1. Coordinates, Map Projection,andDatums

4. Coordinatesare measured AllmeasurementscontainerrorsAllmeasurementscontainerrors Tuesday, September 10, 13

5. Howdowemeasurethelatitude/longitudesofpointstoday?Howdowemeasurethelatitude/longitudesofpointstoday? 1-Ground surveys 2- Astronomical surveys 3-Galacticradiowaves3-Galacticradiowaves 5- Alloftheabove5- Alloftheabove 4-GPS

6. Whenwedescribe geographiccoordinates (lat/long),whydowe havedifferentkinds? WGS84,NAD27,NAD83, NAD83(HARN), NAD83(2007)

7. OurNetworkandmethods improvewithtime

8. 11 Different modifiers And, different coordinate values

9. Adatum is a referencesurface Both Horizontal and Vertical Datums exist Datums are defined by an Ellipsoid (Model of the Earth), including how the model ties into the earth datums are like amoebas,they evolve throughtime, as our measurementsand models improve so.....................everyLat./Long. hasatimestamp

10. OurEarthisn’treallyshapedasa sphere(oid),orevenanellipse(oid)

11. 14 Why? Law of Gravity: Force = g x Mass a x mass b r 2 Larger mass, stronger pull,and the Earth’s mass isn’t uniform throughout, e.g., near mountains, subsurface ore, deep mantle density variation

12. 15 In Addition: Isostatic Rebound

13. 16

14. DifferentElllipsoidsMatchBestinDifferentPlacesDifferentElllipsoidsMatchBestinDifferentPlaces

15. Satellites/computer Revolution of the 1970s - 1990s Before: datums were national/continental, rarely recalculate origin at a geometric center of the Earth fixed a station or measurements, and propagated error from there Assumed a static network of points After: datums were generally global, recalculated often origin at the mass center distributed error across the network could incorporate/measure point velocities 18

16. Measurementshave gottensogood,we canassimilate continentaldrift-

17. Plates Are Moving,soLat/ Lonofany pointchanges withtime! GA Tech Dept of GephysicsGA Tech Dept of Gephysics

18. Wecalculatenewpositionsfrombetter,more,and morerecentmeasurements,andthings “shift”

19. Thereare ThreeMainFamiliesofDatums

20. You have been provided raster land use data in NAD27 on a 1 km grid, and want to combine it with data in NAD83(86). Do you need to worry about datum differences? (yes, no) YourGPSis givingyoureadingsinthelatest WGS84coordinates,accuratetothenearest 10cm.Therestofyourdataarein NAD83(96)coordinates. Doyouneedto transformdatums? (yes,no)

21. You have data in NAD83(HARN) coordinates, accurate to the nearest 10 cm. The rest of your data are in NAD83(2007) coordinates. Doyouneedtotransformdatums?(1=yes, 2=no) YouhavedatainITRF88coordinates, accuratetothenearest50 cm.Therestof yourdataareinITRF00coordinates. Doyou need totransformdatums?(yes,no)

22. Heights-threekinds

23. Cangeoidalheightsbenegative(yes, no)?

25. Canellipsoidalheightsbenegative(yes, no)? Canorthometricheightsbenegative(yes, no)?

26. Canellipsoidalheightsbenegative(yes, no)? Canorthometricheightsbenegative( yes, no)?

27. Howdowegofrom latitude/longitudetoa gridcoordinatesystem? Weuseapro jection

28. A “DevelopableSurface” canbeFittothe Globe,thenCutandUnrolled

30. Thereis ALWAYSDistortion

31. Whereisthedistortionmostpronounced?Whereisthedistortionmostpronounced?

35. WeManageDistortionby: -Placementofintersectinglines -Sizeoftheareaprojected

36. ProjectionsCauseShape andDistanceDistortion

37. CommonCoordinatesCommonCoordinates Projected: -County -StatePlane -UTM Unprojected: -Geographic,NAD27 -Geographic,NAD83(1986) -Geographic,NAD83(96) -WGS84(xxxx) -ITRFxx

38. 13 StatePlaneisCommonfor multi- countyandsingle- countydata

39. UTMiscommonforstatewidedataUTMiscommonforstatewidedata

40. MSTM Mississippi Transverse Mercator The MSTM projection is a customized Transverse Mercator projection designed to more evenly distribute convergence and scale- factor, and is based on the North American Datum of 1983 (NAD83). The projection parameters of the MSTM projection are as follows: Scale Factor: 0.9998335 Central Meridian: -89 45' 00'' Central Latitude: 32 30' 00'' False Northing: 1,300,000 meters False Easting: 500,000 meters Projection Units: meters Spheroid: GRS80 Datum: NAD83

41. So,ifyou’renotinthe rightdatum,you maybeofffroma fewinches to6feet (unlessyourinNAD27,thenitmaybe 100’sof feet). Wecanfixthis withadatum transformation,either......aspartofamapprojection