Features of Point Clouds Synthesized from Multi-View ALOS/PRISM Data and Comparisons with LiDAR Data in Forested Areas

LiDAR waveform data from airborne LiDAR scanners (ALS) e.g. the Land Vegetation and Ice Sensor (LVIS) havebeen successfully used for estimation of forest height and biomass at local scales and have become the preferredremote sensing dataset. However, regional and global applications are limited by the cost of the airborne LiDARdata acquisition and there are no available spaceborne LiDAR systems. Some researchers have demonstrated thepotential for mapping forest height using aerial or spaceborne stereo imagery with very high spatial resolutions.For stereo imageswith global coverage but coarse resolution newanalysis methods need to be used. Unlike mostresearch based on digital surface models, this study concentrated on analyzing the features of point cloud datagenerated from stereo imagery. The synthesizing of point cloud data from multi-view stereo imagery increasedthe point density of the data. The point cloud data over forested areas were analyzed and compared to small footprintLiDAR data and large-footprint LiDAR waveform data. The results showed that the synthesized point clouddata from ALOSPRISM triplets produce vertical distributions similar to LiDAR data and detected the verticalstructure of sparse and non-closed forests at 30mresolution. For dense forest canopies, the canopy could be capturedbut the ground surface could not be seen, so surface elevations from other sourceswould be needed to calculatethe height of the canopy. A canopy height map with 30 m pixels was produced by subtracting nationalelevation dataset (NED) fromthe averaged elevation of synthesized point clouds,which exhibited spatial featuresof roads, forest edges and patches. The linear regression showed that the canopy height map had a good correlationwith RH50 of LVIS data with a slope of 1.04 and R2 of 0.74 indicating that the canopy height derived fromPRISM triplets can be used to estimate forest biomass at 30 m resolution

The 2011 Eco3D Flight Campaign: Vegetation Structure and Biomass Estimation from Simultaneous SAR, Lidar and Radiometer Measurements

The Eco3D campaign was conducted in the Summer of 2011. As part of the campaign three unique and innovative NASA Goddard Space Flight Center airborne sensors were flown simultaneously: The Digital Beamforming Synthetic Aperture Radar (DBSAR), the Slope Imaging Multi-polarization Photon-counting Lidar (SIMPL) and the Cloud Absorption Radiometer (CAR). The campaign covered sites from Quebec to Southern Florida and thereby acquired data over forests ranging from Boreal to tropical wetlands. This paper describes the instruments and sites covered and presents the first images resulting from the campaign

The Use of Sun Elevation Angle for Stereogrammetric Boreal Forest Height in Open Canopies

Stereogrammetry applied to globally available high resolution spaceborne imagery (HRSI; less than 5 m spatial resolution) yields fine-scaled digital surface models (DSMs) of elevation. These DSMs may represent elevations that range from the ground to the vegetation canopy surface, are produced from stereoscopic image pairs (stereo pairs) that have a variety of acquisition characteristics, and have been coupled with lidar data of forest structure and ground surface elevation to examine forest height. This work explores surface elevations from HRSI DSMs derived from two types of acquisitions in open canopy forests. We (1) apply an automated mass-production stereogrammetry workflow to along-track HRSI stereo pairs, (2) identify multiple spatially coincident DSMs whose stereo pairs were acquired under different solar geometry, (3) vertically co-register these DSMs using coincident spaceborne lidar footprints (from ICESat-GLAS) as reference, and(4) examine differences in surface elevations between the reference lidar and the co-registered HRSI DSMs associated with two general types of acquisitions (DSM types) from different sun elevation angles. We find that these DSM types, distinguished by sun elevation angle at the time of stereo pair acquisition, are associated with different surface elevations estimated from automated stereogrammetry in open canopy forests. For DSM values with corresponding reference ground surface elevation from spaceborne lidar footprints in open canopy northern Siberian Larix forests with slopes less than10, our results show that HRSI DSM acquired with sun elevation angles greater than 35deg and less than 25deg (during snow-free conditions) produced characteristic and consistently distinct distributions of elevation differences from reference lidar. The former include DSMs of near-ground surfaces with root mean square errors less than 0.68 m relative to lidar. The latter, particularly those with angles less than 10deg, show distributions with larger differences from lidar that are associated with open canopy forests whose vegetation surface elevations are captured. Terrain aspect did not have a strong effect on the distribution of vegetation surfaces. Using the two DSM types together, the distribution of DSM-differenced heights in forests (6.0 m, sigma = 1.4 m) was consistent with the distribution of plot-level mean tree heights (6.5m, sigma = 1.2 m). We conclude that the variation in sun elevation angle at time of stereo pair acquisition can create illumination conditions conducive for capturing elevations of surfaces either near the ground or associated with vegetation canopy. Knowledge of HRSI acquisition solar geometry and snow cover can be used to understand and combine stereogrammetric surface elevation estimates to co-register rand difference overlapping DSMs, providing a means to map forest height at fine scales, resolving the vertical structure of groups of trees from spaceborne platforms in open canopy forests

A STUDY OF THE ANGULAR REFLECTANCE CHARACTERISTICS OF CORN AND SOYBEAN CANOPIES

Understanding the characteristics of vegetation canopy reflectance is important if remotely sensed data is to be fully exploited to monitor the amount and status of agricultural resources. Since vegetation canopies are not simple Lambertian reflectors the effects of illumination and viewing geometry must be understood to enhance the interpretation of the data. In this study the diurnal and seasonal directional reflectance characteristics of two important agricultural crops: corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) were examined. Spectral reflectance factor (RF), agronomic and biophysical measurements were acquired on three dates during 1980 for soybeans and on five dates in 1982 for corn over a wide range of sun and view angles. A pronounced sun-view angle effect on soybean canopy RF was observed, especially in the 0.6 - 0.7(mu)m (red) wavelength band for canopies with well defined row structure. Near infrared (near-IR) RF and the transformed variable greenness showed a less noticeable dependence for both incomplete and complete canopies. The effect of sun angle was greatest for view directions perpendicular to the canopy rows. Analysis of the data for apparent Lambertian behavior yielded very few off- nadir view angles that approximated straight down measurements for the red band, while near-IR and greenness had extended off-nadir view angle ranges. A geometric optics model was developed to examine the effect of shadows on the red band RF for incomplete soybean canopies. Comparison of modeled and measured results indicated that for nadir view the model explained up to 95% of the variation in the data. A solar zenith angle dependence for visible, near-IR and middle-IR RFs was noted for corn canopies with a low leaf area index (LAI). A decrease in the spectral contrast between vegetation and soil due to shadows was cited as the cause of the dependence. Sun angle dependence was least for canopies with higher LAIs. RFs were maximized for coincident sun and view angles and minimized when the sensor view was towards the sun. Evaluation of three linear transformations (greenness, near-IR/red ratio and normalized difference) suggested they may be useful for correcting the sun angle dependence for nadir acquired data. A strong dependence with view angle was noted for all three transformations for low LAI canopies. Greenness and near-IR/red ratio varied with view zenith angle, normalized difference did not. The asymptotic nature of normalized difference as LAI increases was cited as the reason for the lack of variation with view angle