Geodetic control is extremely important in the production and quality control of topographic data sets, enabling elevation results to be referenced to an absolute vertical datum. Global topographic data with improved geodetic accuracy achieved using global Ground Control Point (GCP) databases enable more accurate characterization of land topography and its change related to solid Earth processes, natural hazards and climate change. The multiple-beam lidar instrument that will be part of the NASA Deformation, Ecosystem Structure and Dynamics of Ice (DESDynI) mission will provide a comprehensive, global data set that can be used for geodetic control purposes. Here we illustrate that potential using data acquired by NASA's Ice, Cloud and land Elevation Satellite (ICEsat) that has acquired single-beam, globally distributed laser altimeter profiles (+/-86deg) since February of 2003 [1, 2]. The profiles provide a consistently referenced elevation data set with unprecedented accuracy and quantified measurement errors that can be used to generate GCPs with sub-decimeter vertical accuracy and better than 10 m horizontal accuracy. Like the planned capability for DESDynI, ICESat records a waveform that is the elevation distribution of energy reflected within the laser footprint from vegetation, where present, and the ground where illuminated through gaps in any vegetation cover [3]. The waveform enables assessment of Digital Elevation Models (DEMs) with respect to the highest, centroid, and lowest elevations observed by ICESat and in some cases with respect to the ground identified beneath vegetation cover. Using the ICESat altimetry data we are developing a comprehensive database of consistent, global, geodetic ground control that will enhance the quality of a variety of regional to global DEMs. Here we illustrate the accuracy assessment of the Shuttle Radar Topography Mission (SRTM) DEM produced for Australia, documenting spatially varying elevation biases of several meters in magnitude

Carabajal, Claudia C.

Harding, David J.

Suchdeo, Vijay P.

NASA Technical Reports Server

ICESAT LIDAR AND GLOBAL DIGITAL ELEVATION MODELS: 
APPLICATIONS TO DESDYNI 
Claudia C. Carabajal l , David J. Hardini and Vijay P. Suchdeo l 
I Sigma Space Corp. @ NASA Goddard Space Flight Center, Planetary Geodynamics Laboratory, Code 
698, Greenbelt, MD 20771, USA. 
2 NASA Goddard Space Flight Center, Planetary Geodynamics Laboratory, Code 698, Greenbelt, MD 
20771 , USA. 
ABSTRACT 
Geodetic control is extremely important in the production 
and quality control of topographic data sets, enabling 
elevation resu lts to be referenced to an absolute vertical 
datum. Global topographic data with improved geodetic 
accuracy achieved using global Ground Control Point 
(GCP) databases enable more accurate characterization of 
land topography and its change related to solid Earth 
processes, natural hazards and climate change. The 
mu ltiple-beam lidar instrument that will be part of the 
NASA Deformation, Ecosystem Structure and Dynamics of 
[ce (DESDynI) mission will provide a comprehensive, 
global data set that can be used for geodetic control 
purposes. Here we illustrate that potential using data 
acquired by ASA's Ice, Cloud and land Elevation Satellite 
(ICEsat) that has acquired single-beam, globally distributed 
laser altimeter profiles (± 86°) since February of 2003 [1, 2]. 
The profiles provide a consistently referenced elevation data 
set with unprecedented accuracy and quantified 
measurement errors that can be used to generate GCPs with 
sub-decimeter vertical accuracy and better than 10 m 
horizontal accuracy. Like the planned capability for 
DESDynl, ICESat records a waveform that is the elevation 
distribution of energy reflected within the laser footprint 
from vegetation, where present, and the ground where 
illuminated through gaps in any vegetation cover [3]. The 
waveform enables assessment of Digital Elevation Models 
(DEMs) with respect to the highest, centroid, and lowest 
elevations observed by ICESat and in some cases with 
respect to the ground identified beneath vegetation cover. 
Using the ICESat altimetry data we are developing a 
comprehensive database of consistent, global, geodetic 
ground control that will enhance the quality of a variety of 
regional to global DEMs. Here we illustrate the accuracy 
assessment of the Shuttle Radar Topography Mission 
(SRTM) DEM produced for Australia, docmnenting 
spatially varying elevation biases of several meters in 
magnitude. 
Index Terms- ICESat, global geodetic control, laser 
altimetry, DEM accuracy, elevation errors, DESDynI 
1. INTROD UCTION 
Accurate laser altimeter elevation profiles contribute to a 
number of Solid Earth science and applied objectives. A 
primary contribution is the independent characterization of 
systematic and random elevation errors in Digital Elevation 
Models (DEMs) produced by photogrammetric and 
Interferometric SAR techniques. This kind of quality 
assessment enables OEMs to be used quantitatively for 
purposes for which their accuracy is appropriate. In addition 
to evaluating DEM accuracy, laser altimeter profiles can be 
used in the correction of systematic errors in OEMs, im-
proving their uti lity for detection of elevation change 
observed by differencing DEMs obtained at different times. 
NASA's Deformation, Ecosystem Structure and Dynamics 
of Ice (DESDynl) mission, scheduled for launch later this 
decade, will provide globally distributed laser altimeter 
profiles well suited for these purposes. It will 
simultaneously acquire multiple profiles with 25 m 
dianleter, nearly-contiguous laser footprints along the 
profiles. Although NASA' s Ice, Cloud and land Elevation 
Satellite CICESat) has less dense sampling (a single profile 
with - 50 m footprints spaced 175 m apart), it illustrates the 
capabi lities for accuracy assessment and contro l of DEMs 
that will be significantly expanded by DESDynI. 
We have previously used ICESat data to evaluate the 
accuracy of the 90 m resolution, near-global DEM produced 
by the Shuttle Radar Topography Mission (SRTM) [4, 5, 6] 
and the more recent global ASTER DEM [7]. The 
Geoscience Laser Altimeter System (GLAS) aboard ICES at 
acquired data from 2003 to 2009, during month-long 
observant periods that were conducted three, and later two, 
times a year. Figure 1 shows an example assessment of 
SRTM accuracy using an ICESat profi le. The negative-
elevation differences reveal that the SRTM DEM is biased 
high relative to an abso lute datum by several meters, on 
average, across western Australia. In addition, the along-
profile variations reveal undulating elevation errors in the 
SRTM DEM at the 100s ofk:ilometer length scale and - 5 m 
amplitude. ICES at data documents the spatia l structure of 
these undulations, which can then be removed from the 
DEM. The comprehensive, near global (± 86°) ICESat 
https://ntrs.nasa.gov/search.jsp?R=20100026470 2019-08-30T10:16:03+00:00Z
coverage across continents enables correction of long-
wavelength DEM errors not previously made possible by 
other means. Here we present results for along-profile and 
gridded elevation differences between ICES at geodetic 
contro l data and the SRTM DEM for Australia, 
documenting tbe spatia l pattern and magnitude of elevation 
errors in that DEM. We do this using each ICESat 
observation period separately, documenting that the ICESat 
results are bighly reproducible. 
Western Auslraiia. 
17 Nov 2003 1051101 1:()2 Pass 21030020397 ! =~. ICE Sa £: .81 >II Pr I . 
g 10)00 tJ 
~ 40000 · ... A .. 
J1 >DO .. S lart and End Lal: -33.919N. · 20.703N 
w 00000 ~ Start and End Lon 11 8.4geE. 117 432E 
E 
1223:JIIOO00 I~OO I~OO 
J2OOOUTC_ 
i 10000 ICE at . RTM Elevalioru 
~ 50000 
~ O~~~~~~~~~~~I'" 
I.....,. .. I~OO 12233I8l0000 
J2OOOVTC ..... I~" 
MODIS False-<:Otor ComposIte 
- 1300 km (Aqua. 20051200 · 07119 at 05.55 UTC) 
Figure I. Elevation measurements (top right) along an TCESat 
profile in western Australia (left, red line on MODIS image) and 
differences with the SRTM DEM (bottom right) for individual 
ICESat laser footprints. 
2. DEVELOPMENT OF A GLOBAL ICESAT 
GEODETIC CONTROL DATABASE 
Using the ICESat Land/Canopy Elevation data product 
(GLAI4), we are producing a global set of GCPs in a 
project supported by NASA's Earth Surface and Interior 
Program. We app ly stringent editing criteria in order to 
produce the highest quality ground control. For tbis paper, 
we have used the latest public release of the ICESat 9 I-day 
repeat track data (Release 31). Like the DESDynI lidar will 
do, the GLAS instrument records a waveform, the height 
distribution of energy reflected from illuminated vegetation 
and ground surfaces in the laser footprint. In addition to 
providing the latitude, longitude and derived elevations for 
each ICESat waveform referenced to the Topex/Posseidon 
(TIP) ellipsoid, the GLAI4 product also includes the SRTM 
DEM elevation at the footprint location. The publically 
distributed finished SRTM DEM contains orthometric 
elevations produced using the EGM96 geoid [8). On the 
GLA14 Release 31 product that we are using the SRTM 
elevations have been converted to ellipsoidal values, but by 
incorrectly applying the more recent EGM08 geoid. We 
therefore correct the SRTM ellipsoidal heights in the 
products using the SRTM reference geoid, EGM96. We then 
compute the elevation differences between the SRTM radar 
phase center elevation and four elevations derived from the 
ICES at waveform: the highest observed surface (wavefornl 
start), the waveform centroid (average elevation), the lowest 
observed surface (waveform end) and, where possib le, the 
ground elevation inferred from the lowest Gaussian fit 
derived from waveform modeling. We also obtain a land 
cover classification for eacb laser footprint from tbe MERIS 
Globcover land cover regional product (51 possible classes), 
derived by an automatic and regionally-tuned classification 
of a MERIS FR time series [9]. The MERIS product is 
generated at 300 m resolution using data from the period 
December 2004 - June 2006. 
We apply stringent editing criteria to yield a high 
quality GCP database. We exclude ICESat data identified as 
returns from water (MERIS land cover = 210). We also 
exclude ICESat retums inferred to be from clouds using a 
threshold 50 m above the SRTM surface at tbe laser 
footprint location. The cloud flag in the ICESat products 
indicating a clear atmosphere proved too severe for editing 
and was not reLiable for periods when the transmit laser 
energy was low. One of the main criteria for obtaining the 
most accurate ground control data is selection of waveforms 
from relatively non-vegetated areas with low relief. We do 
so by limiting the width of the selected waveforms (signal 
start to signal end) to 5 m. For these narrow waveforms the 
elevations for the waveform centroid and tbe ground lowest 
Gaussian fit essentially coincide. 
We also apply editing based on instrumental 
parameters. Weak returns are excluded by only using returns 
with maximum ampl itudes greater than 0.15 Volts. Where 
the return energy exceeds the receiver's dynamic range, the 
GLAS waveforms become saturated. Saturation broadens, 
distorts and truncates the received waveforms. Although 
laboratory calibrations developed as a function of receiver 
gain and observed received energy exist to correct 
elevations and receive energies, we only keep non-saturated 
to minimally saturated data where the saturation index (the 
number of waveform bins with an amplitude greater than the 
saturation threshold) is two or less. The accuracy of the 
ICESat data is degraded with increasing incidence angle 
between tbe laser beam vector and the normal to the surface 
slope, causing waveform broadening. To minimize this error 
source, we exclude data acquired when the laser beam was 
pointed off from nadir by more than 10. Rigorous analysis 
has shown that for low relief locations the ICESat data meet 
the accuracy requirements of 6 m horizontal and J 0 cm 
vertical [So Luthcke, pers. corum.]. Because we use 
stringent editing criteria we expect that our GCPs are of 
equivalent accuracy. 
Table J presents elevation difference between our 
lCESat GCPs and SRTM, computed as ICES at minus 
SRTM, for Australia. Resu lts are ~iven for ICES at 
observation periods acquired with the 2n (L2) and 3rd (L3) 
laser transmitters, with tbeir start and end dates and average 
laser transmit energies ind icated (for each laser tbe transmit 
energy started high, causing substantial saturation, and then 
dropped significantly during the course of the mission) . 
Figure 2 illustrates ICESat minus SRTM difference 
histograms for ICESat's highest, centroid and lowest 
elevations using a representative observation period (L3E), 
showing well -defined normal distributions. Like the other 
periods, this period shows an SRTM elevation bias with 
respect to the lCESat centroid of - -2 m . 
Laser Period D N* N M S R 
L2A H 121981S 106868 0.66 3.53 2.46 
10101103· 11119103 C 8.71\'" I06R6X - 1.86 3.68 2.96 
E (fJ )·· G 106868 - 1.92 3.66 3.00 
70.7 L 106868 -3 .87 3.53 4.50 
L2B H 891 185 274844 0.27 5 .72 2.28 
2117/04-3/2 1104 C 30.84% 274844 - 1.96 5.46 2.98 
E (fJ) G 274844 - 1.98 5.48 2.99 
45.6 L 274844 -3.96 5.72 4.56 
L2C H 9O~5 11 -t73S96 -0 .10 8.03 237 
5/1 1l1O-l-6/21IQ4 C 52. lOt;< 473896 -2.11 7.75 3. 16 
£ (fJ) G 473896 -2.14 7.70 3.19 
12.5 L 473896 -3 .89 8.03 456 
L3A II 1098000 327543 0 .32 4.S9 2.26 
10/3/04- 1I181O-t C 29 .83'K 327543 - 1.95 4.71 2.95 
£ (fJ ) G 327543 -1.96 4.72 2.96 
63.7 L 327543 -3 .98 4.89 4.56 
L3B H 979204 316537 0.19 3. 13 2.23 
2117/05-3/24105 C 32.33£K 316537 -2.0 1 3.04 2.98 
E(fJ) G 316537 -2.0 1 3.02 2.98 
59.1 L 3 16537 -4 .02 3.13 4.58 
L3C H 9-t8062 387 123 0. 11 2.74 2.23 
5/20/05-6/23/05 C -to.83'k 387123 - 1.97 2.7-t 2.96 
£ (fJ ) G 387 123 - 1.96 2.73 2.96 
45 .5 L 387 123 -3.94 2.7-t 4.52 
L3D H 99516.t 452400 0 .00 3.92 2.23 
10/21/05-11124/05 C -t5.46't 452400 2.07 3.71 1.05 
£ (f.l ) G 452400 -2.(16 .1.69 .>.()..I 
39.4 L -t52.tOO -I.O-t 3.92 -t .63 
L3E H 83570 1 380954 -0 .06 5.03 2.23 
2122106-3/28106 C 45.5S% 380954 -2 .08 -t .70 3.05 
E(fJ) G 38095-t -2 .07 4 .67 3.05 
34.1 L 3S()954 -4 .05 5.03 4.63 
L3F H 1036357 -t38~87 0.09 5.09 2.25 
5124/06-6/26/06 C 42 .36'k -t38987 -2.00 4.8-t 3.01 
E (fJ ) G -t38987 - 1.99 -t .83 3.00 
30.8 L 43X9R7 -4JI I 5.()9 4.59 
L3G H 969972 455341 om 3.58 2.18 
10/25106- 1 1127106 C 40 .94c,,· 455.14 1 -2.0n 3.49 3.07 
E (fJ ) G 455341 -HJ5 3.47 3.06 
27. 1 L 45534 1 -1M 3.58 4.65 
L3H H 883219 422979 -0 .05 5.05 2.16 
3/ 12107-4114107 C 47.89% 422979 -2.09 4.89 3.08 
E(f.J) G 422979 -2.08 4.87 3.08 
22.6 L 422979 -4 .06 5.05 4 .65 
L3/ H 967652 488675 -0.15 3.85 2?7 
1012107-H /S/07 C 5050<:f 4S8675 -2 .15 3.69 3.13 
E r G 48M675 -1. 13 3.69 3.12 
20.5 L 488675 -4.12 3.85 .\ .72 
L3J H 922899 447482 -0.13 4. 16 2.24 
2117/08-3/2 1/08 C 48.49<;;- 447482 -2.13 3.89 3.09 
E(f.f) G 447482 -2.1 1 3.87 3.08 
17.7 L 447482 -4.1 1 4.16 4.69 
L3K H 450592 231 p' -0.23 7.10 2.38 
10/4108-I 011 9/08 C 51.291i- 231122 -2.20 6.72 3.23 
E ([I i G 231122 -2 .17 6.70 3.22 
15.6 L 231122 -4 .16 7. 1() -t .7'> 
L2D H 554068 37405 1 -0 .44 7.00 2.44 
11125/08-12/17/08 C 67.51 % 37.t05 1 -2.3.\ 7.34 3.36 
£ r G 374051 -2.38 7 '9 3.39 
5.4 L 374051 -1 .06 7.00 -t .73 
Table J. ICESat - SRTM E levation Difference Statistics 
D = Differences (H: Highest; C: Centroid; L: Lowest; G: ground) 
N* = Number ofretums classified as be from the Earths's surface 
and the percent retained after editing 
= Number of returns after editing 
M =Mean elevation difference; S = standard deviation; R = RMSE 
E (fJ)** = Average transmit energy corrected for receiver field of view 
(FOY) shadowing 
1 2 
centroid 
highest 
lowest 
~ 8 
g 
~ 
u: 
4 
0 
Ali ~ 
- 12 -8 -4 0 4 8 12 
'CESa' . SRTM (ml 
Figure 2. Elevation difference histograms for lCESat higbest, 
centroid and lowest minus SRTM for tile L3E laser ob ervation 
period (See Table J for complete statistics). 
Examination of the centroid differences for all laser 
periods (Figure 3) shows very consistent mean e levation 
difference resu lts, a demonstration of ICESat's highly 
accurate and reproducible abso lute elevations . There is a 
slightly decreasing trend with laser energy decay, especially 
for Laser 2 (L2), potentially related to the large number of 
edited saturated returns during early, high energy periods. 
This could cause a bias by preferentially removing areas of 
high surface reflectivity. The origin of this L2 drift and the 
associated increase in standard deviation requ ires further 
investigation. 
6 .r------=~----------------------------__. 
4 
-6 
-6 
-10L-----~~~--------------------______ ~· 
1.2A L2B L2C L20 L3A L3C laO L3E L3F l3G L3H L31 L3J l3K 
ICESat laser observation periods 
Figure 3. Mean TCESat centroid minus SRTM elevation 
differences and standard deviations fOf all ICESat observation 
periods (See Table I for complete statistics). 
3. DER[VED CORRECTION SURFACES FOR 
AUSTRALIA 
After selecting the ground contro l database for each 
ICESat period, we obtain a representation of the undulations 
in ICESat centroid minus SRTM differences in two different 
ways. First, we apply a sliding boxcar fi lter along every 
edited ICESat profile at 10 intervals (Figure 4, top). 
Alternative ly, we divide the region in 10 by 10 ti les, and 
compute the mean differences and their standard deviations 
for the points within each tile (Figure 4, lower 3 panels). 
Both the along-profile and gridded representations h ighl ight 
long wavelength e levation-error undul ations in tbe SRTM 
DEM of several meters magnitude. 
L3E ICESat centroid - SRTM 
110· 120· 130· 140· 150· 
-10· -10· 
6 E 4 
-20· -20· ~ 2 a: 
f-
-30· -30· 0 
Cf) 
-2 til 
Cf) 
-40· -40· -4 w 0 
-6 
L3E number of points 
110· 120· 130· 140· 150· 
-10 · -10· 1000 
800 N 
-20· -20· 
600 P 
0 
-30· -30· 400 i n 
t 
-40· -40· 200 s 
0 
L3E mean differences 
110· 120· 130· 140· 150· 
-10· -10· 
6 E 
4 Cf) 
-20· -20· Q) 
2 () c: 
-30· -30· 0 
Q) 
& 
-2 is 
c: 
-40· -4 ro -40· Q) 
-6 ~ 
L3E differences sdev 
110· 120· 130· 140· 150· 
-10· -10· 5 
4 
-20· -20· 
3 
.s 
-30· -30· > 2 Q) -0 
Cf) 
-40 · -40· 
o 
Figure 4. Mean ICES at centroid - SRTM differences fo r the L3E 
observation period. Differences computed us ing a 1° along track 
sliding box-car fil ter are shown on the top, and 1° gridded averages 
and thei r standard deviations for the indicated number of retu.rns in 
a cell are sbown in the three lower panels. Colors indicate ranges 
shown by the color bars. 
4. CONCLUDING REMARKS 
AND APPLICA TlON TO THE DSDYNI MISSION 
P rofi ling spacefl ight laser a ltimeters make important 
contributions to the soLid Earth Sc ience by providing 
accurate, global, consis tently referenced , geodeti c e levatio n 
data. Expand ing upon the Austra.lia work described here, 
our ICESat GCP dataset is contributing to a g loba l, 
coordinated and integrated D EM database produced from 
diffe rent sources, embedded into a consiste nt, high 
accuracy, and long term stable geodetic reference frame. It 
wi U be particularly useful in northern and southern latitudes 
above and below ± 60°, wbere bigb-reso lution top ographic 
data assets are not available and topographic contro l is 
scarce. M etbodo logies develo ped to use ICESat data fo r 
g lobal geodetic contro l purposes are a pathfinder fo r s imilar 
use of data to be produced by the lidar co mponent of the 
DESDy nI m iss ion. With substan tia lly improved sampling 
as compared to ICESat, D ESDynl w ill provide a more 
comprebensive set of g loba l GCPs from it's multi-beam, 
bigher-reso lution e levation profiles. 
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