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Site Calibration for 3D GPS Operations

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Presentation on theme: "Site Calibration for 3D GPS Operations"— Presentation transcript:

1 Site Calibration for 3D GPS Operations
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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

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?

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