GNSS and Heighting, Practical Considerations. A Parker National Geo-spatial Information Department of Rural Development and Land Reform

Transcription

1 GNSS and Heighting, Practical Considerations A Parker National Geo-spatial Information Department of Rural Development and Land Reform

2 GNSS Global Navigation Satellite Systems (GNSS) Global Positioning Systems (GPS) Galileo (EU) GLONASS (Russian Federation) Compass (China) Gagan (India) Augmentation Systems Quasi Zenithal Satellite Systems (QZSS) EGNOS WAAS

3 GNSS Heighting GNSS heighting dependant on: 1. Accuracy of vertical datum 2. Accuracy of GNSS measurement 3. Accuracy of Geoid model/modeling

4 Vertical Datum: South African LLD NGI is mandated by the Land Survey Act (Act 8 of 1997) to establish and maintain a National Control Survey Network in South Africa. The official horizontal datum in South Africa is the Hartebeesthoek94 (ITRF1991, epoch 1994) The official vertical datum is the Land Levelling Datum (LLD).

5 The South African Land Levelling Datum Reference surface: Adopted mean sea level Vertical Origin and Orientation: (TSO 1956) Based on MSL determined from tidal observations at Cape Town. Verified by MSL determinations from tidal observations at Durban, Port Elizabeth and East London, over varying periods of time.

6 The LLD is realised by: Numerous precisely levelled bench marks, Realisation of LLD Town survey marks (> unofficial), Approximately Trigonometrical beacons

7 Distribution of NGI Benchmarks

8 Trig Beacons Accuracy of monuments realising LLD The accuracy of adjusted orthometric height coordinates shall not exceed 0.30 meters at the 95% confidence level (some serious outliers!!) FBM s, BM s and TSM s: The accuracy of orthometric height differences between adjacent benchmarks is 0.3mm x sub-section distance (in kilometers) at the 95% confidence level. i.e 10km = 3mm

9 Accuracy of Trigonometrical Beacons Comparison of issued trig beacon heights and ellipsoidal/geoid derived orthometric heights Total number of points compared = 1539 Mean diff = 0.22 m Std dev = 0.47 m Min = m Max = 4.59 m

10 Connection to LLD Currently, most users, especially surveyors that require precise heighting, connect to the South African LLD. The exception to this rule would be: in cases where the project is located too far away from a precise leveling route to justify the cost, or in applications where connection to the LLD is not required or only relative heights are required.

11 TRIGNET TrigNet coordinates defined on: 1. Hartebeesthoek 94 Datum/LLD; 2. ITRF2008 (epoch ) reference frame which is 3D (X,Y,Z) accurate to <1cm. Ellipsoidal heights can be determined relative to TrigNet <2cm using commercial software.

12 TRIGNET

13 Spirit vs GNSS Levelling

14 Spirit vs GNSS Levelling Spirit (and precise) levelling can be an extremely accurate method of height transfer. Both a labour intensive and time consuming procedure. GNSS provides a fast and expedient method of height transfer, albeit with less accuracy over short distances. GNSS provide fully three-dimensional positions (latitude, longitude and height).

15 Characteristics of GNSS Levelling GNSS heighting 2-3 times < precise than the horizontal components. Residual atmospheric biases are the source of the greatest uncertainty. GNSS levelling involves additional calculations for geoid height determination, of which only the geometric method is relatively simple.

16 GNSS and Heighting The reference surface used is the WGS84 ellipsoid, and computed heights are heights above this surface, i.e., ellipsoidal heights (h). In precise surveys where GNSS is used in differential carrier phase mode, GNSS outputs precise ellipsoidal height differences ( h).

17 GNSS & Heighting GNSS measures heights and height differences above ellipsoid, in some cases..sufficient. With GNSS heights, water can flow uphill????? Require relation to physically meaningful surface such as the geoid, or local observed MSL. Such physically meaningful heights take the form of orthometric heights.

18 GNSS & Heighting: Technique The typical (vertical) accuracy of GNSS is: Vertical Accuracy (RMS) Autonomous 1m-10m DGPS 0.5m + 1ppm(post processing or real-time) RTK1Hz 2.0 cm + 1 ppm ( BL) 3.0 cm (10 km) PPK1Hz 1.5 cm +1 ppm ( BL) 2.5 cm FStatic 1.0 cm +0.5 ppm ( BL) 1.5 cm Static 0.5 cm +0.5 ppm ( BL) 1.0 cm

19 ATCHUNG!!!!... Small print Measurement precision, accuracy and reliability are dependent upon various factors including number of satellites, geometry, obstructions, observation time, ephemeris accuracy, ionospheric conditions, multipath etc. GPS and GLONASS can increase performance and accuracy by up to 30% relative to GPS only. A full Galileo and GPS L5 XX constellation will further increase measurement performance and accuracy.

20 Factors influencing GNSS Height Accuracy Multipath (effect up to 15cm on L1, stay away from reflective surfaces, ) Long occupation times can mitigate..to an extent Most receivers have correlators to filter out multipath Re-initialisation no guarantee of mitigation

21 Factors influencing GNSS Height Accuracy Antenna phase centre models Electrical phase centre not fixed (effect up 10 cm, careful of mixing antennas, use correct APC model) Absolute APC s as well as relative APC s Eg: Trimble uses relative APC s by default and absolute as optional When you use an antenna that does not have an APC file it will use a default antenna or null info.

22 Factors influencing GNSS Height Accuracy Relative Antenna phase centre models Relative to Dorne Margolin Model T NGSCorrTable=lat504_leis.ngs (in TGO/TBC) ;PCT converted from <ant_info.003> <MLM-04/01/23=156> ;Processor name : pctconvert v1.02 ;Creation time : Fri Feb 27 09:05: ;Calibrated antenna : LEI AT504 w/leis Dome ;Mean phase center (mm) North East Up L1NominalOffset = L2NominalOffset = ;Elevation range (deg) Start Stop Step, ElevationRange = ;Azimuth step size (deg), AzimuthStep = 0, ;Azimuth/elevation corrections (mm), AZ=0 ;L ;L

23 Factors influencing GNSS Height Accuracy Absolute Antenna phase centre models ;LEIAT504 NONE ;Processor name : pctconvert v1.8 ;Creation time : Thu Dec 10 14:52: ;Calibrated antenna : LEI AT504+cr, Calibrated antenna : LEI AT504 w/leis Dome ;Mean phase center (mm) North East Up, L1NominalOffset = L2NominalOffset = ;Elevation range (deg) Start Stop Step, ElevationRange = ;Azimuth step size (deg), AzimuthStep = 5, ;Azimuth/elevation corrections (mm) AZ=0 ;L1: , ;L2: AZ=5 ;L1, , ;L2,

24 Factors influencing GNSS Height Accuracy Ionosphere Can have significant effect Dual frequency (careful of solar max) Troposphere use good model increase occupation times for large height differences Careful of micro climates

25 Factors influencing GNSS Height Accuracy Length of Baseline (for RTK < 5km for less acc work, RTK control < 1km, use precise orbits if PP, for static 1 hour per 50km) Ocean tide loading Effect can be up to 10cm! Don't use coastal base for inland surveys eg DRBN and PMBG Satellite Geometry Advice (PDOP < 3)

26 Factors influencing GNSS Height Accuracy Measuring height of instrument (measure 2 ways) Incorrect height of target use fixed pole

27 Ellipsoid GNSS & Reference Surface A smooth, mathematically defined model of the earths surface North pole b a Geoid Equatorial plane Elements of the ellipsoid a = Semi Major Axis b = Semi Minor Axis f = Flattening = (a-b)/a Ellipsoid

28 GNSS & Reference Frame Global Navigation Satellite Systems Global Positioning Systems (GPS) and Galileo makes reference to WGS84 reference frame ITRF Glonass (Russian) reference frame PZ90 Compass (Chinese) will use ITRF

29 Geoid GNSS & Reference Surface A surface of equal gravitational pull best fitting the average sea surface over the whole globe. That equipotential surface that on average coincide with mean sea level.

30 Magnitude of Geoidal Height (N) The separation between the geoid and the ellipsoid is the geoidal height (N) and varies spatially (globally) from - 107m to +86m.

31 Relationship between Geoid, Ellipsoid and Mean Sea Level

32 GNSS & Heights A elevation 50 m B elevation 41 m Geoid Height = N Ellipsoid Height = h Orthometric Height = H Earths Surface H=50 m H=41 m Geoid N=34m h=84m Ellipsoid h=73m N=32m Orthometric( H) : 50 m - 41m = 9m Ellipsoidal( h) : 84 m - 73m = 11m

33 SA Geoid 2010 In 2007, the Chief Directorate: National Geospatial Information (NGI) commissioned a study to develop an accurate geoid model for South Africa. This model had to be capable of converting ellipsoidal heights determined using NGI's TrigNet to orthometric heights on the South African Land Levelling Datum, to an accuracy of 10 cm (design requirements). Accuracy is 7cm absolute and relative <2cm + GNSS related error.

34 Magnitude of Geoidal Height in SA

35 Variation of Geoidal Heights in SA Figure 3.2 Geoidal heights over DS2826A ( N = 0.45 m over 50km x 50km) Figure 3.3 Geoidal heights over DS2530B (( N = 4.5 m over 50 km x 50km)

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38 GNSS Surveys in areas with large variations in N P 1 H= orthometric height h= ellipsoidal height N= Geoidal Height P 2 P 3 H 1 H 2 H 3 geoid MSL terrain h 1 h 2 h 3 ellipsoid N 1=14.6 N 2=15.3 N 3=16.1

39 GNSS Surveys in areas with small variations in N H= orthometric height h= ellipsoidal height N= Geoid Height P 1 P 2 P 3 H 1 H 2 H 3 geoid MSL h 1= h 2 terrain h 3 ellipsoid N 1=30.15 N 2=30.20 N 3=30.17

40 Ignoring the Geoid (no Geoid), constraining to here point P 1 P 2 H= orthometric height h= ellipsoidal height N= Geoid Height P 3 H 1 H 2 H 3 Terrain h 1 h 2 h 3 Geoid MSL N 1 =0 N 2 =0 N 3 =0 Ellipsoid

41 Ignoring the Geoid (no Geoid), constraining to orthometric height P 1 P 2 H= orthometric height h= ellipsoidal height N= Geoid Height P 3 H 1 H 2 H 3 Geoid MSL h 1 h 2 h 3 Terrain Ellipsoid N 1 =0 N 2 =0 N 3 =0

42 Geometric Geoid Modelling The previous slide was a single point shift/calibration/transformation to model geoid Some do multiple points which in effect does a tilted plane (local geoid) The clever surveyor uses a higher order parameters

43 Geometric Geoid Modelling: Dangers

44 Use of Geoid in relative sense In this case the user sets up at a point with known orthometric height. The base station would compute the ellipsoidal height and transmit this to the rover. At the rover(p 2 ) H 2 = (H 1 N 1 ) + h + N 2

45 Use of Geoid in absolute sense In this case the user sets up at a point with no known orthometric height. Determines ellipsoidal height from TrigNet (RTK 2-3cm or static post processing 1-2cm), depending on the proximity to TrigNet and other factors. Surveys rover points as previous, but could be a few cm out of sync with nearby vertical control. At the rover (P 2 ) H 2 = h 1 + h + N 2

46 Determining h from TrigNet

47 Determining h from TrigNet

48 Best Practice SAGEOID 2010 better than EGM2008 and and order of magnitude better than EGM96 Always use SAGEOID 2010, even if you set up on a known point! Geometric Geoid modelling appropriate only when surrounded by dense network of control points such as TSM s or working in small area with with little change in geoid.

49 Thank You