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 Metadata Elements Defining accuracy

Datum Shifts Datum Shifts Mismatching Datums #1 reason for mismatching GPS / GIS data is… WGS84 I’m in NAD83 # 1 reason for mismatching GPS and GIS data is the mismatching of datums. Mismatching Datums

NAD27: Origin in Kansas Never Export to NAD27! Datum Origin on Surface of Earth – Ideal for Local Datum As I see it, NAD27’s biggest problem is that the datum origin is on the surface of the earth and it’s second biggest problem is that the Clark 1866 ellipsoid just doesn’t work as you leave the vicinity of Meades Ranch. Never Export to NAD27!

WGS84: Origin Center of Earth Origin at Center of Earth - Ideal for a Global datum Military uses WGS84

NAD83 : Origin Center of Earth Not exactly equal to WGS84 - Ideal for North America NAD83 is the Standard!

Latest Datum Frame Adjustments refinements realization version reference frames flavors epochs WGS84 (G1150) WGS84 1987 - Original WGS84 (G730) 1992 WGS84 (G873) 1997 NAD83 (CORS96) NAD83 (1986) - Original NAD83 (1992) NAD83 (2002) Just when you thought all you had to learn was a Datum name like WGS84… It’s a bit more complicated. As a datum is more precicesly defined or datums become adjusted over time as we learn more about the center of the earths mass. Satellite, gravity measurements all enhance the level of precision that we can measure things. Datums therefore are adjusted or realized using the term epochs. These datum reference frames are noted by a year (1986) or (1992). Whats happened is that the most current realization of NAD83 and WGS84 differ by up to a meter. Errors can be introduced into a map by assuming these are the same

WGS84 World Geodetic System of 1984 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) Reference Frame of 1994 (ITRF94) +/- 10 cm in each component (Proceedings of the ION GPS-97 pgs 841-850) WGS84 (G1150) 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 Scottie (http://www.nima.mil/GandG/pubs.html)

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 David Zezula Published coordinates due March 2007

Difference Between NAD83 (CORS96) and NSRS2007

Simplified Concept of WGS84/ITRF00 vs. NAD 83 h83 Earth’s Surface h00 ITRF00 / WGS84 Origin 2.2 m ITRF (International Terrestrial Reference Frame) just has an origin; take NAD83 shaped ellipsoid centered at the ITRF origin to derive ITRF97 ellipsoid heights. Ellipsoid heights NAD83 vs. ITRF97 - Defined origins are best estimate of the center of mass; NAD83 is not geocentric. Move origin; move ellipsoid surface as illustrated. Ellipsoid height differences reflect the non-geocentricity of NAD83. NAD 83 Origin Identically shaped ellipsoids (GRS-80 & WGS 84) a = 6,378,137.000 meters (semi-major axis) 1/f = 298.257222101 (flattening) – GRS80 1/f = 298.257223563 (flattening) – WGS84

Over Time Datums Grew Apart WGS84 = ITRF00 (2cm) NAD83 3.8 feet NAD83 = WGS84 This slide works great. Shows the old and new (1986 what we knew then) and What we know now.

Difference between NAD 83 and WGS 84 (G1150) at 2002.0 Horizontal difference (feet) The amount of shift will depend on where you are… and this is all mathematical and has nothing to do with fault lines. Graphic: Michael L. Dennis, P.E; GPS, Geodesy, and the Perils of Modern Positioning

Trimble ProXH Accuracy Test Using Survey Benchmark

NDGPS National 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 collection Disadvantages: Requires direct ‘line of sight’ connection to the satellites Satellites are geo-stationary, and low in the horizon WAAS 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.html This 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 3.8 feet NAD83 Reverse the sequence…

+ ESRI Shapefile Pathfinder Office Objective .shx - index .shp - vector data .dbf - attribute name attribute value (Each Feature Name) ESRI Shapefile .ssf (cor) (All Features) Pathfinder Office Objective of this section is to take a COR file (an SSF file that has been differentially corrected) to an ESRI data format called shapefile. The Shapefile format is a universal data exchange format that must be comprised of at least 3 files. Each feature name in the DDF is translated to a unique shapefile. The INF and TXT files found in the export directory will house metadata or information regarding the Export parameters. These are really important files. Lets Get started. Ssf - Standard Storage Format Shx creates an index out of the vector and attribute files (just like index at back of a book) so that when a search is performed, the program doesn’t have to search the entire database, it goes to the index + .inf + .txt Information files

Open Export Utility Launching the Export Utility

Summary Input Files Output Files Export Setup This shows a My Parks Export Setting.

Creating an Export Setup Setup Name: MyParksExport Settings

Review Properties Tabs

Position Filter Tab If mistakes are made in the field, here is your last chance to filter out bad data. Non GPS Positions - created in PFO, not collected by a rover

Coordinate System Tab This tab controls how Pathfinder Office transforms and projects GPS data into the coordinate system of your choosing. In many cases you will need to change the default settings. Controls how Pathfinder Office transforms and projects GPS data into the coordinate system of your choosing

Shifts Will Happen 0.89meters Northing = 4299598.78 m Easting = 279910.29 m This is easily confirmed in PFO. These delta shifts of 0.89 meters for Manassas Virginia. How To: Take any point dataset and differentially correct against a CORS station, using the reference from base provider option. This means your COR file is in ITRF00 reference frame. Then export the same COR file twice, switching the Datum choice from NAD 1983 (Conus) and NAD 1983 (Conus) CORS96. Assign them both as NAD83 in ArcCatalog. Bring both into the same reference frame. The same point, will now appear shifted the difference between ITRF00(the export using NAD 1983 (Conus) and NAD83 CORS96. 0.89meters Northing = 4299599.65 m Easting = 279910.10 m

Attributes Tab – Selecting Metadata Elements for Output Shapefile

Shift Summary Different Datums: Different Realizations of 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) WGS84 (G1150) ITRF 2000 = Autonomous GPS WAAS OPUS CORS Post Processed * NDGPS (Beacon) OMNISTAR (Satellite) OPUS CORS Post Processed * Up To You!

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 transformation ITRF94 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. 32

How do GPS or GIS software handle datum transformations ESRI uses the EPSG transformations. This one is the ESRI _5 transformation and is equivalent to the NGS ITRF96 to NAD83(CORS96, epoch 1997) transformation 33

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