A Merged Global Digital Topographic Data Set - Progress Report.pdf

A Merged Global DigitalTopographic Data Set

Jet Propulsion LaboratoryCalifornia Institute of Technology

IGARSS 2011

Dr. Michael KobrickDr. Robert CrippenDr. Thomas Farr

Jet Propulsion Laboratory

July 27, 2011



Objective:Generate best quality global digital elevation data set at highest possible resolution by combining best existing data

Supported by NASA *MEaSUREs program (M. Maiden)for enhancement of SRTM data set

* Making Earth Science Data Records for Use in Research Environments



Objective• Produce DEM of 80% of Earth surface! 30 meter posting! Complete coverage ± 60° latitude! WGS84 EGM96 geoid datum! 16 m vert., 20 m horiz. accuracy at 90% level

Mission

• Partnership between NASA, NGA, DLR and ASI• C-band single-pass interferometry

• X-band interferometer by German and Italian space agencies• Mission duration: Feb., 11 – Feb. 22, 2000 (11 days, 5 hours)

SRTM: Shuttle Radar Topography Mission



• SRTM coverage• 30 meter samples• ~ 80% coverage• Excellent continuity

• DTED1 coverage• 90 meter samples• ~ 60% coverage• Poor continuity

Topographic Coverage



Accuracy resultsAccuracy tests• NGA verification with DTED, other data sources• USGS comparisons to National Elevations Data sets, GPS arrays• Comparison to static and dynamic GPS elevations • Scores of open literature reports comparing to local data sets

All show data exceed 16 m vertical accuracy specs by factor of ~3

Rondonia, southwest Brazil

Elevations clearly show patterns of tropical forest clear-cutting with height differences of 4-5 meters



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Data distributed by Land Processes DAAC at EDC



Data Restrictions

Re original NASA/NGA Memorandum of Understanding, DEMs better than 3 arcsec outside U.S territory may not be distributed outside NASA

3 arcsec DEMs and any derived, ancillary or associated products may be distributed at any resolution, as long as they cannot be reverse-engineered to better than 3 arcsec DEMs



Under MEaSUREs NASA has funded effort to create SRTM-derived, enhanced, merged global data set

! • Void filled DEMs

! • Enhanced DEM control using ICESat profiles

! • Individual and mosaiced radar image data

! • Raw radar and ancillary data set

NASA MEaSUREs Program

NASA has funded effort to create SRTM-derived, enhanced, merged global data set







SRTM voids - cell average 0.52%, median 0.0023%




SRTM voids - cell average 0.52%, median 0.0023%

Many SRTM DEMs have voids

Voids filled with ASTER DEMOriginal SRTM

• Voids (areas with no data) caused by high slopes or extremely low SNR

• Can be filled with supplementary DEMs

• National Elevation Data (NED), 30m- Excellent quality- Available only for U.S., Mexico

• Canadian Digital Elevation Data (CDED), ~ 20m- Available for virtually all of Canada

• Other sources - various resolutions- SRTM / X-SAR- TanDEM-X- SPOT- Australian GEODATA Topo

• ASTER GDEM- Excellent coverage- Matches SRTM geoid, format, sampling- Some voids and artifacts



Potential Void Fill Sources

!

NED

CDED

X-SAR



With NASA support Silcast Corp. processed several million ASTER stereo pairs to produce a global DEM

GDEM1 released in mid-2009

- Matches SRTM in posting, datum, formatting- Coverage exceeds SRTM- Resolution ~ 120m, versus 50-60m for SRTM- Numerous artifacts due to cloud mask, stacking method

GDEM2 to be released in mid-2011

- Resolution improved by using smaller convolution kernel- Many artifacts remain

GDEM



SRTM coverage

GDEM coverage



GDEM2 - SRTM

Global LandCover

EGM96



SRTM ASTER GDEM

ComparisonSRTM ASTER GDEM1



SRTM GDEM1

Comparison



SRTM w/ void GDEM1



SRTM GDEM1



SRTM ASTER GDEM1



GDEM includes ancillary NUM file showing number of pairs contributing to each sample

GDEM1 GDEM NumSRTM

Capetown area, South Africa



GDEM2 ImprovementGDEM1: Original version delivered mid-2009

- Suffered from low resolution (~120 m), cloud and data stacking induced artifacts

GDEM2: Beta version being evaluated now- Improved resolution with smaller convolution kernel, many artifacts remain- Included quality files allow identification of most(?) artifacts

SRTM GDEM1 GDEM2 GDEM2 N>2



• Voiding samples with NUM < 3 seems to eliminate most artifacts

• Deletes ~ 7% of GDEM samples

• GDEM2 and SRTM voids only loosely correlated

NUM = 3

Distribution of NUMs in GDEM2



SRTM GDEM2 N>2

SRTM/GDEM2 Combo

Voids



• Comparisons of ICESat profiles w/ SRTM data have revealed possible bias at latitude extremes• Other published work suggests possible SRTM bias in high elevation glaciated terrain (this deserves more attention)• SRTM ocean residuals (should center on zero) show curious latitude dependence• Residuals are well within accuracy specs, but systematic nature is obvious• Important to study of glacier changes with implications for climate change

Malaspina

SRTM ocean residuals from unedited data Residual latitude dependence

ICESat Data



ICESat Data• M. Simard working with Carabajal, Sauber et al at GSFC to develop database of ICESat

topographic profiles for additional ground truth

• Requires careful modeling of laser echoes, differentiation of vegetation, bare Earth returns

• Will be applied as additional ground control for void-filled DEMs

• Processing using new control data to begin mid-2011



• Rasterized version of SRTM Water Body Data (significant improvement to World Vector Shoreline) - complete, ~15000 ESDRs to be shipped Aug., 2011

‘Bonus Data’



Void-filling Summary

• Plan as proposed: Void fill SRTM on best effort basis (~20% of tiles)

• GDEM1 available: Void fill much of SRTM with manual artifact detection

• GDEM2 available: Void fill most of SRTM with semi-automated artifact detection

• Will allow data generation greatly exceeding original MEaSUREs plan

and

• Expanded plan...



Expanded Plan

• Original NASA-NGA MOU has expired, creating possibility of distributable full-resolution data set

• At NASA request generated set of options for SRTM data “reprocessing”

• A task plan for an expanded effort is now under review at NASA Hq

• Plan will involve additional resources and an additional year beyond end of MEaSUREs program in 2012

• Resulting data set to be called NASADEM



Elements of NASADEM• First step is interferometric reprocessing of SRTM raw data with improved algorithms, faster

computers- Better handling of Beam Adaptive Tracker and other ancillary data- Improved phase unwapping algorithms- Leverage NASA ʻCloudʼ computing resources

• ICESat database will provide vastly improved ground control

• GDEM2 quality incorporating NUM files is sufficient to merge (not just void-fill) with SRTM, most voids will disappear

• Remaining voids will be filled with new ASTER acquisitions or other data sets- EDC has agreed to provide additional ASTER DEMs as required from archive or new

acquisitions- R. Crippen has developed technique for detecting and eliminating artifacts in individual

ASTER DEMs

• SRTM water mask will allow incorporation of NGA coastline and other water body information



SAR processing (updated)

Continental Adjustment

Raw radar echoes

Updated strip DEMs

ʻNewʼ SRTM Global DEM

Artifact removal

Modified GDEM2

ASTER GDEM2

Merge corrected DEMs

NASADEM + associated products:Slope, Aspect, Curvature, Images,

Errors, Correlation

Current SRTMGlobal DEM

IceSATData Set

SRTM WaterBody Data

Key:Pink: New data set or process

Yellow: Existing data set or processProcess

Void-fill SRTM

Metric Data

(mostly) void-filled SRTM (ver3)

NED, CDED, et al

Remainingvoid map

Acquire newASTER DEMs

Artifact removal

New ASTER DEMs

Modified DEMs

LPDAAC

NASADEM



Elements of NASADEM

• Data will be distributed and freely available at full 30 meter posting!

• New data types included- Correlation maps- Error metrics- Terrain corrected images, slopes, et al

• Processing will be continent-by-continent, with release throughout latter half of 2013









50.02a 151.07d



34.02a 50.02a 135.07d 151.07d

Images(per subswath)

incidence angles

Data take number (may be from 2 to ~20)

• Includes image, incidence angle map for each subswath crossing each cell• Subswath inclusion allows differentiating polarizations• Numerous files/cell, total data set ~ 700 gigabytes (compressed!)• Distributable at 1 arcsec• Complete: > 365,000 ESDRs en route to Land Processes DAAC

Individual data take stacks for each cell

Radar Image Task Status

Basic Image Set



SRTM Image Data Example Application

User-definedregion of interest

Image Incidence angle

Scatter plot generates backscatter curve



Radar Image Task StatusCentral Canada

Single-swath mosaic

Average of all swaths



GILF KEBIRTM SRTM

Radar Image Task Status



SRTM Radar Image of Lake Baikal

A Merged Global Digital Topographic Data Set - Progress Report.pdf

Technology

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