Establishing A Gravity Plan Ohio Department of Transportation October 14, 2011.
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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.
http://www.scintrexltd.com/internal.php?storeCategoryID=1&subcatID=9&s_page=Gravity#A-10
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
http://www.scintrexltd.com/internal.php?storeCategoryID=1&subcatID=9&s_page=Gravity#A-10
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
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