FREQUENTLY ASKED QUESTIONS

When will the new adjustment be finished?
The modified master plan has initiation of the Helmert Block retrievals in June 2005 with completion on February 10, 2007.

Could it be completed sooner?
Yes, if all software is completed and tested for beginning the block retrievals in June 2005, it is possible that the new coordinates could be computed and available in 12-15 months.

When will I see new coordinates in my area?
Because Helmert Blocking is essentially a method of doing a simultaneous adjustment of all data, all coordinates will be available upon completion of the project-scheduled to be no later than February 10, 2007.

When will I know if the contingency adjustment is executed for my area?
There are currently no plans for contingency adjustments; however, if the option becomes necessary because of delays in the implementation of Helmert Blocking software, the National Readjustment Web Page will have an announcement and a link showing the contingency schedule.

When will I see the new network and local accuracies?
These fields will be populated in the data sheets at the same time as the new coordinates appear. However, should the contingency option be necessary, these adjustments will not provide the new accuracy values.

What is the deadline for submission of a GPS UDN project for the readjustment?
The deadline for submission of a GPS UDN project will be June 1, 2005. Projects received by that date and ready for database loading will participate in the new adjustment. Projects received after that date or which cannot be immediately loaded in the database, will not participate in the adjustment. These projects will be loaded and published and then readjusted upon completion of the National Readjustment itself.

What about classical horizontal control surveys?
The Readjustment of the NSRS will contain only GPS projects. At the completion of the project, datum transformation grids will be developed to support those legacy data.

Will the currently published coordinates for the classical triangulation network stations still be available to the public?
NGS has no intention of "abandoning" the classical triangulation network stations. These stations will still be published as part of the National Spatial Reference System; they just will not participate in The Readjustment of the NSRS. When you look at the level of distortion that is anticipated to be removed by this adjustment, it's on the order of 3-7 cm. That's in the noise of the positional uncertainty of the classical network, which in most cases is good to 10-40 cm. The transformation algorithm will allow us to model the positions of the classical network to fit more closely with the GPS data.

What happens afterwards?
First, a report on the project will be written - probably in the 3 months following the completion of the adjustment. Then work will proceed on building datum transformation grids between the newer and older coordinates. Plans will be developed and executed for adjustment of the Hawaiian, Alaskan, and Pacific Island GPS networks. We continue reference frame maintenance work for both NAD83 and ITRF coordinate sets. And, of course, we will be training our users on the use of network and local accuracies.

Why not abandon NAD 83 and use ITRF instead?
The legislation of many states specifies NAD 83. A burden would be imposed on the states if they were requested to change their laws. Instead, final coordinates will be published in both the NAD 83 and ITRF reference frames. By maintaining both reference frames, the greatest number of users will be served.

What are local and network accuracies?
A local accuracy of a control point is a value that represents the uncertainty of its coordinates relative to other directly connected, adjacent points at the 95-percent confidence level. It will be an approximate average of the individual local accuracy values between this control point and other observed control points used to establish its coordinates. A network accuracy of a control point is a value that represents the uncertainty of its coordinates with respect to the geodetic datum at the 95-percent confidence level. The datum is considered to be best expressed by the Continuously Operating Reference Stations (CORS). A more detailed explanation can be found in the following FGCD document:

In what coordinate system (NAD 83 (NSRS), or ITRF, or both) will the adjustment actually be done?
The adjustment will be performed in NAD 83 (NSRS) and the final coordinates will be transformed to ITRF. Just prior to the Readjustment of the NSRS a transformation of the latest ITRF (200x) Continuously Operating Reference Stations (CORS) coordinates to NAD 83 (NSRS) will be made. Coordinates for CORS stations that have participated in the GPS reductions of any GPS survey included in the Readjustment of the NSRS will be constrained.

What will the NAD 83 coordinates from the Readjustment of the NSRS be called?
NAD 83 coordinates from the Readjustment of the NSRS will be labeled NAD 83 (NSRS). Date tags such as those previously used (e.g. NAD 83 (19xx)) will not be used.

What is Helmert Blocking?
Helmert Blocking is a method of network adjustment in which the network is broken up into a hierarchy of smaller networks(blocks). An adjustment is carried out first in each of the smallest members of the hierarchy, which are then combined by means of common points into next larger members which are also adjusted. The process continues until the entire network has been adjusted. The final result is the same as that of a simultaneous least squares adjustment of the entire network. Helmert blocking is a special form of solving by partitioning of the normal matrix, where the observations and unknowns are partitioned geographically. The method is especially applicable to the large, sparse systems of equations typical of large geodetic networks.