Vol. 11, No. 1, 74-86, 2012

Abstract
As an active remote sensing system airborne laser scanning (ALS) is well suited to achieve normalized digital surface models (nDSMs) by subtracting digital terrain models (DTMs) from digital surface models (DSMs). The nDSM represents object heights and is an important data source for the derivation of various forest parameters such as tree height, stem volume or biomass. The validation of the derived results as well as the comparison of the results from different study areas is often a challenging task due to different sampling designs and accuracies of forest inventory data being used as ground truth data.

In this study we use 17 fully callipered samples, covering an area of 4.55 ha in total, to assess the accuracies of stem volume and biomass maps for different Austrian test sites. For the callipered samples all trees with a diameter at breast height ≥10.5 cm were measured. For the calibration of the stem volume and biomass models available national forest inventory (NFI) as well as local forest inventory (LFI) data are used, which are both based on angle count sampling plots. This verification approach guarantees firstly the independency of calibration and validation data and secondly it allows accuracy analyses for different reference units. For the study area Montafon the relative differences of stem volume and biomass range between -20.0% and 57.4% and between -16.3% and 56.2% respectively for twelve coniferous dominated sample areas with ∼0.25 ha each. For a reference unit with an area of ∼3.0 ha the relative differences decrease to 15.7% and 19.3% for stem volume and biomass respectively. For the study area Tyrol deciduous and coniferous models were applied. The calculated relative differences of stem volume and biomass vary between -25.8% and -10.3% and -18.5% and 3.1% respectively for the two coniferous dominated sample areas with an area of ∼0.38 ha each. For the two deciduous dominated sample areas with an area of ∼0.38 ha, both the relative difference of stem volume and biomass vary between -10.0% and 0.6% and -3.3% and 1.1% respectively. The average relative differences for all sample areas of the Tyrol study area with a total area of ∼1.5 ha is -1.2% and -1.9% for the stem volume and the biomass, respectively. As the estimations of the stem volume and biomass maps are based on federal state wide data sets (ALS and NFI) the findings of this study are of high practical relevance for integrating ALS derived forest parameters into operational forest inventories.