Presentation on theme: "Site Calibration for 3D GPS Operations"— Presentation transcript:
1 Site Calibration for 3D GPS Operations Presenter Name
2 GPS Site Calibration Site reconnaissance Performing the Calibration Control point availabilityGPS base station locationPerforming the Calibration
3 Control point availability Control points are measured to “calibrate” the site for GPS-based systemsDo control points exist on site?Check the site plans for control point lists and locationsYou need a minimum of 3 and 5 or more are recommendedFor more information, contact:Land SurveyorEngineering firm
4 GPS base station locations – considerations ObstructionsSetup 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 limitGlonass 65° latitude limitAvoid sources of multipath (deflection of the GPS satellite signal)Chain link fenceTreesFlat, reflective surfaces – metal roofs, glass windows, water, etcSetup GPS base radio link for maximum broadcast rangeElevate the radio antenna to increase rangeCorrect 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 beenreflected The reflected GPS signal interferes with and corrupts the direct GPS signal, causing position errors
6 NEE GPS Site Calibration What is a site calibration? GPS Coordinates A measurement procedure that defines the relationship between GPS coordinates and local coordinatesGPS in Latitude, Longitude, and Ellipsoid HeightLocal Coordinates in Northing, Easting, Elevation (MSL)We are pairing coordinates on a sphere surface and to those on a flat planar surfaceGPSCoordinatesNEE
7 GPS Site Calibration Why is a site calibration required? Allows GPS-based rover systems to work in your local site coordinate systemWhat is needed for site calibrationOnsite control base on local coordinates= GPS observation= Control Point
8 GPS Site Calibration The calibration locally adjusts the ProjectionIncludes shift grids, projection grids, datum gridIncludes Azimuth orientation (e.g.North or South)DatumSite Calibration is comprised of 2 partsHorizontal adjustmentRotate, Translate, & ScaleVertical adjustmentBlock shift & Tilted PlaneGeoid
9 GPS Site Calibration Measuring a Site Calibration will assume Projection – Transverse MercatorDatum – WGS84If 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 & YSame Amount & Direction1 Control Point
11 Horizontal RotationRotation about project centroid2 control points
12 Horizontal ScaleRatioGPS to Local Coordinates2 Control Points
13 ResidualsResidualsBest effort translation between pairs (Control & WGS-84)SCS900 has a Tolerance for CalibrationValue should be 50% of acceptable project toleranceTolerance used to test calibration result worst residualWe 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 RequirementsRequires a single, 3D control point – known or “arbitrary”Single point defines coordinate system orientationUsed when control does not existRecommended for initial site topos and for quick stockpile or volume topos etcRecommend measuring control points to tie to design reference frame laterNot recommend on long linear projects= GPS observation
15 Calibration Requirements Multi-point site calibration RequirementsMinimum of 3 (3D) control pointsRecommend 5+ control points with good geometry for better resultsCombination of horizontal and vertical points - Minimums3 horizontal control points & 3 vertical control points– or –1 vertical + 3 horizontal + Geoid modelUsed when control exists and references a design frame= GPS observation= Control Point
16 Control Point Locations 3 pointswill work, but yields only 3 baselinesGeometry could be weakControl should encompass the entire site
17 Control Point Locations 4 PointsBetter, 6 baselines4th point - independent height checkGeometry is stronger
18 Control Point Locations Optimal control-Multiple control pointsGeometry strongest, balancedPoints encompass the site.This example - 7 points, 18 baselines.
19 Control Point Locations Control point network geometry is key= Poor network geometry= Ideal network geometryFor 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 resultsEnclose the project area with controlMore 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 siteMinimizes height residuals on control points after block shiftUser selectable when to occur in SCS900SCS900 has a minimum of 3 points, default is 5With only 3 points there is no check and it will zero all residuals to create the tilted planeOccurs after 2 points in Survey Controller to eliminate height residuals after block shiftSCS900 and Survey Controller will only agree when the tilted plane has been applied in bothITEMS 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 PointsProvided local coordinatesThe difference between themSome high, some low compared to average shiftBlock Shift from measured to local using average shiftRotate i.e. Tilt the plane through the measured points to minimize residuals and get a best fit of measured control to local controlIn an ideal world we would get a perfect fitWe 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 LevelStore multiple Geoids in Trimble Geodata folderIf 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 calibrationGeoids allow for fewer vertical control pointsProvides better detection of errors in controlRecommended on long linear projectsITEMS TO MENTION:Rule of Thumb: If surveyor or anyone utilized a site calibration and geoid, then you should use that same calibration & GeoidWith 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 GeoidsA geoid height is the separation between the Ellipsoid and the Geoid at any location on the earths surfaceH = Orthometric Heighth = Ellipsoidal Height (WGS84)H = h - NN = Geoid Height (GEOID 03)hEllipsoid(WGS84)NGeoidGeoid Height(GEOID03)TOPOGRAPHIC SURFACEHAB
26 Geoids Geoid Model (Nm) Geoid Values (N) ∆N at each Control Point Approximates GeoidGeoid Values (N)Computed from BM Elevations (h-H=N)∆N at each Control Point
28 Shift Grids SCS900 now supports SHIFT GRIDS There are three different types of Shift Grids in EuropeBelgium and Netherlands use SHIFT GRID FILESUK uses PROJECTION GRID FILESFrance 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 shiftsPROJECTION GRID FILESA 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 shiftsDATUM GRID FILESThe 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 pointsChange toleranceEdit 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 processesITEMS 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 problemAfter calibration – location requirementMust be moved to control pointControl used during the calibrationControl measured before moving based and using calibrationSame rules for calibration obtained under VRS
32 Conclusion In conclusion, remember these key points Good Base Station location for good observablesAdequate number of control points (5+)Good Geometry among control points
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