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.

January 23, 2012 3MAPPS 2012 Winter Conference

January 23, 2012 MAPPS 2012 Winter Conference 4

January 23, 2012 MAPPS 2012 Winter Conference 5

NGS Provides the Geospatial Foundation for a Wide Variety of Activities

Geodetic ControlAerial Imagery

Elevation

Boundaries

Surface Waters

Transportation

Land Ownership

January 23, 2012 6MAPPS 2012 Winter Conference

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

January 23, 2012 7MAPPS 2012 Winter Conference

• 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>

January 23, 2012 8MAPPS 2012 Winter Conference

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

January 23, 2012 9MAPPS 2012 Winter Conference

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

January 23, 2012 10MAPPS 2012 Winter Conference

• 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)

January 23, 2012 11MAPPS 2012 Winter Conference

12

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)

+

+

January 23, 2012 MAPPS 2012 Winter Conference

Geoid Slope Validation Survey

January 23, 2012 MAPPS 2012 Winter Conference 13

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

January 23, 2012 MAPPS 2012 Winter Conference 14

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)

January 23, 2012MAPPS 2012 Winter Conference

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

16January 23, 2012 MAPPS 2012 Winter Conference

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

17January 23, 2012 MAPPS 2012 Winter Conference

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

18January 23, 2012 MAPPS 2012 Winter Conference