Site Calibration for 3D GPS Operations Presenter Name

Site Calibration for 3D GPS Operations

Presenter Name

GPS Site Calibration

Site reconnaissance– Control point availability– GPS base station location

Performing the Calibration

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

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)


What is it…

Requirements


What is a site calibration?– 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

GPSCoordinates NEE


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


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


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

Horizontal Translation

Points shifted X & Y– Same Amount & Direction

1 Control Point

Horizontal Rotation

Rotation about project centroid 2 control points

Horizontal Scale

Ratio– GPS to Local Coordinates

2 Control Points

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

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

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

Control Point Locations

3 points – will work, but yields only 3

baselines– Geometry could be weak– Control should encompass

the entire site


4 Points– Better, 6 baselines– 4th point - independent

height check– Geometry is stronger


Optimal control-– Multiple control points– Geometry strongest,

balanced– Points encompass the

site.

– This example - 7 points, 18 baselines.

Control Point Locations Control point network geometry is key

= Poor network geometry

= Ideal network geometry

• Enclose the project area with control• More control points and good network geometry can improve site

calibration results and identify problems early

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

Block Shift and Tilted Plane ProcessIn Cross Section View

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.

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

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

Geoids

– A geoid height is the separation between the Ellipsoid and the Geoid at any location on the earths surface

H

H = Orthometric Height

H = h - N

TOPOGRAPHIC SURFACE

h = Ellipsoidal Height (WGS84)

N = Geoid Height (GEOID 03)

h

Ellipsoid(WGS84)

N

GeoidGeoid Height (GEOID03)

AB

Geoids

Geoid Model (Nm)– Approximates Geoid

Geoid Values (N)– Computed from BM

Elevations (h-H=N)

∆N at each Control Point

Geoids

Tilted Plane through ∆N

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

Shift Grids

SHIFT 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


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

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

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

Questions?

Documents

Site Calibration for 3D GPS Operations Presenter Name