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

Establishing A Gravity PlanEstablishing A Gravity Plan

Ohio Department of Transportation

October 14, 2011

What is Gravity ?What is Gravity ?

A force that attracts two objects A force that attracts two objects relative to their mass.relative to their mass.

Increase in mass = increased in forceIncrease in mass = increased in force

Simple enough ?????????????Simple enough ?????????????

Why do we care about gravity?Why do we care about gravity?

It keeps you from flying off the planetIt keeps you from flying off the planet

Gravity is a key component in Gravity is a key component in determining a geoid model.determining a geoid model.

How is gravity used to How is gravity used to create geoid models?create geoid models?

Why should we care about Why should we care about geoid models?geoid models?

The geoid model is used to obtain an The geoid model is used to obtain an orthometric height from an ellipsoid orthometric height from an ellipsoid heightheight

Necessary when using GPS/GNSS for Necessary when using GPS/GNSS for positioning (vertical) positioning (vertical)

Geoid / Ellipsoid / OrthometricGeoid / Ellipsoid / OrthometricHeight RelationshipHeight Relationship

Do we already have Do we already have a geoid model?a geoid model?

Hybrid GeoidsHybrid GeoidsGEOID09GEOID09GEOID03GEOID03GEOID99GEOID99GEOID96GEOID96

Gravimetric GeoidsGravimetric GeoidsUSGG2009USGG2009USGG2003USGG2003G99SSSG99SSSCARIB97CARIB97MEXICO97MEXICO97

Why have different Why have different geoid models?geoid models?

A geoid model is based on a specific A geoid model is based on a specific reference framereference frame

New reference frame may yield a New reference frame may yield a new geoid modelnew geoid model

Pursuit of increased accuracyPursuit of increased accuracy

Why should we be concerned about Why should we be concerned about a higher quality geoid model?a higher quality geoid model?

GPS/GNSS positioning provides GPS/GNSS positioning provides increased production and reduces increased production and reduces costcost

NGS plans to migrate to a NGS plans to migrate to a gravimetric geoidgravimetric geoid

How is gravity measured?How is gravity measured?

Gravity is measured in Gals (acceleration)Gravity is measured in Gals (acceleration)• Gal= 1cm/s2

• mGal= 1 x 10-3 cm/s2

• µGal= 1 x 10-6 cm/s2

Gravity at surface of earthGravity at surface of earth • 976-983 Gal (cm/s2)• 9.81 m/s2 (approximately)

What equipment is used to What equipment is used to measure gravity?measure gravity?

Marine/Airborne GravimetersMarine/Airborne GravimetersTAGS Air III Gravity Meter(Turnkey Airborne Gravity

System)http://www.microglacoste.com/tags.htm

Micro-g LaCoste Air Sea Gravity System IIhttp://www.microglacoste.com/airsea.htm

Absolute GravimetersAbsolute 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.

http://www.scintrexltd.com/internal.php?storeCategoryID=1&subcatID=9&s_page=Gravity#A-10

http://www.microglacoste.com/fg5system.htm

Absolute GravimetersAbsolute 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.


Absolute GravimeterAbsolute Gravimeter

JILA2

Relative GravimeterRelative Gravimeter

GWR OSG Superconducting Gravimeter

Superconducting gravimeters are considered some of the most accurate relative gravimeters to date

Relative GravimetersRelative 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


Micro-g LaCoste aPhone Gravimeter

Relative GravimetersRelative Gravimeters

Scintrex CG-5 Relative GravimeterLaCoste&Romberg (LCR) G-type relative

gravimeter

What Relative Gravity Data What Relative Gravity Data Currently Exists?Currently Exists?

Number of Gravity Points Scale

What Gravity Currently ExistsWhat Gravity Currently Exists

Gravity Data for SaleGravity Data for Sale

What Absolute Gravity What Absolute Gravity Currently Exists?Currently Exists?

Location of absolute gravity stations

Comparison of Gravity DataComparison of Gravity Data

Comparison of Gravity DataComparison of Gravity Data

Approximate vertical resolution of gravity

3.0 µGal = 1 cm (or) 0.30 mGal = 1 m

Benchmark ChecksBenchmark ChecksHeight Conversion Discrepancies at 50 Benchmarks

(Compared Using VRS Positioning)

What is NGS doing?What is NGS doing?

What else could be done?What else could be done?

Tie the gravity surveys togetherTie the gravity surveys together• Relate the existing absolute gravity Relate the existing absolute gravity

stations to both the gravity station used stations to both the gravity station used in the airborne survey by NGS for Ohio in the airborne survey by NGS for Ohio and any new gravity surveys that might and any new gravity surveys that might be performed.be performed.

• How? How?

Additional Gravity “Base Stations”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 LinesExisting Level Lines

Relative Gravity Data CollectionRelative Gravity Data Collection Perform relative gravity surveys from Perform relative gravity surveys from

established gravity stationsestablished gravity stations

Minimize travel time to maximize Minimize travel time to maximize productivity.productivity.

Ensure sufficient point densityEnsure sufficient point density

Establish quality control proceduresEstablish quality control procedures

Gravity Data DensityGravity Data Density

Points/CountyPoints/County TotalTotal Approx. SpacingApprox. Spacing

11 8888 30 mi.30 mi.

44 352352 1515

99 792792 1010

1616 1,4081,408 7.57.5

2525 2,2002,200 66

3636 3,1683,168 55

4949 4,3124,312 4.34.3

6464 5,6325,632 3.83.8

8181 7,1287,128 3.33.3

100100 8,8008,800 33

What else is needed?What else is needed?

Gravity collection specificationsGravity collection specifications

Detailed plan with QA/QC proceduresDetailed plan with QA/QC procedures

Funding sourceFunding source

ContractContract

Questions or CommentsQuestions or Comments

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

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