1. Site Calibration for 3D GPS Operations
Presenter Name

2. GPS Site Calibration Site reconnaissance Performing the Calibration
Control point availability
GPS base station location
Performing the Calibration

3. Control point availability
Control points are measured to “calibrate” the site for GPS-based systems
Do control points exist on site?
Check the site plans for control point lists and locations
You need a minimum of 3 and 5 or more are recommended
For more information, contact:
Land Surveyor
Engineering firm

4. GPS base station locations – considerations
Obstructions
Setup GPS base station antenna with 360° view of the sky. If limited try to set up with clear visibility to the south.
GPS 55° latitude limit
Glonass 65° latitude limit
Avoid sources of multipath (deflection of the GPS satellite signal)
Chain link fence
Trees
Flat, reflective surfaces – metal roofs, glass windows, water, etc
Setup GPS base radio link for maximum broadcast range
Elevate the radio antenna to increase range
Correct antenna (high gain and low gain)
Avoid sources of RF interference (Microwave, Power lines, etc)
Multipath occurs when a GPS antenna receives a GPS signal both directly from the GPS satellite and after it has been
reflected The reflected GPS signal interferes with and corrupts the direct GPS signal, causing position errors

5. GPS Site Calibration
What is it…
Requirements

6. NEE GPS Site Calibration What is a site calibration? GPS Coordinates
A measurement procedure that defines the relationship between GPS coordinates and local coordinates
GPS in Latitude, Longitude, and Ellipsoid Height
Local Coordinates in Northing, Easting, Elevation (MSL)
We are pairing coordinates on a sphere surface and to those on a flat planar surface
GPS
Coordinates
NEE

7. GPS Site Calibration Why is a site calibration required?
Allows GPS-based rover systems to work in your local site coordinate system
What is needed for site calibration
Onsite control base on local coordinates
= GPS observation
= Control Point

8. GPS Site Calibration The calibration locally adjusts the
Projection
Includes shift grids, projection grids, datum grid
Includes Azimuth orientation (e.g.North or South)
Datum
Site Calibration is comprised of 2 parts
Horizontal adjustment
Rotate, Translate, & Scale
Vertical adjustment
Block shift & Tilted Plane
Geoid

9. GPS Site Calibration Measuring a Site Calibration will assume
Projection – Transverse Mercator
Datum – WGS84
If you want to use an alternative to this, then a DC file containing that information should be created and used as the starting point e.g. State Plane etc.
Can then be localized through site calibration

10. Horizontal Translation
Points shifted X & Y
Same Amount & Direction
1 Control Point

11. Horizontal Rotation
Rotation about project centroid
2 control points

12. Horizontal Scale
Ratio
GPS to Local Coordinates
2 Control Points

13. Residuals
Residuals
Best effort translation between pairs (Control & WGS-84)
SCS900 has a Tolerance for Calibration
Value should be 50% of acceptable project tolerance
Tolerance used to test calibration result worst residual
We use a least squares fit – Control (assumed correct) & GPS WGS-84(errors)
ITEMS TO MENTION:
SCS900 has a Tolerance for the Site Calibration – this value should be at a maximum 50% of what your accepted tolerance for the project is. This tolerance is used to test the calibration result worst residual. If the calibration passes then you are good to go (but cannot try to improve the cal further – if you want to do better than the tolerance you need to change the tolerance to a tighter value)
If SCS900 fails, what it does is compute the calibration n-1 times (where n is the number of points observed). Each computation uses all but one of the control points. The best calibration result will not include the point that has the most negative affect on the calibration (Hz or Vt). The worst point and worst error is flagged by the software (Hz or Vt) and that component of the point can be rejected. At that time SCS900 recomputes the Site Cal again, and checks to see if it passes tolerance. If it does you are good to go, if it fails it repeats the same process with the remaining points. It will do this loop until it gets a cal in tolerance or until the user accepts a lower tolerance. Once you have rejected a point component, it cannot be added back in to the solution. The maximum tolerance you can put into the site calibration is 0.2’.

14. Calibration Requirements
Single point site calibration Requirements
Requires a single, 3D control point – known or “arbitrary”
Single point defines coordinate system orientation
Used when control does not exist
Recommended for initial site topos and for quick stockpile or volume topos etc
Recommend measuring control points to tie to design reference frame later
Not recommend on long linear projects
= GPS observation

15. Calibration Requirements
Multi-point site calibration Requirements
Minimum of 3 (3D) control points
Recommend 5+ control points with good geometry for better results
Combination of horizontal and vertical points - Minimums
3 horizontal control points & 3 vertical control points
– or –
1 vertical + 3 horizontal + Geoid model
Used when control exists and references a design frame
= GPS observation
= Control Point

16. Control Point Locations
3 points
will work, but yields only 3 baselines
Geometry could be weak
Control should encompass the entire site

17. Control Point Locations
4 Points
Better, 6 baselines
4th point - independent height check
Geometry is stronger

18. Control Point Locations
Optimal control-
Multiple control points
Geometry strongest, balanced
Points encompass the site.
This example - 7 points, 18 baselines.

19. Control Point Locations
Control point network geometry is key
= Poor network geometry
= Ideal network geometry
For multi-point site calibrations, control points geometrically arranged in a “line” or “clustered” in one area on the construction site is undesirable.
Error in position dramatically increases the distance traveled outside calibrated site area.
It is ideal to have 5 or more control points, good “geometry” of control, in the center and around the extents of the project for better site calibration results
Enclose the project area with control
More control points and good network geometry can improve site calibration results and identify problems early

20. Tilted Plane Tilted Plane
Effectively models the effect of the local Geoid i.e. local variations in gravity over the site
Minimizes height residuals on control points after block shift
User selectable when to occur in SCS900
SCS900 has a minimum of 3 points, default is 5
With only 3 points there is no check and it will zero all residuals to create the tilted plane
Occurs after 2 points in Survey Controller to eliminate height residuals after block shift
SCS900 and Survey Controller will only agree when the tilted plane has been applied in both
ITEMS TO MENTION:
With SCS900 we will have a height residual after 2 points, no residual at 3 points and then residuals again at 4 points. In SC you will have residuals at the 4th point only

21. Block Shift and Tilted Plane Process In Cross Section View

22. Details of how this works
Measured Points
Provided local coordinates
The difference between them
Some high, some low compared to average shift
Block Shift from measured to local using average shift
Rotate i.e. Tilt the plane through the measured points to minimize residuals and get a best fit of measured control to local control
In an ideal world we would get a perfect fit
We don’t have an ideal world so we get residuals after the tilted plane – those residuals dictate whether we are in tolerance for the Site Calibration or not.

23. Geoids Trimble GeoData folder added in SCS900 v2.3
Geoids will apply at Site Level
Store multiple Geoids in Trimble Geodata folder
If no Geoids exist in the Trimble Geodata folder then you will not be asked this question

24. Geoids Should I use a Geoid model? Geoids will never hurt
Geoids will allow you to go outside your project calibration
Geoids allow for fewer vertical control points
Provides better detection of errors in control
Recommended on long linear projects
ITEMS TO MENTION:
Rule of Thumb: If surveyor or anyone utilized a site calibration and geoid, then you should use that same calibration & Geoid
With a Geoid Model loaded, the user can work with one point only for height if required. We recommend 2 height points as a minimum.

25. Geoids
A geoid height is the separation between the Ellipsoid and the Geoid at any location on the earths surface
H = Orthometric Height
h = Ellipsoidal Height (WGS84)
H = h - N
N = Geoid Height (GEOID 03) h
Ellipsoid
(WGS84) N
Geoid
Geoid Height
(GEOID03)
TOPOGRAPHIC SURFACE H A B

26. Geoids Geoid Model (Nm) Geoid Values (N) ∆N at each Control Point
Approximates Geoid
Geoid Values (N)
Computed from BM Elevations (h-H=N)
∆N at each Control Point

27. Geoids
Tilted Plane through ∆N

28. Shift Grids SCS900 now supports SHIFT GRIDS
There are three different types of Shift Grids in Europe
Belgium and Netherlands use SHIFT GRID FILES
UK uses PROJECTION GRID FILES
France uses DATUM GRID FILES

29. Shift Grids SHIFT GRID FILES PROJECTION GRID FILES DATUM GRID FILES
A standard projection is used to get grid coordinates and then the shift grids are applied to get the correct national coordinates.  One .sgf file contains both northings and eastings shifts
PROJECTION GRID FILES
A standard projection is used to get grid coordinates and then the shift grids are applied to get the correct national coordinates.  One .pgf file contains northings and another .pgf file contains eastings shifts
DATUM GRID FILES
The shift is applied before the projection.  One .dgf file contains the datum shift

30. GPS Site Calibration Improving my calibration results
Continue to calibrate on additional control points
Change tolerance
Edit Calibration Components after measuring all control points for the calibration by switching on/off Hz or VT components…
Careful, it is extremely risky to remove one component of a point unless Horizontal and Vertical components of control were established under separate processes
ITEMS TO MENTION:
When removing items from the calibration you should talk about what you are going to do about the point, since you have determined the point is wrong. If an inspector sets up on that point with a total station you are going to have trouble. Appropriate parties should be notified of conflict.

31. Moving the GPS base station
Before calibration – no problem
After calibration – location requirement
Must be moved to control point
Control used during the calibration
Control measured before moving based and using calibration
Same rules for calibration obtained under VRS

32. Conclusion In conclusion, remember these key points
Good Base Station location for good observables
Adequate number of control points (5+)
Good Geometry among control points

33. Questions?