Spaceborne light detection and ranging (LiDAR) enables us to obtain information about vertical forest structure directly, and it has often been used to measure forest canopy height or above-ground biomass. However, little attention has been given to comparisons of the accuracy of the different estimation methods of canopy height or to the evaluation of the error factors in canopy height estimation. In this study, we tested three methods of estimating canopy height using the Geoscience Laser Altimeter System (GLAS) onboard NASA’s Ice, Cloud, and land Elevation Satellite (ICESat), and evaluated several factors that affected accuracy. Our study areas were Tomakomai and Kushiro, two forested areas on Hokkaido in Japan. The accuracy of the canopy height estimates was verified by ground-based measurements. We also conducted a multivariate analysis using quantification theory type I (multiple-regression analysis of qualitative data) and identified the observation conditions that had a large influence on estimation accuracy. The method using the digital elevation model was the most accurate, with a root-mean-square error (RMSE) of 3.2 m. However, GLAS data with a low signal-to-noise ratio (≤ 10.0) and that taken from September to October 2009 had to be excluded from the analysis because the estimation accuracy of canopy height was remarkably low. After these data were excluded, the multivariate analysis showed that surface slope had the greatest effect on estimation accuracy, and the accuracy dropped the most in steeply sloped areas. We developed a second model with two equations to estimate canopy height depending on the surface slope, which improved estimation accuracy (RMSE = 2.8 m). These results should prove useful and provide practical suggestions for estimating forest canopy height using spaceborne LiDAR.
衛星に搭載されたレーザ高度計により森林の鉛直構造に関する情報が得られるため、樹冠高や地上のバイオマスの計測に使われる。しかし樹冠高の推測には異なる手法による精度の違いや誤差の評価に注意を要する。私たちの研究では北海道の苫小牧と釧路の森林において、NASAの衛星ICESat＊に搭載されたレーザ高度計（GLAS）を利用して複数の方法を比較し、さらに精度に影響を与えるいくつかの要素を評価した。その結果、地表面の勾配が推測の精度に大きな影響を及ぼすことが確認されたため、勾配に応じて異なる推測式を利用することを提案した。この方法を使うと、平均2.8mの精度で樹冠高を推測できることが確認された。
- ＊
- ICESat: 氷、雲、標高を計測するNASAの地球観測衛星（Ice, Cloud, and land Elevation Satellite: ICESat）
