stereo radargrammetry with radarsat 2 in the canadian arctic a digital elevation model dem is generated using a radarsat 2 r 2 high resolution stereo pair acquired over challenging icefields and fjords site in the canadian arctic 80 of ice covered areas and almost 50 of 40 circ slopes the stereo dem was produced using a new hybrid radargrammetric model which did not need any reference cartographic data the accuracy of the stereo dem was quantified with a topographic 1959 dem and ice cloud and land elevation satellite icesat data over and outside the icefields and as a function of slope the method could be applied to ice covered areas with 1 sigma 25 or 18 m accuracy over less than 30 circ or 5 circ slopes respectively in addition a systematic elevation lowering of around 10 m computed between 1959 dem and recent elevation data r 2 and icesat could be due to icefield wastage over the last 50 years

remote sensing estimates of glacier mass balances in the himachal pradesh western himalaya india abstract although they correspond to an important fraction of the total area of mountain glaciers 33 000 km2 out of 546 000 km2 himalayan glaciers and their mass balance are poorly sampled for example between 1977 and 1999 the average area surveyed each year on the field was 6 8 km2 only no direct mass balance measurement is available after 1999 to contribute to fill this gap we use remote sensing data to monitor glacier elevation changes and mass balances in the spiti lahaul region 32 2 n 77 6 e himachal pradesh western himalaya india our measurements are obtained by comparing a 2004 digital elevation model dem to the 2000 srtm shuttle radar topographic mission topography the 2004 dem is derived from two spot5 satellite optical images without any ground control points this is achieved thanks to the good on board geolocation of spot5 scenes and using srtm elevations as a reference on the ice free zones before comparison on glaciers the two dems are analyzed on the stable areas surrounding the glaciers where no elevation change is expected two different biases are detected a long wavelength bias affects the spot5 dem and is correlated to an anomaly in the roll of the spot5 satellite a bias is also observed as a function of altitude and is attributed to the srtm dataset both biases are modeled and removed to permit unbiased comparison of the two dem on the 915 km2 ice covered area digitized from an aster image on most glaciers a clear thinning is measured at low elevations even on debris covered tongues between 1999 and 2004 we obtain an overall specific mass balance of 0 7 to 0 85 m a water equivalent depending on the density we use for the lost or gained material in the accumulation zone this rate of ice loss is twice higher than the long term 1977 to 1999 mass balance record for himalaya indicating an increase in the pace of glacier wastage to assess whether these ice losses are size dependant all glaciers were classified into three samples according to their areal extent all three samples show ice loss the loss being higher for glaciers larger than 30 km2 in the case of the benchmark chhota shigri glacier a good agreement is found between our satellite observations and the mass balances measured on the field during hydrological years 2002 2003 and 2003 2004 future studies using a similar methodology could determine whether similar ice losses have occurred in other parts of the himalaya and may allow evaluation of the contribution of this mountain range to ongoing sea level rise

glacier changes in southeast alaska and northwest british columbia and contribution to sea level rise 1 the digital elevation model dem from the 2000 shuttle radar topography mission srtm was differenced from a composite dem based on air photos dating from 1948 to 1987 to determine glacier volume changes in southeast alaska and adjoining canada srtm accuracy was assessed at 5 m through comparison with airborne laser altimetry and control locations measured with gps glacier surface elevations lowered over 95 of the 14 580 km2 glacier covered area analyzed with some glaciers thinning as much as 640 m a combination of factors have contributed to this wastage including calving retreats of tidewater and lacustrine glaciers and climate change many glaciers in this region are particularly sensitive to climate change as they have large areas at low elevations however several tidewater glaciers that had historically undergone calving retreats are now expanding and appear to be in the advancing stage of the tidewater glacier cycle the net average rate of ice loss is estimated at 16 7 4 4 km3 yr equivalent to a global sea level rise contribution of 0 04 0 01 mm yr

multispectral remote sensing as stratigraphic and structural tool wind river basin and big horn basin areas wyoming stratigraphic and structural analyses of the wind river basin and big horn basin areas of central wyoming are in progress one result has been the development of an approach to stratigraphic and structural analysis that uses photogeologic and spectral interpretation of multispectral image data to characterize the attitude thickness and lithology of strata new multispectral systems that have only been available since 1982 are used with topographic data to map upper paleozoic and mesozoic strata exposed on the southern margin of the bighorn mountains landsat acquired thematic mapper tm data together with topographic data are used to map lithologic contacts measure dip and strike and develop a stratigraphic column that is correlated with conventional surface and subsurface sections aircraft acquired airborne imaging spectrometer ais and thermal infrared multispectral scanner tims data add mineralogic information to the tm column including the stratigraphic distribution of quartz calcite dolomite smectite and gypsum results illustrate an approach that has general applicability in other geologic investigations that could benefit from remotely acquired information about areal variations in attitude sequence thickness and lithology of strata exposed at the earth s surface application of our methods elsewhere is limited primarily by availability of multispectral and topographic data and quality of bedrock exposures

combination of srtm3 and repeat aster data for deriving alpine glacier flow velocities in the bhutan himalaya abstract the present study evaluates the fusion of dems from the advanced spaceborne thermal emission and reflection radiometer aster instrument and the shuttle radar topography mission srtm the study area consists of high elevation glaciers draining through the rough topography of the bhutan himalayas it turns out that the aster derived and srtm3 dems have similar accuracy over the study area but the srtm3 dem contains less gross errors however for rough topography large sections of the srtm3 dem contain no data we therefore compile a combined srtm3 aster dem from this final composite master dem we produce repeat aster orthoimages from which we evaluate the dem quality and derive glacier surface velocities through image matching the glacier tongues north of the himalayan main ridge which enter the tibet plateau show maximum surface velocities in the order of 100 200 m year 1 in contrast the ice within the glacier tongues south of the main ridge flows with a few tens of meters per year these findings have a number of implications among others for glacier dynamics glacier response to climate change glacier lake development or glacial erosion the study indicates that space based remote sensing can provide new insights into the magnitude of selected surface processes and feedback mechanisms that govern mountain geodynamics

penetration depth of interferometric synthetic aperture radar signals in snow and ice digital elevation models of glaciated terrain produced by the nasa jet propulsion laboratory jpl airborne interferometric synthetic aperture radar insar instrument in greenland and alaska at the c 5 6 cm wave length and l band 24 cm frequencies were compared with surface elevation measured from airborne laser altimetry to estimate the phase center of the interferometric depth or penetration depth p on cold polar firn at greenland summit p 9 2m at c and 14 4m at l band on the exposed ice surface of jakobshavn isbrae west greenland p 1 2 m at c and 3 3 m at l band except on smooth marginal ice where p 15 5 m on colder marginal ice of northeast greenland p reaches 60 to 120 m at l band on the temperate ice of brady glacier alaska p is 4 2 m at c and 12 6 m at l band with little dependence on snow ice conditions the implications of the results on the scientific use of insar data over snow ice terrain is discussed

late cretaceous climate changes recorded in eastern asian lacustrine deposits and north american epieric sea strata abstract cretaceous climate data of the long lived cretaceous songliao basin sb in eastern asia is correlated and compared with the western interior seaway wis on the northern american plate in order to understand better the dynamics of the earth s past greenhouse climates nearly continuous late cretaceous terrestrial deposition in the songliao basin is represented by two cores totaling 2431 m in length the turonian maastrichtian age of the section is based on integrated stratigraphy and is comparable in age with upper cretaceous strata in the wis being consistent with global trends the dynamic late cretaceous climates of both the sb and wis gradually cooled from the warmest albian cenomanian time to the end of the maastrichtian with several intervening warm periods as did the global climate however regional differences existed the songliao basin climate was humid to semi humid warm temperate subtropical and the western interior seaway was in the humid warm temperate zone and experienced only moderate climatic changes the shifts of oxygen isotope data in the songliao basin were frequent and abrupt whereas wis records more gradual change affected mainly by fresh water runoff mixing with southern tethyan and northern arctic waters sedimentary cycles of eccentricity obliquity and precession bands are recorded in both the sb and wis basins the sedimentary cycles in the wis and sb are interpreted to be related to variations of the wet dry runoff cycles which indicate that orbital forcing played an important role in global climate change in late cretaceous the most favorable condition for organic carbon burial in both the sb and wis basin was bottom water anoxia regardless of the cause of the anoxia but the organic carbon burial rate was usually much higher in the songliao lake than in the wi epeiric sea suggesting that giant lakes may serve as important sinks of atmospheric co 2 in both basins organic rich deposits formed during a rise in water level and incursion of saline waters the integration of paleoclimate data from cretaceous marine deposits and terrestrial sedimentary record will promote our understanding of the cretaceous greenhouse climate change and may provide insights for a future greenhouse world

contribution of the patagonia icefields of south america to sea level rise digital elevation models of the northern and southern patagonia icefields of south america generated from the 2000 shuttle radar topography mission were compared with earlier cartography to estimate the volume change of the largest 63 glaciers during the period 1968 1975 2000 these glaciers lost ice at a rate equivalent to a sea level rise of 0 042 0 002 millimeters per year in the more recent years 1995 2000 average ice thinning rates have more than doubled to an equivalent sea level rise of 0 105 0 011 millimeters per year the glaciers are thinning more quickly than can be explained by warmer air temperatures and decreased precipitation and their contribution to sea level per unit area is larger than that of alaska glaciers

potential of radarsat 2 stereo radargrammetry for the generation of glacier dems the study of glaciers and ice caps in remote and cloudy regions remains difficult using current remote sensing tools here the potential of stereo radargrammetry srg with radarsat 2 wide ultra fine images is explored for dem extraction elevation changes and mass balance calculations on barnes ice cap nunavut canada over low relief terrain surrounding barnes a vertical precision of 7 m 1 confidence level is measured as well as an average vertical bias of 4 m moreover we show that the c band penetration depth over the ice cap is insignificant at this time of the year i e late ablation season this is likely due to a wet surface and the presence of superimposed ice that leads to a surface radar response comparing the srg dems with other datasets an historical glacier wide mass balance of 0 52 0 19 m w e a 1 is estimated for 1960 2013 whereas it decreases to 1 06 0 84 m w e a 1 between 2005 and 2013 this clear acceleration of mass loss is in agreement with other recent studies given its all weather functionality and its possible use without ground control points the radarsat 2 srg technology represents an appropriate alternative for glacier monitoring in cloudy and remote regions

icesat measurement of greenland ice sheet surface slope and roughness the ice cloud and land elevation satellite icesat in its 8 day repeat orbit mode provided data not only on the along track surface slope but also on the cross track surface slope from adjacent repeat ground tracks during the first 36 days of operation four to five such repeat orbits occurred within 1 km in the cross track direction this provided an opportunity to use icesat data to measure surface slope in the cross track direction at 1 km scale an algorithm was developed to calculate the cross track surface slope combining the slopes in the cross track and along track directions gives a three dimensional surface slope at 1 km scale the along track surface slope and surface roughness at 10 km scale are also calculated a comparison between icesat surface elevation and a european remote sensing satellite ers 1 5 km digital elevation model shows a difference of 1 2 m in central greenland where the surface slope is small and 20 m at the edge of greenland where the surface slope is large the large elevation difference at the edge is most likely due to the slope induced error in radar altimeter measurement accurate surface slope data from icesat will help to correct the slope induced error of radar altimeter missions such as geosat ers 1 and ers 2

icesat antarctic elevation data preliminary precision and accuracy assessment 1 since first light on february 20th 2003 nasa s ice cloud and land elevation satellite icesat has derived surface elevations from 86 n to 86 s latitude these unique altimetry data have been acquired in a series of observation periods in repeated track patterns using all three geoscience laser altimeter system glas lasers here we focus on antarctic ice sheet elevation data that were obtained in 2003 2004 we present preliminary precision and accuracy assessments of selected elevation data and discuss factors impacting elevation change detection we show that for low slope and clear sky conditions the precision of gla12 laser 2a release 21 data is 2 1 cm and the relative accuracy of icesat elevations is 14 cm based on crossover differences

a new high resolution digital elevation model of greenland fully validated with airborne laser altimeter data a new digital elevation model of the greenland ice sheet and surrounding rock outcrops has been produced at 1 km postings from a comprehensive suite of satellite remote sensing and cartographic data sets height data over the ice sheet were mainly from ers 1 and geosat radar altimetry these data were corrected for a slope dependent bias that had been identified in a previous study the radar altimetry was supplemented with stereophotogrammetric data sets synthetic aperture radar interferometry and digitized cartographic maps over regions of bare rock and where gaps in the satellite altimeter coverage existed the data were interpolated onto a regular grid with a spacing of 1 km the accuracy of the resultant digital elevation model over the ice sheet was assessed using independent and spatially extensive measurements from an airborne laser altimeter that had an accuracy of between 10 and 12 cm in a comparison with the laser altimetry the digital elevation model was found to have a slope dependent accuracy ranging from 1 04 1 98 m to 0 06 14 33 m over the ice sheet for a slope range of 0 0 1 0 the mean accuracy over the whole ice sheet was 0 33 6 97 m over the bare rock areas the accuracy ranged from 20 to 200 m dependent on the data source available the new digital elevation model was used as an input data set for a positive degree day model of ablation the new elevation model was found to reduce ablation by only 2 compared with using an older 2 5 km resolution model which suggests that resolution induced errors in estimating ablation are no longer important

spot5 hrs digital elevation models and the monitoring of glacier elevation changes in north west canada and south east alaska abstract monitoring the response of land ice to climate change requires accurate and repeatable topographic surveys the spot5 hrs high resolution stereoscopic instrument covers up to 120 km by 600 km in a single pass and has the potential to accurately map the poorly known topography of most glaciers and ice caps the acquisition of a large hrs archive over ice covered regions is planned by the french space agency cnes and spotimage france during the 2007 2008 international polar year ipy here we report on the accuracy and value of hrs digital elevation model dem over ice and snow surfaces a dem is generated by combining tools available from cnes with the pci orthoengine se software using hrs images acquired in may 2004 over south east alaska usa and northern british columbia canada the dem is evaluated through comparison with shuttle radar topographic mission srtm dem and icesat data on and around the glaciers a horizontal shift of 50 m is found between the hrs and srtm dems and is attributed to errors in the srtm dem over ice free areas hrs elevations are 7 m higher than those of srtm with a standard deviation of 25 m for the difference between the two dems the 7 m difference is partly attributed to the differential penetration of the electromagnetic waves visible for hrs microwave for srtm in snow and vegetation we also report on the application of sequential dems srtm dem in february 2000 and hrs dem in may 2004 for the monitoring of glacier elevation changes we map the topographic changes induced by a surge of one tributary of ferris glacier maximum surface lowering of 42 10 m and rising of 77 10 m are observed in the 4 years time interval thinning rates up to 10 2 5 m yr are observed at low altitudes and confirm the ongoing wastage of glaciers in south east alaska

diagenetic uptake of rare earth elements by bioapatite with an example from lower triassic conodonts of south china abstract the distribution patterns of rare earth elements rees are frequently used as proxies for ancient seawater chemistry or paleomarine environmental conditions however recent work has shown that diagenesis can lead to remobilization and inter elemental fractionation of rees and that these effects often occur in conjunction with redox reactions in sediment porewaters here we review existing literature on the diagenetic fluxes of rees in marine sediments and porewaters in order to systematize existing knowledge on this subject rees undergo significant redistribution among sediment phases during both early and late diagenesis as a consequence of adsorption and desorption processes remobilization of rees commonly leads to inter elemental fractionation variously leading to enrichment or depletion of the light middle or heavy ree fractions further ree remobilization can be facilitated by redox changes e g through reductive dissolution of host phases in suboxic and anoxic porewaters characteristic ree distribution patterns develop through these processes 1 a flat distribution signifying predominantly terrigenous siliciclastic influence 2 a middle ree bulge probably due to adsorption of light and heavy rees to mn and fe oxyhydroxides respectively and 3 heavy ree enrichment indicative of hydrogenous seawater influence note all patterns in this paper are normalized to the ree composition of average upper continental crust or ucc in the second part of this study we undertake an analysis of the ree distributions in conodonts and whole rock samples from west pingdingshan a permian triassic boundary section in south china using ree th and y ho ratios we show that almost all of the conodont samples have a strong diagenetic overprint and that the hydrogenous ree fraction is small and not isolatable furthermore the conodonts contain two diagenetic ree components one characterized by low ree 100 300 ppm high ree th ratios 1000 strong middle ree enrichment and eu eu ratios of 1 5 2 0 and the second by high ree 300 2000 ppm low ree th ratios 20 30 little or no middle ree enrichment and eu eu ratios of 1 0 the first component exhibits a pronounced middle ree bulge that represents an early diagenetic signature associated with suboxic conditions possibly related to adsorption of rees onto fe mn oxyhydroxides in the shallow subsurface environment the second component shows a flat ree distribution that is similar to both our whole rock samples and average ucc indicating derivation from rees released from detrital siliciclastics e g clay minerals probably at a range of burial depths from shallow to deep failure of the conodont samples to yield an isolatable hydrogenous component demonstrates that bioapatite does not always preserve a primary marine ree signature given that bioapatite rees have been widely used for this purpose often on the assumption of minimal or no diagenetic influence our findings are likely to necessitate a re evaluation of the results of many earlier studies in general we counsel caution in inferring a hydrogenous origin for rees in bioapatite owing to frequent diagenetic alteration of ree distributions

stratigraphic correlation by integrating photostratigraphy and remote sensing multispectral data an example from jurassic eocene strata northern somalia integrated analyses of aerial photographs and multispectral remote sensing images were used for stratigraphic correlation in mainly carbonate and evaporitic rocks these rocks crop out in an area of northern somalia characterized by an arid climate by the aerial photo analysis we recognized photostratigraphic logs and stratal patterns and established correlations based on the tracing of physical surfaces with chronostratigraphic significance such as photohorizons and photostratigraphic discontinuities a limited number of field sections provided the lithological interpretation of the packages of strata delineated in aerial photos by satellite multispectral landsat thematic mappertm data analysis we identified image facies that represent packages of strata with different lithological characteristics to interpret the image facies we compared the responses in the thematic mapper tm bands with the laboratory spectroscopic properties of rock samples from the study area and interpreted the absorption features by p trographic analysis the mesozoic and tertiary strata analyzed herein are part of several formations deposited on a passive margin preceding the oligocene miocene gulf of aden rifting and initial drifting following this approach a number of stratigraphic units were recognized and mapped on aerial photos and a framework of photostratigraphic correlation surfaces was delineated over significantly wide areas these surfaces approximate time surfaces and are traced both within and across the lithostratigraphic units improving existing maps this method represents a mapping tool preliminary to more detailed field work and is particularly useful in areas of difficult access

preliminary spectral and geologic analysis of landsat 4 thematic mapper data wind river basin area wyoming a landsat 4 thematic mapper tm image covering the wind river basin area wyoming is under evaluation for stratigraphicand structural mapping and for assessment of spectral and spatialcharacteristics using the six visible near infrared and short wavelengthinfrared bands the image path 36 row 30 id 40128 17232 was acquired on november 21 1982 the data were obtained from the nasa goddard space flight center in radiometrically and geometrically correctedfullscene magnetic tape format to estimate equivalent lambertian surface spectral reflectance tmradiance data must first be calibrated to remove atmospheric andinstrumental effects reflectance measurements for homogeneousnatural and cultural targets in the scene were acquired during the period october 27 november 3 1983 about one year after satellite dataacquisition scatterplots were prepared of image dn versus reflectancefor these sites the scatterplots show the tm sensor system response islinear for the conditions of image acquisition and for the mix of terraincover types encountered low tm offset and gain settings result in encoded spectral data thatdo not occupy the full dynamic range 256 gray levels of the tm calibration data acquired during the study were used to calculate new gainsand offsets that would improve scanner response for earth scienceapplications analysis demonstrates that principal component images provideuseful structural and stratigraphic information principal componentscalculated from the correlation matix result from linear transformationsof ground reflectance in images prepared from this transform theseparation of spectral classes is largely independent of systematicatmospheric and instrumental factors

recent rapid thinning of the mer de glace glacier derived from satellite optical images the rapid wastage of mountain glaciers and their contribution to sea level rise require worldwide monitoring of their mass balance in this paper we show that changes in glacier thickness can be accurately measured from satellite images we use spot image pairs to build digital elevation models dems of the mont blanc area french alps for different years to register the dems we adjust their longitude latitude and altitude over motionless areas the uncertainty of the thickness change measurement is greatly reduced by averaging over areas covering altitude intervals of 50 m comparisons with topographic profiles and a differential dem from aerial photographs obtained on the mer de glace indicate an overall accuracy of 1 m for the thickness change measurement below 2100 m satellite dems show an evolution of the thinning rate from 1 0 4 m a years 1979 1994 to 4 1 1 7 m a 2000 2003

mobility of the high field strength rare earth elements and yttrium during late diagenesis authigenic titanite occurs as a secondary pore filling cement in the permian sandstones of the cock of arran scotland the titanite is apparently not associated with in situ alteration of a specific mineral phase or phases suggesting it crystallized from fluids migrating through these porous and permeable sandstones the titanite contains significant concentrations of zr nb p ce nd gd and y implying that these elements were also carried in solution probably in the form of halogen or carbonate complexes associated with hydrothermal circulation related to the intrusion of the adjacent northern granite

effects of basic intrusions on ree mobility of sandstones and their geological significance a case study from the qianjiadian sandstone hosted uranium deposit in the songliao basin abstract ree geochemistry of sandstones and their carbonate cements from the upper cretaceous yaojia formation in the qianjiadian uranium deposit southern songliao basin are illustrated in order to reveal effects of basic intrusions on ree mobility of sandstones and their geological significance compared with the sandstones far away from the diabase dikes the sandstones near the diabase dikes are obviously depleted in rees lree content hree content and ree content all increase with an increase of the distance away from the diabase dikes carbonate cements in the sandstones comprise of dolomite siderite ankerite and calcite which are successively formed from the early to late stage in the sandstones near the diabase dikes the dolomite and siderite are partly dissolved and the ankerite is accumulated lree content and hree content are both gradually decreasing in turn from the dolomite to siderite to ankerite and to calcite dolomite content is positively correlated with hree content siderite content is positively correlated with lree content or hree content and ankerite content is inversely correlated with lree content dissolution of the dolomite and siderite and formation of the ankerite were aroused by intrusion of the diabase dikes resulting in the sandstones near the diabase dikes being depleted in rees the study suggests that high grade uranium mineralization may be attributed to the diabase dikes ree depletion of the sandstones can be severe as a new prospecting indicator and sandstones affected by igneous intrusions should be avoided during sampling for sandstone provenance by rees

rare earth element geochemistry of australian paleozoic graywackes and mudrocks provenance and tectonic control abstract in the paleozoic graywackes of eastern australia there is an increase in the total abundance of rare earth elements ree a light to heavy ree ratio and a decrease in the chondrite normalized eu anomaly with the increase in sio2 al2o3 and k2o na2o ratios this is due to the change in the dominant source rocks from andesite to dacite to granite gneiss and sedimentary rocks similarly an increase in total ree and a decrease in eu anomaly is also seen in the associated mudrocks with the increase in total clay content this correlation in paleozoic sedimentary rocks suggests that the ree characteristics of terrigenous sedimentary rocks reflect the depositional tectonic setting of the basin the oceanic island arc type graywackes are characterised by their lower total ree abundance only slight enrichment of lree over hree and the absence of a negative eu anomaly on chondrite normalised plots the continental island arc type graywackes are discriminated by their higher ree abundance and la yb ratio the presence of a small negative eu anomaly on the chondrite normalised plots and the significant enrichment of eu compared to paas post archean average australian shale the sedimentary rocks deposited on andean type active continental margins passive margins platform and cratonic basins are all characterised by the high enrichment of lree over hree and the presence of a pronounced negative eu anomaly on chondrite normalised plots and can not be discriminated from each other by ree patterns alone

thinning of the south dome of barnes ice cap arctic canada over the past two decades between 1970 and 1984 ground surveys were carried out along a flowline extending from the top of the south dome of barnes ice cap to the margin over this time span the ice cap thinned an average of 1 7 m or 0 12 m yr 1 by comparing the 1984 survey with elevations derived from satellite imagery in 2006 we find that it has now thinned an additional 16 8 7 7 m or an average of 0 76 0 35 m yr 1 laser altimeter profiles show that between 2004 and 2006 the thinning rate was 1 0 0 14 m yr 1 a correlation between mass balance and mean summer temperature at nearby weather stations developed over the period of the ground surveys permits independent estimates of the thinning rate these estimates are in excellent agreement with those based on satellite imagery the acceleration in thinning is consistent with meteorological records documenting an increase in the number of positive degree days atmospheric warming in the region

icesat validation of srtm c band digital elevation models 1 the geoscience laser altimeter system glas on the ice cloud and land elevation satellite icesat provides a globally distributed data set well suited for evaluating the vertical accuracy of shuttle radar topography mission srtm digital elevation models dems the horizontal error 2 4 7 3 m and vertical error 0 04 0 13 m per degree of incidence angle for the icesat data used are small compared to those for srtm using glas echo waveforms we document differences between the srtm c band phase center and the highest centroid and lowest elevations within icesat laser footprints in the western united states in areas of low relief and sparse tree cover the mean and standard deviation of elevation differences between the icesat centroid and srtm are 0 60 3 46 m the differences are 5 61 15 68 m in high relief sparse tree cover areas and 3 53 8 04 m in flat areas with dense tree cover the largest differences occur in rugged densely vegetated regions