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ICESat satellite laser altimetry provides an unprecedented set of global elevation measurements of the Earth, yielding great detail over ice, land and ocean surfaces. Coastal regions in particular, including seamless land-water transitions, benefit from the small footprint (50 to 90 m), high resolution (40 Hz, ∼170 m along-track), and high precision (2 to 3 cm) of ICESat. We discuss the performance and character of ICESat data in three example coastal scenarios: continental coast (Louisiana-Mississippi Gulf Coast, USA, including Lake Pontchartrain), open ocean island (Funafuti, Tuvalu), and an inland river (confluence of Tapajos and Amazon rivers, Brazil). Water elevations are compared to tide gauge heights and to TOPEX and Jason-1 radar altimetry. In demonstrating the utilization of ICESat, we also present examples of: laser waveform shapes over a variety of surface types (water, land, and vegetation); vegetation canopy heights (detecting large-scale destruction from Hurricane Katrina comparing data before and after); sub-canopy surface water; measurements of waves; and examination of along-stream river slope and comparisons of river stage to hydrologically-driven GRACE geoid change. [ABSTRACT FROM AUTHOR]

Abstract: This paper focuses on developments in topographic data acquisition, including airborne remote sensing, digital photogrammetry, differential GPS, and laser profiling (e.g. LiDAR) technologies that allow rapid acquisition of high resolution and high precision topographic data sets over a range of spatial scales. These developments have offered new opportunities for investigating spatial and temporal patterns of morphological change in gravel-bed rivers and have contributed to revitalization in three key areas: (1) morphometric estimates of sediment transport and sediment budgeting; (2) boundary conditions for numerical models, including computational fluid dynamics and cellular modelling; (3) three dimensional characterisation of morphology that is independent of flow stage. The potential is clear but there remain a number of significant challenges, including quality control and the effects of error on specific applications and morphologies. This paper presents results from two investigations, representing field and laboratory analyses of gravel-bed river morphology at different spatial scales and for different applications. Case study 1 is concerned with monitoring and modelling morphological change in a large, braided gravel-bed river, using ground-based GPS survey and digital photogrammetry derived from airborne imagery. Case study 2 is an investigation of the mechanisms for infiltration of fine sediment into gravel-bed rivers, which applies close range laser altimetry in flume experiments to derive very high resolution digital elevation models (DEMs) that are used to quantify and analyse changes in bed texture. These case studies highlight the strengths and weaknesses of specific technologies and approaches to analysis of channel and floodplain morphology and change, and suggest key areas that remain to be fully resolved. In particular, the critical need to define a specific threshold level of detection associated with each acquisition method, and for different fluvial settings (e.g. bar surface, sub-aqueous zone), is emphasised. [Copyright &y& Elsevier]

Knowledge of sea ice thickness is critical for the prediction of future climate, and for assessing the significance of changes in thickness. Sea ice thickness can be calculated from radar or laser satellite altimetry measurements of freeboard. However, a lack of knowledge of snow depth introduces significant uncertainties into these calculations. This paper compares the first coincident airborne laser and radar altimetry data over sea ice, collected during the Laser Radar Altimetry (LaRA) field campaign. LaRA was a flight of opportunity that provided valuable data to explore techniques to validate satellite measurements of ice freeboard, and the possibility of combining laser and radar measurements over snow covered sea ice to calculate the snow depth. Two new methods were created to analyse these data sets: a new radar retracker and a radar power simulator, which models radar returns from the laser data. We present the first quantitative analysis of data from the LaRA laser and radar altimeters, and demonstrate the potential of combining laser and radar altimetry to estimate snow depth. LaRA elevation estimates compare well with elevations from the radar altimeter onboard ERS-2 at the sub-meter level and the study provides lessons for future validation of satellite altimetry data over sea ice. Laser elevations are consistently higher than the radar elevations over snow covered sea ice. As LaRA was a flight of opportunity, no coincident in-situ measurements were available. Nevertheless, the difference between the reflecting surface of the laser and radar is consistent with snow depth from climatology and the analysis techniques developed in this paper will be useful for future radar and laser altimetry comparisons. [Copyright &y& Elsevier]

Abstract: The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser–INS–GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of −3.1±1.0 m a−1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of −7.7±1.0 m a−1 at the calving front at 50 m a.s.l. and minimum values of between −1.0 and −2.0±1.0 m a−1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at ∼600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of −3.2 m a−1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239–247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation changes in Patagonia, where an ice thinning acceleration trend has been observed during recent years, presumably in response to warming and possibly also drier conditions. [Copyright &y& Elsevier]

Abstract: The BepiColombo Laser Altimeter (BELA) has been selected for flight on board the European Space Agency''s BepiColombo Mercury Planetary Orbiter (MPO). The experiment is intended to be Europe''s first planetary laser altimeter system. Although the proposed system has similarities to the Mercury Laser Altimeter (MLA) currently flying on board NASA''s MESSENGER mission to Mercury, the specific orbit and construction of the MPO force the use of novel concepts for BELA. Furthermore, the base-lined range-finding approach is novel. In this paper, we describe the BELA system and show preliminary results from some prototype testing. [Copyright &y& Elsevier]

Abstract: The direct retrieval of canopy height and the estimation of aboveground biomass are two important measures of forest structure that can be quantified by airborne laser scanning at landscape scales. These and other metrics are central to studies attempting to quantify global carbon cycles and to improve understanding of the spatial variation in forest structure evident within differing biomes. Data acquired using NASA''s Laser Vegetation Imaging Sensor (LVIS) over the Bartlett Experimental Forest (BEF) in central New Hampshire (USA) was used to assess the performance of waveform lidar in a northern temperate mixed conifer and deciduous forest. Using coincident plots established for this study, we found strong agreement between field and lidar measurements of height (r 2 =0.80, p <0.000) at the footprint level. Allometric calculations of aboveground biomass (AGBM) and LVIS metrics (AGBM: r 2 =0.61, PRESS RMSE=58.0 Mg ha−1, p <0.000) and quadratic mean stem diameter (QMSD) and LVIS metrics (r 2 =0.54, p =0.002) also showed good agreement at the footprint level. Application of a generalized equation for determining AGBM proposed by to footprint-level field data from Bartlett resulted in a coefficient of determination of 0.55; RMSE=64.4 Mg ha−1; p =0.002. This is slightly weaker than the strongest relationship found with the best-fit single term regression model. Relationships between a permanent grid of USDA Forest Service inventory plots and the mean values of aggregated LVIS metrics, however, were not as strong. This discrepancy suggests that validation efforts must be cautious in using pre-existing field data networks as a sole means of calibrating and verifying such remote sensing data. Stratification based on land-use or species composition, however, did provide the means to improve regression relationships at this scale. Regression models established at the footprint level for AGBM and QMSD were applied to LVIS data to generate predicted values for the whole of Bartlett. The accuracy of these models was assessed using varying subsets of the USFS NERS plot data. Coefficient of determinations ranged from fair to strong with aspects of land-use history and species composition influencing both the fit and the level of error seen in the predicted relationships. [Copyright &y& Elsevier]

Meeting the accuracy demand of products derived from spaceborne laser altimetry requires post-launch calibration/validation procedures. Over general surfaces, such as the ones land or ice offer, calibration methods that suite undulating topography should be applied. Whereas the estimation of calibration parameters usually refers to the derivation of an error model, the problem here extends beyond that. Not knowing in advance the offsets between the recorded and actual footprint locations implies that the offsets and the calibration parameters should be estimated jointly. In addition, concerns arise regarding the reliability and quality of the estimates, namely avoiding convergence to a false solution and avoiding estimates that are highly correlated and with a low level of accuracy. The correlation and the accuracy can indeed be estimated in retrospect, but the preference is to learn in advance what factors affect them and under what circumstances they can be optimized. This paper analyzes the calibration problem along the lines of correctness and reliable estimation and shows that the error recovery model provides ample information that can lead to a solution that is both efficient and an integral part of a calibration model. Analysis of the model shows that over natural topography relatively short sites are sufficient for achieving a reliable calibration. The paper then shows that by applying optimization criteria on the estimation, rules for predicting the quality of the solution can be derived as well as general measures for evaluating sites considered for calibration. [ABSTRACT FROM AUTHOR]

Abstract: Small footprint LiDAR data were used to detect and characterize vegetation in a semi-arid sagebrush steppe environment in southeastern Idaho. Processing the raw data in individual flightlines maintained the high relative accuracy of the data set and allowed for the detection of sub-meter vegetation. First return LiDAR pulse data were used to both determine the ground surface as well as calculate vegetation heights. Surface roughness maps based on vegetation heights were found to best capture the variability of the canopy and accurately distinguish burned and unburned areas. Field validation of a sagebrush presence and absence classification based on a single roughness threshold value indicates an overall accuracy of 86%. The LiDAR-determined vegetation heights are moderately well correlated to those measured in the field, although the LiDAR heights uniformly underestimate the field heights. This underestimation is believed to be due to signal threshold limits within the LiDAR sensor, producing heights corresponding to the interior of the shrub canopy rather than the top. [Copyright &y& Elsevier]

Abstract: This study used airborne laser altimetry (LiDAR) to examine the surface morphology of two canyon-rim landslides in southern Idaho. The high resolution topographic data were used to calculate surface roughness, slope, semivariance, and fractal dimension. These data were combined with historical movement data (Global Positioning Systems (GPS) and laser theodolite) and field observations for the currently active landslide, and the results suggest that topographic elements are related to the material types and the type of local motion of the landslide. Weak, unconsolidated materials comprising the toe of the slide, which were heavily fractured and locally thrust upward, had relatively high surface roughness, high fractal dimension, and high vertical and lateral movement. The body of the slide, which predominantly moved laterally and consists mainly of undisturbed, older canyon floor materials, had relatively lower surface roughness than the toe. The upper block, consisting of a down-dropped section of the canyon rim that has remained largely intact, had a low surface roughness on its upper surface and high surface roughness along fractures and on its west face (unrelated to landslide motion). The upper block also had a higher semivariance than the toe and body. The topographic data for a neighboring, older and larger landslide complex, which failed in 1937, are similarly used to understand surface morphology, as well as to compare to the morphology of the active landslide and to understand scale-dependent processes. The morphometric analyses demonstrate that the active landslide has a similar failure mechanism and is topographically more variable than the 1937 landslide, especially at scales >20 m. Weathering and the larger scale processes of the 1937 slide are hypothesized to cause the lower semivariance values of the 1937 slide. At smaller scales (<10 m) the topographic components of the two landslides have similar roughness and semivariance. Results demonstrate that high resolution topographic data have the potential to differentiate morphological components within a landslide and provide insight into the material type and activity of the slide. The analyses and results in this study would not have been possible with coarser scale digital elevation models (10-m DEM). This methodology is directly applicable to analyzing other geomorphic surfaces at appropriate scales, including glacial deposits and stream beds. [Copyright &y& Elsevier]

Abstract: Consistent and accurate measurements of forest structure at the landscape scale are required by forest ecologists and managers for a variety of applications. Lidar remote sensing has proven to be a valuable tool for measuring these attributes in many ecosystems, including tropical, boreal, and mid-latitude forests. However, there have been few studies in montane forests. Here, we examine the ability of a large footprint lidar system to retrieve forest structural attributes in the highly variable terrain and canopy conditions of the Sierra Nevada mountains in California. Specifically, we examined the impact of slope, elevation, aspect, canopy cover, crown shape, and the spatial arrangement of canopy-forming trees on the accuracy of a large footprint lidar system in retrieving canopy height, canopy cover, and biomass. We found good agreement between field and lidar measurements of height, cover, and biomass at the footprint level, and canopy height and biomass at the stand level. Differences between field and lidar measurements are mainly attributable to the spatial configuration of canopy elements and are less sensitive to topography, crown shape, or canopy cover. The accuracy of canopy cover retrieval was highly sensitive to estimates of ground cover reflectivity and to ground sampling density. The accuracy of biomass retrieval was also good, and comparable to previous efforts in other biomes. [Copyright &y& Elsevier]

Abstract: Worldwide, rivers and streams are negatively impacted by sedimentation. However, there are few broad scale techniques for quantifying the sources of sediment, i.e. upland vs. river bank erosion. This research was designed to evaluate the use of airborne LIDAR for characterizing sediment and phosphorus contributions from river bank erosion. The evaluation was done on the main stem of the Blue Earth River in southern Minnesota. Detailed topographic data were collected on an annual basis in April 2001 and 2002 over a 56 km length of the river with a helicopter mounted Topeye laser system. The raw database included X, Y, Z coordinates of laser returns sampled from the river valley with a density of 1–3.3 elevations per m2. Uniform 1 m bare earth digital elevation models were constructed by stripping vegetation laser returns and interpolation. The two models were differenced to determine volume change over time, which was then converted to mass wasting by multiplying volume change with bulk density. Mass wasting rates were further converted to sediment load based on percentage of transportable material in the bank strata. The average difference between LIDAR measured elevations and RTK GPS surveyed elevations on 5 highway bridge surfaces was 2.5 and 8.8 cm for the 2001 and 2002 scans, respectively. The elevation errors were quasi-normally distributed with standard deviation of 6.7 and 6.1 cm for 2001 and 2002, respectively. No elevation or planimetric corrections were made to the laser data before calculating mass wasting rates because it was not possible to determine the source of error or if it was uniform within and between scans. The mass wasting estimate from the LIDAR surveys varied from 23% to 56% of the sediment mass transported past the downstream gauging station depending on the range of textural material that was entrained once in the river. These estimates are in the range of values reported in the literature. Total P contribution due to bank erosion from the river reach was estimated to be 201 t/yr. [Copyright &y& Elsevier]

Abstract: Laser scanning has become the standard technique for the acquisition of digital elevation models. With the increasing point densities point clouds acquired by laser scanning have the potential to also become valuable data for the production of three-dimensional topographical databases. This paper discusses the possibilities to use airborne laser scanning for two tasks: change detection and semi-automated 3D mapping of urban environments. While laser scanning is demonstrated to be useful for detecting changes as well as mapping errors, the implementation of mapping rules to avoid false alarm errors needs to be improved. The creation of 3D city models with laser altimetry data in a semi-automated way is considered efficient. A higher level of automation seems to be feasible to further improve the efficiency of three-dimensional mapping. [Copyright &y& Elsevier]

A conceptual model describing why laser height metrics derived from airborne discrete return laser scanner data are highly correlated with above ground biomass is proposed. Following from this conceptual model, the concept of canopy-based quantile estimators of above ground forest biomass is introduced and applied to an uneven-aged, mature to overmature, tolerant hardwood forest. Results from using the 0th, 25th, 50th, 75th and 100th percentiles of the distributions of laser canopy heights to estimate above ground biomass are reported. A comparison of the five models for each dependent variable group did not reveal any overt differences between models with respect to their predictive capabilities. The coefficient of determination ( r 2 ) for each model is greater than 0.80 and any two models may differ at most by up to 9%. Differences in root-mean-square error (RMSE) between models for above ground total, stem wood, stem bark, live branch and foliage biomass were 8.1, 5.1, 2.9, 2.1 and 1.1 Mg ha -1 , respectively. [ABSTRACT FROM AUTHOR]

A first-return laser rangefinder deployed from a low-flying helicopter provided inexpensive, repeatable, and high-resolution measurements of the shape of the outer forest canopy in stands of four successional stages on the Maryland coastal plain. The traces of canopy height in these stands revealed structural characteristics such as cover, maximum and mean heights which were consistent with corresponding ground measurements. Differences in the statistics on canopy structure corresponded to general developmental trends in these stands, including the development of maximum height and surface rugosity. Furthermore, some of the outer canopy statistics related to measures of internal organization, such as the shape of the foliage height profile and the leaf area index. These observations suggest that such measurements could be used readily to classify stand structure and developmental stage, and to deduce some aspects of internal organization of vegetation. [Copyright &y& Elsevier]

Remote sensing can potentially provide information useful in improving pollution transport modelling in agricultural catchments. Realisation of this potential will depend on the availability of the raw data, development of information extraction techniques, and the impact of the assimilation of the derived information into models.High spatial resolution hyperspectral imagery of a farm near Hereford, UK is analysed. A technique is described to automatically identify the soil and vegetation endmembers within a field, enabling vegetation fractional cover estimation.The aerially-acquired laser altimetry is used to produce digital elevation models of the site. At the subfield scale the hypothesis that higher resolution topography will make a substantial difference to contaminant transport is tested using the AGricultural Non-Point Source (AGNPS) model. Slope aspect and direction information are extracted from the topography at different resolutions to study the effects on soil erosion, deposition, runoff and nutrient losses.Field-scale models are often used to model drainage water, nitrate and runoff/sediment loss, but the demanding input data requirements make scaling up to catchment level difficult. By determining the input range of spatial variables gathered from EO data, and comparing the response of models to the range of variation measured, the critical model inputs can be identified. Response surfaces to variation in these inputs constrain uncertainty in model predictions and are presented.Although optical earth observation analysis can provide fractional vegetation cover, cloud cover and semi-random weather patterns can hinder data acquisition in Northern Europe. A Spring and Autumn cloud cover analysis is carried out over seven UK sites close to agricultural districts, using historic satellite image metadata, climate modelling and historic ground weather observations. Results are assessed in terms of probability of acquisition probability and implications for future earth observation missions. [Copyright &y& Elsevier]

Light detection and ranging (LiDAR) technology provides horizontal and vertical information at high spatial resolutions and vertical accuracies. Forest attributes such as canopy height can be directly retrieved from LiDAR data. Direct retrieval of canopy height provides opportunities to model above-ground biomass and canopy volume. Access to the vertical nature of forest ecosystems also offers new opportunities for enhanced forest monitoring, management and planning. [ABSTRACT FROM AUTHOR]

ABSTRACT Aim Previous studies have developed strong, site-specific relationships between canopy metrics from lidar (light detecting and ranging) remote sensing data and forest structural characteristics such as above-ground biomass (AGBM), but the generality of these relationships is unknown. In this study, we examine the generality of relationships between lidar metrics and forest structural characteristics, including AGBM, from two study areas in Central America with different precipitation patterns. Location A series of tropical moist forest sites in Panama and a tropical wet forest in Costa Rica. Methods Canopy metrics (e.g. canopy height) were calculated from airborne lidar data. Basal area, mean stem diameter and AGBM were calculated from measurements taken as a part of ongoing forest dynamics studies in both areas. We examined the generality of relationship between lidar metrics and forest structure, and possible environmental effects (e.g. leaf phenology). Results We found that lidar metrics were strongly correlated (R 2: 0.65–0.92) with mean stem diameter, basal area and AGBM in both regions. We also show that the relationships differed between these regions. Deciduousness of canopy trees in the tropical moist forest area accounted for the differences in predictive equations for stem diameter and basal area. The relationships between lidar metrics and AGBM, however, remained significantly different between the two study areas even after adjusting for leaf drop. We attribute this to significant differences in the underlying allometric relationships between stem diameter and AGBM in tropical wet and moist forests. Conclusions Important forest structural characteristics can be estimated reliably across a variety of conditions sampled in these closed-canopy tropical forests. Environmental factors such as drought deciduousness have an important influence on these relationships. Future efforts should continue to examine climatic factors that... [ABSTRACT FROM AUTHOR]

Data acquired by the airborne Scanning Lidar Imager of Canopies by EchoRecovery (SLICER) laser altimeter provided high-resolution digital topographicdata over Puerto Rico, the Dominican Republic and several of the Lesser AntillesIslands. The instrument was developed by the NASA-Goddard Space Flight Center.It has the capability of multibeam resolution of ground elevations beneath densecanopy areas. Data, therefore, can be used to generate a more accurate representation of the ground surface by removing the vegetation cover. Although internal precision is high (10 cm to 1 m), absolute accuracy is difficult to evaluate and depends on several factors, including the post-processed kinematic GPS (KGPS) flight path for the aircraft platform and clear identification of ground returns in the SLICER waveform. We compared topographic profiles from USGS 30 m and 1:250K DEMs for Puerto Rico with those generated by SLICER and with spot elevations derived from static and continuous GPS surveys. SLICER and KGPS surveys cross at six points in western Puerto Rico. Agreement between both elevation data sets is excellent and well fit (r = 0.921) by a linear model with a final residual bias of -0.501 m for SLICER ground returns relative to KGPS elevations. The agreement between SLICER and USGS 30 m DEMs is also very good with the largest errors associated with steep slopes and high vegetation cover. Residuals between KGPS and USGS 30 m DEMs are +1 ± 25 m, assuming a fixed uniform offset of +43.23 m between WGS84 and mean sea level. [ABSTRACT FROM AUTHOR]

For many science applications of laser altimetry, the preciselocation of the point on the Earth's surface from which the laser energy reflects is required.The laser surface return geolocation is computed from the laser altimeter's range observation in combinationwith precise knowledge of spacecraft position, instrument tracking points referenced to thespacecraft center of mass, spacecraft attitude, laser orientation, observation and attitude data timetags. An approach that simultaneously estimates the geometric and dynamic parameters of the orbit andlaser range measurement model by a combined reduction of both spacecraft tracking and laseraltimeter surface range residuals is applied to produce improved pointing, orbit and range bias solutionsand therefore improved geolocation. The data acquired by the Shuttle Laser Altimeter (SLA)-01 and 02missions constitute a valuable pathfinder data set to test algorithms in preparation for the upcoming VCL(Vegetation Canopy Lidar) and ICESat (Ice, Cloud and Elevation Satellite) missions. Results from apreliminary SLA-01 data analysis are presented along with a brief description of the methodology and itsapplication to future spaceborne missions. [ABSTRACT FROM AUTHOR]

Unlike current manned systems, NASA's next generation SLR2000 Satellite Laser Ranging (SLR) station is fully autonomous, eye-safe, relatively compact and inexpensive, and, during daytime tracking, operates at signal-to-noise ratios several orders of magnitude below unity. Tiny, passivelyQ-switched microlasers generate ultra-short pulses with output energies on the order of 100 μJ at few kHz rates to achieve mm-levelranging precision to satellite altitudesof 20,000 km. Special ranging receivers, combined with Poisson statistical analysis of the received photon distribution, enable the system to rapidly and reliably identify and extract the single photon laser echoes from the solar background. The enhanced rate of return, combined with a uniform signal strength, can actually drive down both systematic and random range errors. The new SLR2000 technology has already spawned exciting new applications. Compact microlaser altimeters, capable of mapping the surface of a planet or other celestial body at multikilohertz rates, is one such application, and a high altitude, airborne version is currently being developed under NASA's Instrument Incubator Program. Interplanetary microlaser transponders would be capable of performing decimeter ranging or subnanosecond time transfer to spacecraft throughout the inner Solar System, resulting in improved knowledge of planetary motions and librations and enhanced General Relativity experiments. [ABSTRACT FROM AUTHOR]