The National Geodetic Survey Gravity Program Benefits and Opportunities Juliana Blackwell, Director...
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Transcript of The National Geodetic Survey Gravity Program Benefits and Opportunities Juliana Blackwell, Director...
The National Geodetic Survey Gravity Program
Benefits and Opportunities
Juliana Blackwell, DirectorNational Geodetic Survey (NGS)
What is Geodesy?Geodesy (geodetic control) is a
foundational science that defines position and height.
The Earth is an irregular surface and is difficult to model. Accurate positions
are required for a wide variety of applications.
Why is Geodesy important?
January 23, 2012 2MAPPS 2012 Winter Conference
NGS Mission Statement
To define, maintain and provide access to the National Spatial Reference System (NSRS) to meet our nation’s economic,
social, and environmental needs.
The NSRS is a consistent coordinate system that defines latitude, longitude, height, scale, gravity, and
orientation throughout the United States.
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NGS Provides the Geospatial Foundation for a Wide Variety of Activities
Geodetic ControlAerial Imagery
Elevation
Boundaries
Surface Waters
Transportation
Land Ownership
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The NSRS Supports:
Flood zones for the National Flood Insurance Program Federal Emergency Management Agency
Levee Safety Program to determine levee heights and positionsUnited States Army Corps of Engineers
NSRS gravity data for the geospatial mission of NGA National Geospatial-Intelligence Agency
Topographic Maps and interior water data for the nationUnited States Geological Survey
Nautical charts, among many other geospatial applications National Oceanic and Atmospheric Administration
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• Strictly speaking, a vertical datum is a surface representing zero elevation
•A vertical datum has a:– Definition: Parameters and other
descriptors – Realization: Its physical method
of accessibility
• Traditionally, how heights above the datum are determined is also specified– Orthometric vs. dynamic "topographic map." Online Art.
Britannica Student Encyclopædia. 17 Dec. 2008 <http://student.britannica.com/ebi/art-53199>
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Vertical Datum 101
NGS and the National Vertical DatumNAVD 88 (North American Vertical Datum of 1988)• Definition:
– The surface of equal gravity potential to which orthometric heights shall refer in North America*
– 6.271 meters (along the plumb line) below the geodetic mark at “Father Point/Rimouski” (NGSIDB PID TY5255)
• Realization: – Over 500,000 geodetic marks across North America
with published Helmert orthometric heights – Most heights were originally computed from a
minimally constrained adjustment of leveling and gravity data, holding the geopotential value at “Father Point/Rimouski” fixed.
Father Point Lighthouse Quebec *Not adopted in Canada
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H
Earth’sSurface
The Geoid
NAVD 88 reference level
H (NAVD 88)
Errors in NAVD 88 : ~50 cm average, 100 cm CONUS tilt, 1-2 meters average in Alaska, NO tracking of temporal changes
Errors in NAVD 88
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• Replace the Vertical Datum of the USA by 2022 (at today’s funding) with a gravimetric geoid accurate to 1 cm
• Orthometric heights accessed via GNSS• Three thrusts of project:
– Airborne gravity survey of entire country and its holdings
– Long-term monitoring of geoid change– Partnership surveys
• Working to launch a collaborative effort with the USGS for simultaneous magnetic measurement
• Big benefits: $522 million in estimated annual economic benefits once complete
Gravity for the Redefinition of the American Vertical Datum (GRAV-D)
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Building a Gravity Field
Long Wavelengths:(≥ 350 km)
GRACE and GOCE (not shown)
Intermediate Wavelengths(500 km to 20 km)
Airborne Measurement
Surface Measurement
Short Wavelengths(< 100 km)
+
+
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Geoid Slope Validation Survey
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325 km218 points1.5 km apart
Austin
Rockport
• Observe geoid shape (slope) using multiple independent terrestrial survey methods– GPS + Leveling– Deflections of the Vertical
• Compare observed slopes (from terrestrial surveys) to modeled slopes (from gravimetry or satellites)– With/without new GRAV-D
airborne gravity
Geoid Slope Survey Conclusions• Including airborne gravity data improves geoid
slope accuracy at nearly all distances <325 km• The NGS geoid in the TX survey meets the 1
cm accuracy objective only if airborne data are included– No other model achieved 1 cm accuracy
• Gravimetric geoid models and GPS are a viable alternative to long-line leveling
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GRAV-D Update
15
Alaska FY10-13 West Coast FY11
Great Lakes FY11-13
FY10 = GreenFY11 = BlueFY12 = OrangeFY13 = White
FY12 Texas
15.6% of total area is
surveyed (as of 11-23-11)
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GRAV-D Contracting Expenditures
• Contract employees:– FY11: 24.5% of total expenditures– FY12 (planned): 24.8%
• Contract Surveys– FY11 Fugro: 10.7%– FY12 Fugro (estimated): 20.6%
• Fugro– Provided aircraft and operator in FY11– Exploring option for data products in FY12
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Future Contracting Plans
• Minimum of 10% of annual surveying will be contracted out (~100 flight hours/yr)
• If additional funding can be found, the percentage of contract surveying would be increased
• Explore options for dedicated aircraft enabled for magnetic measurement
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Summary
• GRAV-D project is on course for completion in 2022
• Involvement with contract data services is planned at the 10% level minimum
• The new vertical datum produced by this effort will be invaluable for all geospatial concerns
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