we2.l09 - icesat lidar and global digital elevation models: applications to desdyni

Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010 Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010

1 Sigma Space Corp. @ NASA/GSFC – Planetary Geodynamics Laboratory 2NASA/GSFC - Planetary Geodynamics Laboratory

Claudia C. Carabajal1, David J. Harding2,

and Vijay P. Suchdeo1

2 Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010

Primary Objectives Ice sheet elevation change Sea ice thickness change

Secondary Objectives Cloud and aerosol profiles

Geodetic land topography profiles Forest canopy height sampling

Globally-distributed Repeated Profiles Geoscience Laser Altimeter System (GLAS)

Footprint: ~70 m (lasers 1 & 2), ~50 m (laser 3) Along-track spacing: 170 m

Vertical Precision: 3 cm (flat surfaces) Vertical Accuracy: ~10 cm (flat surfaces)

Horizontal Accuracy: < 6 m

Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010

• The high accuracy of the ICESat elevation measurements in a consistent reference frame provides a unique, globally distributed Ground Control Point (GCP) data set

Vertical Accuracy: 10 cm (flat surface) Horizontal Accuracy: < 6 m

• Three main applications of ICESat geodetic control are:

Independent assessment of the accuracy of DEMs defining their random and systematic error characteristics.

Correction of systematic errors in DEMs improving their utility scientific and applied purposes including detection of elevation change

Use as ground control points in the production of DEMs either by stereo photogrammetric or interferometric SAR techniques


Observation Period

Laser Energy Corrected for FOV Shadowing Effects

2A 2B 2C 3A 3B 3D

3G

3J 3H

3F 3C

3E

2E 2D

3I 3K

2A 1A

Laser 2 – 91 day Laser 3 – 91 day L1 & L2 8-day

ICESat was in a precisely repeated orbit (±86°), acquiring data along the same 491 orbit tracks in ~33-day long periods.

Laser energy dropped significantly during the course of the mission M

ean

per

Puls

e En

ergy

(mJ)

Laser 2

Laser 3


•  ICESat Land/Canopy Product (GLA14), Release 31 GLAS waveform-derived elevations

highest detected signal signal centroid (average) inferred ground peak lowest detected signal

Each Laser 2 and 3 month-long observation periods used separately to assess reproducibility of the results

•  SRTM Finished Product DEM elevation interpolated to laser footprint location, provided on GLA14

geoid corrected to be in ICESat reference frame Elevation standard deviation (relief) from 3 x 3 cells at footprint location

•  ESA’s MERIS Globcover Global land cover at 300 m resolution (Regional products) 51 land cover classes possible

6 Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010 After Harding & Carabajal, 2005.


•  Surface returns not from cloud tops ICESat - SRTM DEM elevations < 50 m

•  Non-saturated returns Saturation index ≤ 2

•  Data acquired near nadir Incidence angle ≤ 1°

•  No potential range delay due to atmospheric forward scattering When correction available, in the mm range

•  No broadened returns from high relief or vegetation cover Width ≥ 0.5 m and ≤ 5 m

Stringent editing applied to identify appropriate returns: Low within-footprint slope and roughness Vegetation absent or very low stature Not impacted by measurement artifacts


•  Negative elevation differences: SRTM biased high relative to ICESat absolute datum by several meters, on average, across western Australia.

•  The along-profile variations reveal undulating elevation errors in the SRTM DEM at the 100s of kilometer length scale and ~5 m amplitude.


•  We quantify differences between ICESat GCP’s and SRTM along the ICESat ground tracks using a sliding 1 degree box-car filter.

•  We compute average 1 degree gridded ICESat-SRTM elevation differences.

•  We evaluate spatial patterns of mean elevation differences (biases) and standard deviations (noise component).

•  We do this using each ICESat observation period separately, testing the reproducibility of ICESat elevation measurements with different laser energies.

•  We include topographic relief and land cover information to establish empirical relationships between ICESat - SRTM elevation differences with respect terrain characteristics.


•  Difference histograms for ICESat’s highest, centroid, inferred ground and lowest elevations show well-defined normal distributions.

•  ICESat centroid and inferred ground are essentially equivalent for the narrow waveforms selected by editing

•  SRTM elevation bias ~ 2 m above ICESat’s centroid.

-10 0 10

Highest Centroid Ground Lowest

0

5

10

15

ICESat – SRTM Elevation (m)

Freq

uenc

y (%

)


The along-profile smoothed differences show long wavelength undulations in the SRTM DEM, of several meters magnitude, that are consistent for all observation periods and lasers.

L3A L3D L3G

L2A L2B L2C


Along-track differences show large wavelength undulations (100s of kilometers) for the various periods, not correlated with relief.

The along-track differences are independent of the ICESat observation period, and are therefore characteristic of SRTM.


Residual height error of the SRTM X-band DEM.

(a)  Error along a particular data take acquired over the pacific for calibration purposes.

Shown is the band of the relative and absolute vertical accuracy requirement.

(b) Schematic distribution of SRTM error sources across spatial scales in azimuth direction.

The largest error contribution comes from roll angle firings used to counteract the torque exerted on the mast by the earth gravity field gradient.

Rabus et al., 2003


points/cell mean st. dev.

minimum rmse maximum

0 1000 -20 20 0 10

0 10 -20 20 -20 20


•  Centroid differences for all laser periods show very consistent means of ~ -2m, a demonstration of ICESat’s highly accurate and reproducible absolute elevations.

•  There is a slightly decreasing trend with laser energy decay, especially for Laser 2. It is not related to editing of saturated returns during high energy periods.

•  The origin of this ICESat L2 drift and the associated increase in standard deviation requires further investigation.

Laser 2 Laser 3

-2 m

-6 m

2 m

18 Carabajal et al. - IGARSS 2010 - Honolulu, HI, July 25-31, 2010 Sparse Vegetation Bare areas

grassland/short stature vegetation

Cropland/grass/shrubs

Water

sparse vegetation

cropland/grass/shrubs

grassland/short stature vegetation bare

areas


Histograms of differences between ICESat and SRTM 90 m elevations at the ICESat footprint locations for bare ground land cover. The Mean and Standard Deviation of the distribution are -1.91 m and 2.12 m, respectively, for a population of 46271 laser returns.

-5m

5m

0m

Mean: -1.91 m St. Dev.: 2.12 m


Waveforms with narrow pulse-widths (0 to 5 m), are consistent with low relief surfaces having no or only short-stature vegetation cover, and are suitable for use as ground elevation control points.

Approximately 30%-35% of the data acquired in North America fits this criteria (however, a large fraction are at higher latitudes where the ground track spacing is smaller).

Narrow ICESat Waveforms L2B (Feb.-Mar., 2003)

5.0

4.5

4.0

3.5

≤3.0

Wav

efor

m P

ulse

Wid

th (m

)


Identify narrow last peaks in broad waveforms that are likely to be returns from the ground beneath the vegetation to

increase the number of global GCPs.

Use of last peaks as GCPs in vegetated terrain must be restricted to areas of low topographic relief due to the complex merging of ground and canopy returns in waveforms from areas moderate to steep relief. (Harding & Carabajal, 2005)


•  Using careful editing of ICESat elevation data, we are developing a Global Geodetic Control database for a variety of Solid Earth applications.

•  Edited data apply to locations of low relief and absent to short stature vegetation cover (< a few meters).

•  As an application of ICESat for Ground Control, we have performed a comprehensive analysis of the spatial distribution and magnitude of the ICESat - SRTM differences for Australia.

•  A negative mean difference of ~ 2 m (SRTM on average higher than ICESat) is observed for Australia, but there are regionally correlated mean differences that vary from about -10m to 5m. These might be associated with differences in land cover type.

•  We have investigated the repeatability of the results for all ICESat observation periods, exploring possible intra-period instrument/pointing biases remaining in the ICESat elevation data.

•  Identification of ground peaks in broadened waveforms will expand the number of GCPs for vegetated regions.


•  Methodologies developed to use ICESat data for global geodetic control purposes are a pathfinder for similar use of the data to be produced by the Lidar component of the DESDynI mission.

•  With substantially improved sampling as compared to ICESat DESDynI will provide a more comprehensive set of global GCPs

- Multiple beams spaced across track by ~ 1 km - Smaller footprints (25 m) that are contiguous along track - Continuous, rather than episodic, operation

•  Differencing the densely sampled DESDynI Lidar data through time with respect to a common DEM should reveal surface elevation changes at the decimeter level during the course of the mission on a local to regional (TBD) scales, including for surfaces that are decorrelated at radar wavelengths

E.G. seasonal snow accumulation; soil loss in agricultural regions

we2.l09 - icesat lidar and global digital elevation models: applications to desdyni

Documents

icesat centroid

icesat ground tracks

icesat gcps

icesat observation period

srtm elevation bias

productdem elevation

negative elevation differences

laser footprint location