Post Processing - Achieving the Best Accuracy from your Field Data

Collection

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

Datum Shifts

#1 reason for mismatching GPS / GIS data is…

Datum Shifts Datum Shifts

I’m in NAD83

Mismatching Datums

WGS84

NAD27: Origin in KansasNAD27: Origin in Kansas

Datum Origin on Surface of Earth – Ideal for Local Datum

WGS84: Origin Center of EarthWGS84: Origin Center of Earth

Origin at Center of Earth - Ideal for a Global datum

NAD83 : Origin Center of EarthNAD83 : Origin Center of Earth

Not exactly equalto WGS84 - Ideal for North America

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

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)

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

Difference Between NAD83 (CORS96) and NSRS2007

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

Over Time Datums Grew Apart

NAD83 = WGS84

WGS84 = ITRF00 (2cm) NAD833.8 feet

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

Trimble ProXH Accuracy Test Using Survey Benchmark

NDGPSNational Differential Global Positioning System

Maintains Continuously Operating Reference Stations(CORS) For Providing Differential Correction

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

• 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)

Steps in Post-Processing

Using Pathfinder Office

You Have to Go Back

WGS84 = ITRF00

NAD833.8 feet

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

Open Export Utility

Summary

Output Files

Export Setup

Input Files

Creating an Export Setup

Setup Name: MyParksExport SettingsSetup Name: MyParksExport Settings

Review Properties Tabs

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

Coordinate System Tab

Controls how Pathfinder Office transforms and projects GPS data into the coordinate system of your choosing

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

Attributes Tab – Selecting Metadata Elements for Output Shapefile

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

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=

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.

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.

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

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