2. GPS Site Calibration

3. Objectives  Explain the Co-ordinate systems used in GPS Surveying.  Explain what a calibration is.  Explain the 5 main process steps of a calibration  Recall the software where a calibration can be performed  List the recommended procedures when performing a Calibration.  Correctly site ideal calibration control based on different survey site situations  Explain the required relationship between GPS Base Stations and calibrations  Recall the common base co-ordinate system of a site calibration After this session, you should be able to:

4. First of all what can you remember about the GPS Co- ordinate System?

5. ECEF Coordinate System +Z -Y +X X Y Z ECEF X = -2691542.5437 m Y = -4301026.4260 m Z = 3851926.3688 m

6. Reference Ellipsoid a b a = semi-major axis b = semi-minor axis b a  H WGS-84 Ellipsoid a = 6378137.000000 m b = 6356752.314245 m 1/f = 298.2572235630

7. ECEF and WGS-84 +Z -Y +X ECEF X = -2691542.5437 m Y = -4301026.4260 m Z = 3851926.3688 m X Y Z b a  H WGS-84  = 37 o 23’ 26.38035” N = 122 o 02’ 16.62574” W H = -5.4083 m

8. What co-ordinate system would you prefer to work in?

9. What can you remember about the heights you get from GPS and the heights you need to know?

10. GPS Heights vs. Elevations e = Orthometric Height H = Ellipsoid Height N = Geoid Height N N N e e e H H H Earth’s Surface Ellipsoid Geoid e = H - N

11. What do you think a calibration is?

12. A Calibration  Word refers to several related items in RTK surveying:  File -- the file used on RTK rovers to transform from WGS84 (L,L,H) to local (flat plane, E,N,E) coordinates  RTK field survey -- the survey conducted in the field to determine the mathematical relationship between WGS84 and site (flat plane) coordinate system  What it is literally: the mathematical relationship between WGS84 and site (flat plane) coordinate system

13. Coordinate System  Converts coordinates from GPS system to our local site (flat plane) coordinate system  Includes datum transformation, map projection, horizontal & vertical adjustment  REQUIRED to work in local (flat plane) coordinates This is the GPS calibration! WGS84 NEE Must Have!!

14. How do you get from WGS84 ECEF to Easting, Northing and Elevation ?

15. First Things First  To relate one co-ordinate system to another you need a set of points on the ground that have co-ordinates in both systems.  Therefore you need a set of WGS84 co-ordinates and a set of local Northing, Easting and Elevation co-ordinates. WGS84 Local Site Co-ordinates

16. Calibration Process 1. Datum Transformation 2. Define Projection 3. Horizontal Adjustment 4.Vertical Adjustment  Geoid Model (optional)

17. Datum Transformation WGS84Local  Usually published parameters  Two basic types:  3 Parameter  7 Parameter

18. 3 vs. 7 Parameter 3 Parameter7 Parameter

19. Calibration Process  Datum Transformation  Define Projection

20. Plane Coordinate Systems

21. Types of Projections Plane Ellipsoid Tangent Plane Local Plane Point of Origin Intersecting Cylinder Transverse Mercator Axis of Ellipsoid Ellipsoid Axis of Cylinder Line of intersection Apex of Cone Intersecting Cone 2 Parallel Lambert Axis of Cone & Ellipsoid Ellipsoid

22. Calibration Process  Datum Transformation  Define Projection  Horizontal Adjustment (on projected plane surface)  Rotation  Translation  Scale

23. Horizontal Adjustment  At least 3 horizontal control points are required  5 points are recommended = GPS observation = Control Point

24. Rotation

25. Translation

26. Scale

27. Calibration Results  After the calibration you will obtain a set of calibration results, which consist of residuals. These need to be understood so you know how good the calibration is. Residua l  Residuals: The difference between the grid co-ordinate and the GPS co- ordinate after the calibration has been applied.

28. Calibration Results  Therefore the smaller the residual the more accurate the calibration parameters are and the better the relationship between the GPS (WGS84 co-ordinates) and the Local Site Easting and Northing co-ordinates. Residua l

29. Calibration Results Residua l  You should be looking for residuals ideally less than 20mm, but 30mm may be acceptable.  When you are looking at final co-ordinates your minimum accuracy is: Standard RTK GPS receiver error plus your largest calibration residuals  Your residuals should be spread evenly between the calibration points.

30. Quick Calibration Summary 1.Start with Latitude & Longitude, in WGS84, as measured with GPS. 2.Perform Datum Transformation to obtain Latitude & Longitude in local datum.  Today this is often a system very similar to WGS84, such as ETRS89. 3.Project local Latitude & Longitude onto a flat predefined plane using a projection. 4.On the projection perform a Horizontal Adjustment to fit the GPS measured points as close as possible to the Local control points in N,E 5.Obtain residuals.  Difference between GPS derived N,E and Local Control N,E

31. What can you remember?

32. Calibration Process  Datum Transformation  Define Projection  Horizontal Adjustment  Vertical Adjustment

33. Vertical Adjustment  At least 4 vertical control points are required  5 points are recommended N N N h h h H H H Earth’s Surface Ellipsoid Geoid

34. Vertical Adjustment (No Geoid Model) Earth’s Surface H H H H H h h h h h N N N N N Inclined Plane H NENE N E = estimated geoid height hEhE h E = H - N E Ellipsoid Geoid

35. Vertical Residuals Earth’s Surface H H H H H h h h h h N N N N N Inclined Plane Ellipsoid Geoid N Inclined Plane Residual

36. Calibration Process  Datum Transformation  Define Projection  Horizontal Adjustment  Vertical Adjustment  Geoid Model (optional)

37. The Geoid Model Geoid  A gridded surface that approximates the geoid  Model of the height difference between the geoid and a specified ellipsoid (normally WGS84)  Some popular global geoid models: Geoid 96 (USA) EGM96 (Global)

38. Vert. Adj. (w/ Geoid Model) Earth’s Surface H H H H H h h h h h N N N N N Geoid Model Ellipsoid Geoid NmNm NmNm NmNm NmNm NmNm N m = modeled geoid height

39. Modeling Errors Earth’s Surface H H H H H h h h h h N N N N N Geoid Model Ellipsoid Geoid NmNm NmNm NmNm NmNm NmNm NN Geoid Model Geoid N NmNm

40. Inclined Plane N N N N N Geoid Model Ellipsoid Geoid NmNm NmNm NmNm NmNm NmNm Inclined Plane NN + - Residual

41. Vertical Adjustment (with Geoid Model) Earth’s Surface H H H H H h h h h h N N N N N Geoid Model Ellipsoid Geoid NmNm NmNm NmNm NmNm NmNm NN H NmNm N C = Geoid Model Correction hEhE h E = H - N m - N C Inclined Plane

42. What can you remember?

43. Recommended Procedures  Observe GPS calibration points for at least:  3 minutes if RTK  8 minutes if Static / Fast Static (depends on baseline length and no. of SV’s)  Use a stable setup (e.g. bipod or Tripod)  Use ground plane to minimize multipath (if possible)  Enclose the project area with control  Use only reliable ENE positions  Use good network geometry  Note: If you have more than 1 base station on a large project you may need more than 1 calibration. Use your residuals to determine this

44. Let’s expand on the placement of control, project area and network geometry

45. Ideal Situations?  If you have the survey area below. Where ideally will your control be situated?  See the following slides 10km 20km

46. Ideal Situations? A.5 Horizontal and Vertical control points in a straight line down the center of the area? 10km 20km

47. Ideal Situations? B.5 Horizontal and Vertical control points in the center of the area? 10km 20km

48. Ideal Situations? C.5 Horizontal and Vertical control points surrounding the outside of the area? 10km 20km

49. Making the best of a Situation?  A customer presents you with the following calibration and complains that their elevations are not correct, in the circular area by up to 10cm. What would you advise them?  See the following slides 15km 30km Hz. Control V. Control Hz.& V. Control

50. Making the best of a Situation? A.The height from GPS is not as accurate as the horizontal, therefore these differences are acceptable. 15km 30km Hz. Control V. Control Hz.& V. Control

51. Making the best of a Situation? B.You have no residuals, therefore you have no idea how good your vertical calibration is. Due to the location of existing points you need to add in at least 3 other vertical control points. As specified below (red squares). 15km 30km Hz. Control V. Control Hz.& V. Control

52. Making the best of a Situation? C.You have no residuals, therefore you have no idea how good your vertical calibration is. You need to add in 1 other vertical control points. As specified below (red square). 15km 30km Hz. Control V. Control Hz.& V. Control

53. Large jobs  A customer presents you with the following calibration and complains that what ever they do they cannot get their greatest residuals down to less than 20cm. What could this issue be? 30km 50km Hz.& V. Control

54. Large jobs  Which points would you include for the 2 calibrations?  See the following slides 30km 50km Hz.& V. Control

55. Large jobs A.2 separate calibrations on either side of the river? 30km 50km Hz.& V. Control

56. Large jobs B.2 overlapping calibrations, either side of the river? 30km 50km Hz.& V. Control

57. Elongated jobs  Roads, Railways, Canals etc.  Why do you think the following placement of control is bad? 2km 20km Hz.& V. Control

58. Elongated jobs  It is the vertical control that is the problem. Take the cross section  Fit the vertical calibration plane though it 2km 20km Hz.& V. Control

59. Elongated jobs  The plane fits the vertical control very well and the residuals are small.  Close to the control points (white area) the elevations produced by the calibration are good.  As the plane is inclined steeply the further you go (blue shaded area) from the control points the greater the error in produced elevations using the calibration.

60. Elongated jobs  Vertical Calibration Planes  Should be no more than 50ppm, ideally <30ppm  How do you ensure this? 30mm 100,000 mm = 1km

61. Elongated jobs  Vertical control has to surround the area and ensure it is at least 1km in width apart.  Horizontal control must surround the area.  If control is only 100m apart from each other with 20mm of standard GPS error in vertical.  By the time you get out to 1km, how much error in elevation do you have? 20mm 10000 mm = 100m

62. Elongated jobs  Based on your knowledge now, how would you improve the calibration control on this job?  See following slides 2km 20km Hz.& V. Control

63. Elongated jobs A.Add in Horizontal control close to either side of the job, to enclose the area. Add in Vertical control to surround the area and create a width of at least 1km around the job. 2km 20km Hz. Control V. Control Hz.& V. Control

64. Elongated jobs B.Add vertical control only to surround the area and create a width of at least 1km around the job. 2km 20km V. Control Hz. & V. Control

65. Elongated jobs C.Add in Horizontal and Vertical control close to either side of the job 2km 20km Hz.& V. Control

66. How do Base Stations and Calibrations relate to each other?

67. Base Stations & Calibrations  Your Base Station must have WGS84 co-ordinates if you do not have a coordinate system defined  A Calibration is a form of co-ordinate system.  Your base station can have N,E,E co-ordinates if a coordinate system is defined  Your base station point does not have to be a Calibration control point. However it must be measured relative to the calibration control points

68. What if….. ….the Base MUST be moved with in the survey area? 1.PRIOR TO MOVING the base, locate new base position. 2.Ensure it is inside the calibration area. 3.Survey from existing base 4.Move old base to new base location 5.Enter new coordinates for new base station 6.Enter new antenna height for new base station 7.Keep the existing calibration 8.Check consistency with previous survey (stakeout) IF THIS IS NOT DONE CORRECTLY, the data collected from your first base will not match the data collected from your second base.

69. Where do you think the co- ordinate system is defined in?

70. Base Stations & Calibrations  What if you can not set your base station up on a known Co- ordinate (N,E,E or WGS84)  Unknown Base Station Co-ordinates  Use ‘here’ position.  Some versions of SC need to have display set to WGS84, not grid  Must perform calibration  To re-use calibration you must  Set-up on same point and use yesterdays ‘here’ co-ordinates OR  Set-up on another point surveyed yesterday with co-ordinates relative to yesterdays base station

71. Which base Co-ordinate System ?  By base co-ordinate system we mean which co-ordinate system do you select in TGO or in the Survey Controller before carrying out a calibration.  Local site co-ordinates (i.e. 1000, 5000)  Generally WGS84 + TM (no projection / no datum)?  No geoid model  Country specific co-ordinates  Generally WGS84 + TM (no projection / no datum)?  There are exceptions depending on country specific systems.  Geoid model – varies from country to country  Will be discussed in more detail in classroom based training sessions

72. In TGO

73. In the Survey Controller

74. What can you remember?

75. Review  The WGS84 / ECEF Co-ordinate systems used in GPS Surveying.  How a calibration converts your GPS WGS84 Co-ordinates into local plane co-ordinates. In the following 5 steps:  Datum Transformation, Projection, Hz Adjustment, Geoid Model (optional), V. Adjustment  Calibrations can be performed in the Survey Controller or TGO software.  The recommended procedures when performing a Calibration.  Observation time of GPS calibration points depending on survey method.  Stable setup with ground plane  Control should have good network geometry, reliable co- ordinates and surround the project area You have now learnt about:

76. Review  How to correctly site ideal calibration control based on different survey site situations  The required relationship between GPS Base Stations co-ordinates and calibrations  The base co-ordinate system of a site calibration:  Normally WGS84 + TM (no projection / no datum)?  There are many exceptions! You have now learnt about:

77. Questions