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Comprehensive Site Calibration with Trimble GNSS

Learn about site calibrations, control requirements, interpreting results, and how to apply them. Understand why we need calibrations, how they work, and how to compute transformation parameters. Dive into coordinate systems, calibration elements, and adjustment parameters.

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Comprehensive Site Calibration with Trimble GNSS

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  1. ST236Site Calibrations with Trimble GNSS Peter Mestemaker Trimble Navigation Westminster, Colorado

  2. Agenda • What is a site calibration? • Calibration – how it works! • Control requirements • Interpreting results • Calibration scenarios

  3. Section 1 – General Overview • Site calibration • Why do we need it? • What does it do? • How is it done? • How is it used?

  4. Site Calibration – Why we need it • Why use a calibration? • GPS with grid coordinates

  5. Z P h Y j l Equator X Greenwich Meridian GPS Coordinates (WGS84) • WGS84 • Latitude (φ) • Longitude (λ) • Height (h) • Cartesian • (X,Y, Z)

  6. Z Y X Surveyors want grid • (X) Easting • (Y) Northing • (Z) Elevation • vertical datum

  7. Site Calibration – What it does • Compute transformation parameters • WGS84 • grid WGS84 Grid

  8. Site Calibration – How it’s done • Control (Grid) • Measure (GPS) • Match point pairs • Calibrate!

  9. Site Calibration – How it’s used • Computes grid • WGS84 measured • Computes WGS84 • Stakeout from grid

  10. Section 2 – How it works! • Coordinate systems • Elements of the calibration • Calibration parameters

  11. Coordinate System • Transform WGS84 to grid • Requires: • Datum transformation • Map projection

  12. WGS84 Local Ellipsoid Datum Transformation • WGS84 to local ellipsoid • Not required if: • Local ellipsoid = WGS-84 • Arbitrary local grid • Known parameters? • Use them!

  13. Map Projection • Projection to local grid (φ, λ) (N, E) • Always required • Not specified? • default TM at project location

  14. Calibration Elements • Horizontal adjustment • Vertical adjustment • Geoid model • Adjustment parameters

  15. = GPS observation = Control Point Horizontal Adjustment • 2 coordinates per control • Measured (projected) • Control grid • Least squares adjustment • Rotation • Translations • scale

  16. Horizontal Rotation • Rotation about project centroid • 2 control points • no redundancy

  17. Horizontal Translations • Points shifted (X,Y) • same amount • same direction • 1 control point • no redundancy

  18. Horizontal Scale Factor • Ratio • GPS to grid distance • 2 control points • no redundancy

  19. Horizontal Residuals • Redundancy = residuals Residual • Residual • GPS vs. control coordinate • 3 control points • minimum

  20. Vertical Adjustment • Least squares best fit • WGS84 heights • Elevations • Parameters • Vertical shift • Vertical tilts (N & E) • Geoid model (optional)

  21. Geoid Model (Optional) • Geoid separation (N) Earth’sSurface Geoid N WGS84Ellipsoid

  22. Adjustment – No Geoid Model • 3 control points • minimum Earth’s Surface WGS-84 Ellipsoid N NP Geoid Inclined Plane • Best fit inclined plane • approximates local Geoid

  23. Earth’s Surface H H H H N h H NP h h Ellipsoid h N N N N e Residual Inclined Plane N Geoid Inclined Plane Geoid Residuals – No Geoid Model • Residuals at all vertical control • 4 benchmarks minimum

  24. Ellipsoid N N N N Nm Geoid Nm N Nm Nm Nm Geoid Model Inclined Plane + DN Residual - Inclined Plane – Geoid Model • Inclined plane through ΔN • Corrections to Geoid model

  25. Geoid Model - Benefits • Improved modeling results when working with a larger calibrated site that incorporates a high degree of geoid undulation • Performing a site calibration along the front range of Colorado

  26. Calibration Results - Applied • Computed using control points • Applied to all points

  27. Section 3 – Control Requirements • Horizontal control requirements • Vertical control requirements • Recommendations

  28. Control Requirements • Minimum redundancy • 3 Horizontal • 4 Vertical • Trimble recommends • 5 Horizontal • 5 Vertical • More is better!

  29. Project Area Control Placement • Critical to success • Cover entire project

  30. Vertical tilts magnified No Survey Here Limits of control Control Placement • Stay inside control • especially vertical • Vertical tilts • magnified outside control

  31. Horizontal and Vertical Control • H & V • may be different • You decide: • H • V • 3D

  32. Horizontal and Vertical Control • Mix and match • as required ▲= Horizontal ■ = Vertical

  33. Site Recommendations • Limit calibration size • minimize scale distortion • Practical limitation • 10 km x 10 km 10 km

  34. Calibration 1 Overlap area Calibration 2 Site Recommendations • Multiple zones • long linear projects • Overlap • common control

  35. ΔH Calibration 2 Calibration 1 Site Recommendations • Large ΔH • scale errors • Split into zones • Minimize ΔH

  36. Section 4 – Interpreting Results • Residuals • Horizontal adjustment parameters • Vertical adjustment parameters

  37. Interpreting Results - Residuals • Residuals • H and V • Large residuals • Control or measurement error

  38. Horizontal Adjustment Parameters • Scale factor • close to 1 • Rotation • match local orientation • Max. H. Residual

  39. Vertical Adjustment Parameters • Slope N & E • vertical tilts • Constant Adjustment • vertical shift • all points • Max. V Residual

  40. Section 5 – Calibration Scenarios • Defined coordinate system • (US state plane zone) • Arbitrary grid system • (local ground coordinates) • 1 point calibration • (scenarios for H & V adjustment)

  41. Calibrating to Pre-defined Grid • Select from library • “GRID”coordinates • Project height • ellipsoid

  42. Calibrating to Pre-defined Grid • Projected to mapping plane Position at ground surface Mapping Plane at Ellipsoid A B Projected Grid Coordinate • Horizontal adjustment • scale ~ 1 • small rotation WGS-84 Coordinate SF ≈ 1.0000 TO CENTER OF ELLIPSOID

  43. Calibrating to Assumed Grid • “no projection / no datum” • “Ground” coordinates • Project height • Geoid model • if available

  44. Calibrating to Assumed Grid • Projected to map grid • Grid scaled to ground Control at ground Project height Scaled to project height SF > 1.0000 A B Projected to grid SF ≈ 1.0000 WGS84 • Horizontal adjustment • scale ~ 1 • rotation – any value • depends on local orientation TO CENTER OF ELLIPSOID

  45. 1 Point Calibration to Ground • “no projection / no datum” • “Ground” coordinates • Project height • Geoid model • if available

  46. 1 Point Calibration to Ground • Projected to map grid • Grid scaled to ground SF > 1.0000 Scaled to Project Height Project Height A B Projected to Grid SF = 1.0000 WGS84 • Horizontal adjustment • scale = 1 • no rotation • geodetic North TO CENTER OF ELLIPSOID

  47. 1 Point Vertical Calibration • With geoid model • maintains shape of Geoid • vertical shift • no vertical tilts Earth’s Surface WGS-84 Ellipsoid H h N Geoid Geoid Model

  48. Summary • GPS site calibration • Computes transformation parameters • WGS84 to grid • Grid to WGS84

  49. Summary • Calibration requires a coordinate system • Datum transformation • WGS84 to local ellipsoid • Map projection • Ellipsoid to map grid • Use published – if available

  50. Summary • Elements of the calibration • Horizontal adjustment • Rotation, translations (2), scale • Vertical adjustment • Vertical shift, tilts (2) • Geoid model - optional

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