Explanation of NAD 1983 (2011)
From the NGS Web site:
“As part of continuing efforts to improve the NSRS, on June 30, 2012, NGS completed the National Adjustment of
2011 Project. This project was a nationwide adjustment of NGS "passive" control (physical marks that can be
occupied with survey equipment, such as brass disk bench marks) positioned using Global Navigation Satellite
System (GNSS) technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83)
latitude, longitude, and ellipsoid heights of NGS Continuously Operating Reference Stations (CORS). The CORS
network is an "active" control system consisting of permanently mounted GNSS antennas, and it is the geometric
foundation of the NSRS. Constraining the adjustment to the CORS optimally aligned the GNSS passive control with
the active control, providing a unified reference frame to serve the nation's geometric positioning needs.”
What does this mean for you?
It is important to understand that this adjustment did not create any new datum, nor did it redefine the relationship
between Lat. / Long. / Ellip. Ht. and your state plane system. When the adjustment was completed the only thing that
changed from previous years was the coordinate values on the passive marks. The CORS stations were held as
control but keep in mind that all CORS data was adjusted in 2011 in the NAD 1983 (IGS08) adjustment. Think of an
error ellipse around a passive mark. Depending on what is constrained (or held as control) in the adjustment will
impact the final values of the passive marks. The below diagram shows an example of the different values by
adjustment year:
If you were to set up your base on the monument above, you will need to know what which coordinate you keyed
in to start your survey.
On an NGS data sheet, near the top, you will see the current values being used by NGS.
Near the middle are the associated State Plane values:
Near the bottom of the sheet are the superseded values from the previous adjustments:
The difference between the 2007 values and the 2011 values for this particular station is 2cm in the easting and
3mm in the northing. Differences of this small would be hard to notice using RTK methods. Below is a map of the
magnitude of this adjustment on the passive marks.
Courtesy of NGS
What if I am using Real Time Network (RTN) data? Well because the 2011 adjustment held the CORS positions as
fixed, then by default you are already on the 2011 adjustment. Maybe?
One thing to consider is that not all RTN broadcast the same values that NGS uses there could be small differences.
Most RTNs are managed by private companies or government agencies that may or may not be corresponding
directly with NGS and comparing values. These private networks are typically established using various methods
such as an average of 30 days of OPUS solutions and/or processed and adjusted by and individual using their post
processing software. These RTNs will start broadcasting their data and then submit their station to become part of
the national CORS. Due to budgetary restraints, NGS is typically behind on establishing their values for their
stations. Because the RTN community is anxious to implement their network and that changes in network values
are inconvenient to the end users, these differences in coordinate values often remain.
So for example, the CORS at our office in Indianapolis is part of the National CORS.
The difference between our broadcasting values that were established in 2007 prior to the 2007 readjustment)
and the 2011 NGS published values are 2cm in the easting, 1mm in the northing and 4mm in the ellipsoidal height.
The difference between the 2007 broadcast values and the NGS superseded values (CORS96) are 7mm in the
easting, 7mm in the northing and 15mm in the ellipsoidal height.
So while theoretically if you are using an RTN, the intent is that you are using NAD 1983 (2011) knowing what
values the RTN is pushing is critical.
In the past Trimble never distinguished between the different adjusments. This was because in NAD 1983 a certain
Lat. and Long will always yield the same Northing and Easting no matter the adjustment year because this is not a
datum change. Keep in mind that Lat. and Longs also have a datum or reference ellipsoid that can very greatly.
Since the beginning Trimble has always call the Lat. and Long. a WGS84 position. This was an incorrect statement
but everyone came can to associate WGS84 (also very very close to ITRF) with NAD83. The reality is that your Lat
and Long have always been NAD83 lat and long. With the newest release of Trimble Business Center and Access,
Trimble tried to address this issue. So the NAD 1983 (2011) option in your data collector and office software now
are assuming that the Lat and Long you are receiving is ITRF (title global) an try’s to convert that position to NAD83
(title local). If you RTN administrator had converted their network to push out ITRF positions, then this would have
worked out fine. As most of you know or are finding out, the RTN administrators have not done that, causing
Trimble software to double correct your lat and long. So until further notice DO NOT USE the NAD 1983 (2011)
option in you Trimble Survey Software (Mapping customers, please consult your Trimble sales associate). For your
information the Trimble VRS Now network is currently pushing the NAD 1983 (COR96) which is one and the same
with NAD 1983 (2007). INDOT has moved to NAD 1983 (2011) has have other DOT’s RTN (please contact your RTN
administrator to be sure). So please continue to select just NAD 1983 if you desire for your values to be on state
plane coordinates and DO NOT use NAD 1983 (2011) option in Trimble software until further notice.
Also please find attached a video on how to update your Trimble Business Center project to new Lat and Long
values. This file is an swf so if you download it and then right click it and tell windows to open it in Windows
Explorer then it will open and play.