysis were local terrain form, terrain surface, ground vegetation height, and terrain slope, because they may be correlated with recruitment patterns of pioneer trees. Based on 426 ground control points collected in a boreal-alpine ecotone, a standard deviation of 0.07–0.08 m was found for the lowest flying altitudes and lowest PRFs. For the highest PRF the standard deviation was 0.13 m. There were statistically significant mean errors for the different terrain forms and ground vegetation heights (−0.11 to 0.13 m).

Highlights

As a result of global warming, the world’s climate will undergo distinct alterations over the coming decades leading to rapid changes in basic growth factors for trees and other vegetation
The overall mean error between the triangulated irregular networks (TINs) elevation and the elevation of the ground control points across all airborne laser scanning (ALS) acquisitions was 0.01 m (Table 3), which was identical to the P50ε value
This study has demonstrated that a terrain model standard deviation of 0.15 m or better can be achieved from ALS data with densities of ~8–10 pulses·m−2 in fairly tree-less areas in the boreal-alpine ecotone with gentle terrain, provided that the laser is operated at a robust pulse repetition frequency and moderate flying altitudes

Summary

Introduction

As a result of global warming, the world’s climate will undergo distinct alterations over the coming decades leading to rapid changes in basic growth factors for trees and other vegetation. This will influence the boreal forest and its transition zones, leading to an increase in productivity [1]. Even a moderate increase in temperature may lead to a rapid increase in growth of existing trees [2,3] as well as colonization of tree-less areas and migration of the alpine tree line [4]. A need exists to monitor vegetation changes in these ecotones [6]

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Discussion

Conclusion