Establishing A Gravity Plan Ohio Department of Transportation October 14, 2011

What is Gravity ? A force that attracts two objects relative to their mass. A force that attracts two objects relative to their mass. Increase in mass = increased in force Increase in mass = increased in force Simple enough ????????????? Simple enough ?????????????

Why do we care about gravity? It keeps you from flying off the planet It keeps you from flying off the planet Gravity is a key component in determining a geoid model. Gravity is a key component in determining a geoid model.

How is gravity used to create geoid models?

Why should we care about geoid models? The geoid model is used to obtain an orthometric height from an ellipsoid height The geoid model is used to obtain an orthometric height from an ellipsoid height Necessary when using GPS/GNSS for positioning (vertical) Necessary when using GPS/GNSS for positioning (vertical)

Geoid / Ellipsoid / Orthometric Height Relationship

Do we already have a geoid model? Hybrid Geoids GEOID09 GEOID09 GEOID03 GEOID03 GEOID99 GEOID99 GEOID96 GEOID96 Gravimetric Geoids USGG2009 USGG2009 USGG2003 USGG2003 G99SSS G99SSS CARIB97 CARIB97 MEXICO97 MEXICO97

Why have different geoid models? A geoid model is based on a specific reference frame A geoid model is based on a specific reference frame New reference frame may yield a new geoid model New reference frame may yield a new geoid model Pursuit of increased accuracy Pursuit of increased accuracy

Why should we be concerned about a higher quality geoid model? GPS/GNSS positioning provides increased production and reduces cost GPS/GNSS positioning provides increased production and reduces cost NGS plans to migrate to a gravimetric geoid NGS plans to migrate to a gravimetric geoid

How is gravity measured? Gravity is measured in Gals (acceleration) Gal= 1cm/s 2 mGal= 1 x cm/s 2 µGal= 1 x cm/s 2 Gravity at surface of earth Gal (cm/s 2 ) 9.81 m/s 2 (approximately)

What equipment is used to measure gravity?

Marine/Airborne Gravimeters TAGS Air III Gravity Meter (Turnkey Airborne Gravity System) Micro-g LaCoste Air Sea Gravity System II

Absolute Gravimeters Micro-g LaCoste FG-5 Recognized standard for measuring gravity. 2-3 microgal accuracy in 24 hour site occupations Uses the free-fall technique of measuring gravity. A test mass is dropped inside a vacuum chamber. The descent rate implies the magnitude of the gravity The mass is monitored using a laser interferometer. catID=9&s_page=Gravity#A-10

Absolute Gravimeters Micro-g LaCoste A10 Known for its fast data acquisition and portability in outdoor applications including harsh field conditions in the sun, snow, and wind. goryID=1&subcatID=9&s_page=Gravity#A-10

Absolute Gravimeter JILA2

Relative Gravimeter GWR OSG Superconducting Gravimeter Superconducting gravimeters are considered some of the most accurate relative gravimeters to date

Relative Gravimeters The Micro-g LaCoste aPhone gravity meter provides microGal repeatability with vey low static drift which makes it the best suited as a monitoring meter. It can be monitored and controlled via the internet for remote operation oryID=1&subcatID=9&s_page=Gravity#A-10 Micro-g LaCoste aPhone Gravimeter

Relative Gravimeters Scintrex CG-5 Relative Gravimeter LaCoste&Romberg (LCR) G-type relative gravimeter

What Relative Gravity Data Currently Exists? Number of Gravity Points Scale

What Gravity Currently Exists

Gravity Data for Sale

What Absolute Gravity Currently Exists? Location of absolute gravity stations

Comparison of Gravity Data

Approximate vertical resolution of gravity 3.0 µGal = 1 cm (or) 0.30 mGal = 1 m

Benchmark Checks Height Conversion Discrepancies at 50 Benchmarks (Compared Using VRS Positioning)

What is NGS doing?

What else could be done? Tie the gravity surveys together Tie the gravity surveys together Relate the existing absolute gravity stations to both the gravity station used in the airborne survey by NGS for Ohio and any new gravity surveys that might be performed.Relate the existing absolute gravity stations to both the gravity station used in the airborne survey by NGS for Ohio and any new gravity surveys that might be performed. How?How?

Additional Gravity “Base Stations” Gravity base stations that could be established. Locations could vary as needed. Some could coincide with existing level lines.

Existing Level Lines

Relative Gravity Data Collection Perform relative gravity surveys from established gravity stations Perform relative gravity surveys from established gravity stations Minimize travel time to maximize productivity. Minimize travel time to maximize productivity. Ensure sufficient point density Ensure sufficient point density Establish quality control procedures Establish quality control procedures

Gravity Data Density Points/CountyTotalApprox. Spacing Points/CountyTotalApprox. Spacing mi , , , , , , ,8003

What else is needed? Gravity collection specifications Gravity collection specifications Detailed plan with QA/QC procedures Detailed plan with QA/QC procedures Funding source Funding source Contract Contract

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