Spatial -

NOAA Datums Videos 2014

Differential correction with NA2011 and the newest epoch of NAD83

See our and Datums Basics and GPS Tutorials for the long-awaited results of months of rigorous testing and peer review

Trimble Positions software has replaced Trimble GPS Analyst in ArcGIS 10.1

ALASKA GPS and GIS users - check out Joel Cusick's Datum and Projections presentation Feb 2013

ArcGIS 10.1 - New and Cool improvements see Presentation from Melita Kennedy, ESRI : Projections and Datums Dec 2012

Home GPS Tutorials

 A. Changes in 2012

Previously, the NGS “Old” ITRF00 coordinates exactly matched Trimble CBS “Offline” ITRF00 coordinates.  Both referenced to ITRF00, values could be compared directly, that is, no transformations were necessary.  In October 2012, the “Offline” coordinates were removed from the Trimble CBS.  With this change, the ability to directly compare coordinates on the CBS list with those on the NGS datasheets was lost.  Our first demonstration shows that even with HTDP transformation, a match can no longer be found of NGS and Trimble CBS coordinates.  

B. Trimble Pathfinder Office

When NGS released the NA2011, the Community Base Station (CBS) list showed coordinates in both –

“NEW”: (CORS, GARLIC (GARL), NEVADA (ITRF00 (1997)-Derived from IGS08 (NEW)).  The IGS08 epoch 2005.0 locations on the NGS datasheet have been transformed via HTDP software to ITRF00 epoch 1997.0, the chosen reference frame of Pathfinder Office

“Offline”: The original ITRF00 epoch 1997 coordinates

C. National Geodetic Survey (NGS)

With the National Adjustment of 2011 (NA2011), NGS realigned all CORS stations with passive controls.  Also in Sept 2011, NAD83(2011), the latest epoch of NAD83 was released.  To help users with this transition, NGS Position and Velocity Coordinate datasheets provide reference station locations in –

“NEW” coordinates in both IGS08 and NAD83(2011).  These are referenced to the latest absolute antennae positions, and

“Old” coordinates in both ITRF00 and NAD83(CORS96).  These preserve the original relative antennae positions.  NGS preserves this information as historical data but they will no longer be updated.  It is expected these will be accessible by ftp only by the end of 2013.

D. GOAL- Find the best workflow in light of these changes

In light of NA2011 and recent changes in PFO, this series of pages documents a months-long exploration aimed at  finding the best workflow for post-processing GPS data by varying the below settings:

Differential correction (.ssf to .cor file)

  • Compare “base file” vs. “base provider” options for Trimble CBS coordinates
  • “Seed” (manually enter) coordinates from NGS datasheets in each of the 4 available reference frames


Export (.cor to .shp file)

  • NAD83(Conus), a null transformation, i.e. all parameters set to 0
  • NAD 1983 (2011), a 7-parameter transform from WGS84 to NAD83(2011)

Given the high level of accuracy expected from all GPS data, sub meter imagery and other GIS datasets, users should be concerned when post-processing their carefully collected GPS data introduces any amount of shift.

E. Recommendations

·         ALWAYS include the epoch of each datum in your metadata

·         Use only CORS base stations for differential correction of GPS data

·         Manually seed NGS NAD83(2011) values in Pathfinder Office differential correction

·         Avoid applying transformations within Trimble Pathfinder Office

·         Assign .prj file to ensure the correct transformation(s) is applied in ESRI ArcGIS

F. Request of Trimble

This series of pages is in no way meant to assign blame to Trimble, NGS or any agency.  Rather it has sparked communication and revealed the mixed blessings brought by the latest high accuracy receivers, processing and imagery. 

Users trust Trimble.  This trust is not misplaced as it is a very powerful product on which many of us have relied upon for years.  As an international company, Trimble literally has uses across the entire globe.  However, a significant portion of the users are based in NAD83.   Given NAD83(2011) has been adopted as the standard by NGS, USCG real-time beacons, OPUS solutions and the US federal government (i.e. NPS, BLM, USFS and BIA).

We trust Trimble will do the 'Right Thing' for North American users and

move from ITRF00 epoch 1997 to the latest NAD83(2011)… please

Title Filter     Display # 
# Article Title
11 Request to Trimble
12 Hoping Trimble will adopt NAD83(2011) or IGS08 standard
13 Why GPS and GIS data are not in the same reference frame
14 Why post-processing is important
15 Key issues when moving GPS data to a GIS
16 Accuracy REALLY does matter
17 Special gratitude for contributions and ongoing review
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