post processing - achieving the best accuracy from your field data collection
TRANSCRIPT
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Post Processing - Achieving the Best Accuracy from your Field Data
Collection
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GPS collection Survey Grade Receivers Mapping Grade Receivers Recreational Grade Receivers
Differential correction Real time CORS beacon WAAS Subscription (Omni-Star) Post-Processing NDGPS OPUS
GIS Program Exporting GPS Data for use in GIS
MetadataElementsDefining accuracy
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Datum Shifts
#1 reason for mismatching GPS / GIS data is…
Datum Shifts Datum Shifts
I’m in NAD83
Mismatching Datums
WGS84
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NAD27: Origin in KansasNAD27: Origin in Kansas
Datum Origin on Surface of Earth – Ideal for Local Datum
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WGS84: Origin Center of EarthWGS84: Origin Center of Earth
Origin at Center of Earth - Ideal for a Global datum
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NAD83 : Origin Center of EarthNAD83 : Origin Center of Earth
Not exactly equalto WGS84 - Ideal for North America
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WGS84 (G1150)WGS84 1987 - OriginalWGS84 (G730) 1992WGS84 (G873) 1997
NAD83 (CORS96)NAD83 (1986) - Original
NAD83 (1992)
NAD83 (2002)
Latest Datum Frame Adjustments
Latest Datum Frame Adjustments
reference frames
flavors
realization
epochs
refinements
version
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WGS84World Geodetic System of 1984
WGS84World Geodetic System of 1984
• WGS84– Released – September 1987– Based on Observations at more than 1900 Doppler Stations
• WGS84 (G730)– Datum redefined with respect to the International Terrestrial– Reference Frame of 1992 (ITRF92) +/- 20 cm in each component – (Proceedings of the ION GPS-94 pgs 285-292)
• WGS84 (G873)– Datum redefined with respect to the International Terrestrial– Reference Frame of 1994 (ITRF94) +/- 10 cm in each component – (Proceedings of the ION GPS-97 pgs 841-850)
• WGS84 (G1150)– Datum redefined with respect to the International Terrestrial– Reference Frame of 2000 (ITRF00) +/- 2 cm in each component – (Proceedings of the ION GPS-02) – http://164.214.2.59/GandG/sathtml/IONReport8-20-02.pdf
(http://www.nima.mil/GandG/pubs.html)(http://www.nima.mil/GandG/pubs.html)
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NAD83 North American Datum of 1983
NAD83 North American Datum of 1983
• NAD83 (1986) – Original Realization
• NAD83 (1992) – HARN Realization
• NAD83 (CORS96) – CORS Realization
• NAD83 (NSRS2007) - Readjustment- Ties together HARN & CORS
- Provides Network & Local Accuracies
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Difference Between NAD83 (CORS96) and NSRS2007
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Simplified Concept of WGS84/ITRF00 vs. NAD 83
2.2 m
NAD 83Origin
ITRF00 / WGS84Origin
Earth’sSurface
h83h00
Identically shaped ellipsoids (GRS-80 & WGS 84)a = 6,378,137.000 meters (semi-major axis)1/f = 298.257222101 (flattening) – GRS801/f = 298.257223563 (flattening) – WGS84
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Over Time Datums Grew Apart
NAD83 = WGS84
WGS84 = ITRF00 (2cm) NAD833.8 feet
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Horizontal difference (feet)Horizontal difference (feet)
Difference between NAD 83 and WGS 84 (G1150) at 2002.0
Graphic: Michael L. Dennis, P.E; GPS, Geodesy, and the Perils of Modern Positioning
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Trimble ProXH Accuracy Test Using Survey Benchmark
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NDGPSNational Differential Global Positioning System
Maintains Continuously Operating Reference Stations(CORS) For Providing Differential Correction
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Real Time Correction using WAAS
Advantages: Provides real-time correction to <7 meter accuracy Inexpensive alternative
Maintains native WGS84 datum in data collectionDisadvantages: Requires direct ‘line of sight’ connection to the satellites
Satellites are geo-stationary, and low in the horizonWAAS does not work well in forest canopy
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• WAAS satellites always reside relatively low in the southeastern sky at 40° in a geostationary orbit at 0° N and 53° W (i.e. near the Brazilian border with French Guiana and Suriname).
• WAAS in Canada: http://members.shaw.ca/pdops/WAAS.html
• http://www.lyngsat.com/tracker/g15.htmlThis website allows one to check the azimuth and elevation of the WAAS satellites relative to local position. For GLKN Network, 92.09W; 46.78N,
Immarsat 3 f4 at 142° : azimuth = 239.1°; elevation = 18.85° (WAAS/NMEA #35)
Galaxy 15: azimuth = 229.14°; elevation = 23.6° (WAAS/NMEA #48)
Anik F1R: azimuth = 200.58°; elevation = 34.98° (WAAS/NMEA #51)
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Steps in Post-Processing
Using Pathfinder Office
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You Have to Go Back
WGS84 = ITRF00
NAD833.8 feet
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Objective Objective
.inf.inf + .txt + .txt Information filesInformation files.inf.inf + .txt + .txt Information filesInformation files
+
.shx - .shx - indexindex
.shp.shp - - vector datavector data
.dbf.dbf - - attribute nameattribute name
attribute valueattribute value(Each Feature Name)(Each Feature Name)
.shx - .shx - indexindex
.shp.shp - - vector datavector data
.dbf.dbf - - attribute nameattribute name
attribute valueattribute value(Each Feature Name)(Each Feature Name)
ESRI ESRI ShapefileShapefile
ESRI ESRI ShapefileShapefile
.ssf (cor).ssf (cor)
(All Features)(All Features)
.ssf (cor).ssf (cor)
(All Features)(All Features)
Pathfinder Pathfinder Office Office
Pathfinder Pathfinder Office Office
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Open Export Utility
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Summary
Output Files
Export Setup
Input Files
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Creating an Export Setup
Setup Name: MyParksExport SettingsSetup Name: MyParksExport Settings
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Review Properties Tabs
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Non GPS Positions - created in PFO, not collected by a rover
Non GPS Positions - created in PFO, not collected by a rover
If mistakes are made in the field, here is your last chance to filter out bad data.
If mistakes are made in the field, here is your last chance to filter out bad data.
Position Filter Tab
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Coordinate System Tab
Controls how Pathfinder Office transforms and projects GPS data into the coordinate system of your choosing
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Shifts Will Happen
Northing = 4299599.65 mNorthing = 4299599.65 m
Easting = 279910.10 mEasting = 279910.10 m
Northing = 4299598.78 mNorthing = 4299598.78 m
Easting = 279910.29 mEasting = 279910.29 m
0.89meters
0.89meters
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Attributes Tab – Selecting Metadata Elements for Output Shapefile
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Shift Summary
• Different Datums:– NAD83 vs WGS84/ITRF00
• Different Realizations of Datums– NAD83 (1986) vs NAD83 (CORS96)
• Know your reference frame and if you use OPUS or post process, there may be more than one option to choose – there is over 1+meter in that choice
– ……..
• Know your Shift
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Which Reference Frame Are You In?
NAD83 (CORS96)
- NDGPS (Beacon)
- OMNISTAR (Satellite)
- OPUS
- CORS Post Processed *
- Autonomous GPS
- WAAS
- OPUS
- CORS Post Processed *
WGS84 (G1150)
ITRF 2000=
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Best Practices• Define the Coordinate System
– Units, datum, projection
• Tie to National Spatial Reference System– Occupy published survey control
• Specify and Test Accuracy– Clearly defined, consistent, robust
• Metadata– Field, processing, and export methods– Contact information
• Archive All Raw Data– Base, rover, metadata files– Avoid uncommon, proprietary formats.
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How do GPS or GIS software handle datum transformations
ESRI uses the EPSG transformations. This one is the ESRI _1 transformationITRF94 to NAD83(CORS94). This example is labeled for NAD83(HARN) but it was also used for NAD83(86) at one time so the epoch date is indeterminate.
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How do GPS or GIS software handle datum transformations
ESRI uses the EPSG transformations. This one is the ESRI _5 transformationand is equivalent to the NGS ITRF96 to NAD83(CORS96, epoch 1997) transformation
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A Few Additional Notes• Remember that when doing real time correction, CORS stations
broadcast NAD83(CORS96).The problem is if you are not able to maintain real time
connection throughout collection.In this case you have to be very careful to avoid mixing
datums
• Updates in New TerraSync (v. 3.20)-Now writes projection file (.prj) to output shapefile-Support for GLONASS – Russian GPS constellation-Real-time correction source datum transformations to WGS84, to avoid datum shifts within the same SSF file
• When doing differential correction, and accuracy is most important:- Real Time Correction uses IGS ultra-rapid (predicted) orbits (near
real-time) - Post-processing within one week, CORS stations use IGS rapid
orbits (one day delay) provides better accuracy than real-time- Post-processing using IGS precise orbits (typical delay 10-14 days)
provides the highest accuracy