TEAM 14 FINAL REPORT
Executive Summary

Team Members:
1. Donna Amoroso
2. David Doyle
3. Steve Frakes
4. Kathy Milbert
5. Don Mulcare
6. Jim Ray
7. Ron Taylor
8. Maralyn Vorhauer
9. Bill Wallace

Team 14 was chartered to address issues raised by the pending completion of the state-wide High Accuracy Reference Networks (HARN). The opportunity this presents for a rethinking of the adequacy of the North American Datum 83 (NAD 83) and the consideration of the adoption of a new datum based on the International Terrestrial Reference Frame (ITRF) was the primary focus of the Team.
Detailed in the report are the following:
* Fact-finding and outreach activities
* Analysis of various adjustment options
* Justification for an ITRF-based datum
* Discussion of implementation issues
* Recommended supporting activities
In recognition of the NGS role in providing users a uniform and homogeneous coordinate reference system, Team 14 recommends the following:
*A nationwide readjustment of higher-order NAD 83 data be performed once all HARNs are
completed. All HARN and Continuously Operating Reference Station (CORS) data should be included to ensure a consistent, national framework reliable at about the 2-centimeter level horizontally. All readjustments should be completed by the year 2000.
* A policy change to eliminate multiple date tags for coordinates should be implemented
immediately. Rather than aiding users, date tags are confusing to them. Instead of a date tag, a tag indicating whether or not a position is consistent with the HARN should be adopted.
* An ITRF-based system should be adopted to ultimately replace NAD 83. The ITRF should be adopted due to its inherent accuracy advantages and its closer linkage to GPS orbit information.
* The shift to an ITRF-system should take place within the context of a large-scale educational and outreach effort. Every effort should be made to involve the user community in the implementation of this shift.
* A Project Director should be appointed to ensure a coherent, focused approach and to ensure project success.

Section 2
Background, Research, and Investigation
The Team was created to consider the impact of the pending completion of the state-wide High Accuracy Reference Networks (HARNs) and the advances in positioning and navigation technology on the current North American Datum of 1983 (NAD 83) reference system. The Team worked to balance user interests in avoiding unnecessary changes to coordinates with the obligation of the National Geodetic Survey (NGS) to provide users a reference frame of sufficient accuracy and homogeneity to meet their needs both now and in the foreseeable future.
Membership of the Team included representatives from the Geodetic Services, Geodetic Research, Observation and Analysis, and Spatial Reference System Divisions. Most members interact daily with users of NGS data products.
The Team sought insight into the implications of various adjustment options. Presentations were made to the Team by the following persons:
* Dr. James Ray discussed ITRF and its development and implementation by the International Earth Rotation Service (IERS). The accuracy and consistency advantages of the ITRF were also highlighted.
* Mr. Stephen Frakes discussed the technical aspects of the definition of NAD 83 and the
magnitude of differences between ITRF and NAD 83. NAD 83 is fundamentally flawed in the
sense that it was implemented prior to the development of GPS.
* Ms. Kathryn Milbert discussed the magnitude of distortions (horizontal) that have been
found between HARNs and the Continuously Operating Reference Station (CORS) network. She also discussed our current procedures for analyzing and resolving network problems.
* Dr. Dennis Milbert discussed his views of various adjustment options, the existence of
continuing problems with the height component in the northern areas of the United States, and developments in the use of real-time orbit predictions for decimeter-level (or better) positioning.
* Mr. William Strange discussed the discontinuity between HARNs and CORS. The need, at
a minimum, to provide users the means to combine CORS and HARN positions through either transformation software or other measures was highlighted.
* Mr. David Zilkoski discussed his experiences implementing the North American Vertical Datum of 1988 (NAVD 88). He highlighted the importance of involving users groups in decision making and implementation issues. In addition to these in-house presentations, the views of other Federal surveying and mapping agencies were solicited, including the Bureau of Land Management (BLM), National Imagery and Mapping Agency (NIMA), U.S. Army Corps of Engineers (USACE) and U.S. Geological Survey (USGS). Of these groups USACE was the only agency to accept the invitation, providing Mr. William Bergen to brief the team concerning the current state of USACE
activities. Mr. Bergen expressed the USACE view that a change in datums will have no significant impact on their activities, as the majority of their operations are still related to the North American Datum of 1927 (NAD 27)!
During the course of the Team's deliberations, team members Dave Doyle and Ron Taylor asked attendees at the annual GIS/LIS Conference in Denver for their opinion of the wide range of technical and political issues such a change would represent. Representatives of the four member organizations of the American Congress on Surveying and Mapping (ACSM), i.e., the National Society of Professional Surveyors, the American Association for Geodetic Surveying, the Geographic and Land Information Society, and the Cartographic and Geographic Information Society, were appraised of the rationale for such a change and their views solicited. There was considerable concern expressed by these groups that an ITRF datum change would lead to "coordinates du jour" and that the surveying and mapping community may not be adequately involved in the process. Every assurance was given that issues related to this process would be addressed at both the national and local levels.
State Geodetic Advisors who will be involved in implementation were also kept abreast of the Team's work. Advisors Taylor and Mulcare were Team members. Other Advisors provided feedback via E-mail and phone conversations. A sense of how State and local agencies will respond to such a change was also obtained. In some cases, such as Minnesota and Montana, a clear willingness to follow the lead of NGS was expressed. Others, such as Colorado, will resist such changes. Most states would probably keep their operations in NAD 83 due to extensive
investments in the current State Plane Coordinate System.
In regard to the implications such a change in datum would have on Canada, team member Steve Frakes contacted the Geodetic Survey of Canada. To ensure there would be no misunderstanding of their position, their response is quoted below:
"(We) would be interested in participating in a new adjustment to a reference frame compatible with the United States. However, we would likely not be interested in moving away from NAD 83. Our plans are to leave our current ACP and CBN coordinates unchanged until the turn of the century. Therefore, regarding timing, we would most likely be interested in a readjustment around the year 2000."

Section 3
Options with Pros and Cons
Team 14 began discussing various options at the third team meeting, after presenting some background information on ITRF and the current status of state HARN adjustments at previous meetings. Discussions on these options continued over the next six meetings, with a consensus reached at the meeting on December 16, 1996. Some further discussion was held at subsequent meetings to clarify issues to be presented in the final report. The following is a summary of the options discussed, the rationale presented, and the consensus reached, including the recommendation to the NGS Executive Steering Committee.
Option 1.
No change to current NGS procedures; when all HARNs and state-wide readjustments are
completed, re-label to NAD 83 (XXXX).
Pros
(1) NAD 83 is already widely accepted and is in legislation for most states.
(2) No new (additional) resources required.
(3) Most "traditional" users, e.g., surveyors and engineers, do not want a 2-meter change in coordinates.
Cons
(1) Distortions of 5 to 6 cm possible.
(2) Changing/improving technologies (e.g., CORS) will increase demand for higher accuracies.

The team realizes that not all "pros" and "cons" for various options have the same weight. In this case, although many traditional users may not favor a 2-meter change in coordinates, the large new set of users in the GIS/LIS area is probably not bothered by such a change. In many cases it will not impact their application. Most would not be opposed to a change as long as it was well-defined and explained. The demand for higher accuracies will have the most weight. Even traditional users will eventually realize that NAD 83 is not best suited for precise GPS computations and the status quo (NAD 83) is not the best way to take advantage of the accuracies GPS can provide.
Option 2.
A. Readjust nationally all B-order and better, holding the CORS sites fixed at their NAD 83 coordinates and re-label the datum as in Option 1.
Pros
(1) Remove remaining distortion (up to 5-6 cm) between the HARN and CORS.
(2) Correct discrepancies between the HARNs caused by piecemeal methods used in their original adjustments; this will allow for improved positional checks (2 cm) over very long lines.
(3) Eliminate the confusion caused by multiple datum tags.
Cons
(1) Most users do not require this level of accuracy and it may be hard to sell.
(2) NAD 83 is not compatible with the ITRF. The ITRF system is better suited for GPS.
(3) By readjusting only B-order and better, discontinuities between HARN and lower-order control will worsen; this will adversely affect areas where first-order GPS densification to the HARN has occurred.

B. Same as A except include down to first-order (GPS data only), as a separate layer.
Pros
(1) Will maintain maximum consistency between HARN and first-order GPS network.
Cons
(1) Will require additional resources to deal with the extra data.
C. Same as A except include down to all first-order as a separate layer.
Pros
(1) Will maintain a total first-order network consistent with the HARN.
Cons
(1) NGS currently does not have software to easily deal with a layer of classical data; this would have to be developed and tested.
(2) The amount of analyses/computation will increase significantly with the addition of classical data.
D. Same as A except include all GPS, as a separate layer.
Pros
(1) Will maintain a GPS network consistent with the HARN.
Cons
(1) There are currently a number of old GPS projects of lower-order with questionable data that could adversely affect the total solution.
(2) Additional analyses and computation will be required to deal with the large data set.
While there is some advantage in removing distortions at any of these levels (A,B,C, or D), if a nationwide readjustment is done on the NAD 83 datum, it suffers the same deficiencies as stated
in Option 1. NAD 83 is not compatible with the ITRF which is better suited for GPS data reductions and orbit computations. Thus the team rejected both Options 1 and 2.
Option 3.
Same as Option 2, including sub-options A, B, C, and D, except use ITRF instead of NAD 83 coordinates to define the datum.
Pros
(1) The new datum will be fully compatible with the ITRF; transformation parameters would be zero at a particular epoch.
(2) The new datum will be fully compatible with precise GPS orbits. (No rotations would need to be applied to the vectors.)
(3) The new datum will be fully compatible with precise GPS data reductions.
(4) The new datum will be fixed and stable on the North American Tectonic Plate, similar to EUREF and the new Mexican datum.
Cons
(1) Legislation has been passed in all but six states that adopts NAD 83 as the official datum for mapping, charting, and surveying. In addition, most Federal agencies and Canada have adopted
NAD 83.
(2) There will be significant position shifts throughout the country. Some examples follow - shifts are based on the transformation parameters from NAD 83 to ITRF94:

Section 4
Justification for ITRF Datum
Team 14 recommends adoption by NGS of a new datum based on ITRF, as maintained by IERS, for the following reasons:
* Increasingly, access to geodetic reference frames and the practice of geodesy and surveying rely on GPS. This trend is likely to accelerate.
* The nature of GPS is that it is global. ITRF is a global Cartesian coordinate system, geocentric to within about 1 centimeter and accurate at the few parts-per-billion (ppb) level. It is defined through combined solutions using VLBI, SLR, LLR, GPS, and DORIS data.
* The ITRF coordinate system supports GPS orbit computations and is consistent with GPS orbit and Earth orientation products, including both broadcast (within the now-equivalent WGS84 system) and precise orbits.
* The development of autonomous positioning capabilities based on real-time precise ephemerides and clock correctors forces consideration of the impact the divergence between the NAD 83 and WGS84 systems will have on this growing user community.
* The ITRF coordinate system provides a basis for more precise GPS data reduction computations than can be realized using the NAD 83 system.
* High-precision users of GPS routinely obtain positioning accuracies better than that supported by NAD 83 (even including recent HARN readjustments).
* The ITRF coordinate system has already been accepted as a basis for new datums in Europe and Mexico.
* Unquestionably, increasing numbers of GPS users will require a coordinate system comparable to ITRF and will not be satisfied with the limitations of NAD 83.

Section 5
Implementation of ITRF-Based Datum
Team 14 recommends that a new datum based on ITRF be adopted by NGS to ultimately replace the existing NAD 83 datum. We recognize that a large established user community is satisfied with NAD 83 and will retain it indefinitely. However, it is equally clear that the widespread and improving application of GPS dictates an improved datum. Therefore, it will be necessary to finalize the current NAD 83 datum and maintain it while transitioning to a new ITRF-based datum. Concerning the completion and maintenance of the NAD 83 datum, we recommend specifically that:
* The current plan of conducting statewide HARN surveys should be continued to completion and the current policy of piecewise readjustment of NAD 83 coordinates when discrepancies exceed 5-6 cm (horizontally) should also be continued. However, we feel this effort should be completed by the year 2000. If this time scale appears unrealistic, additional resources and efficiencies should be implemented to speed the production as may be needed. Furthermore, we recommend that when all HARN data have been collected, there be a fully consistent, national readjustment of the HARN and CORS data to ensure that the basic NAD 83 framework is reliable to about 2 cm (horizontally).
* Starting immediately, the following policy should be adopted for datum tags: there will be
two labels, NAD 83 (Upgrade) and NAD 83 (Original). The NAD 83 (Upgrade) refers to CORS
and HARN coordinates, as well as any other NAD 83 coordinates that are adjusted to be consistent with CORS and HARNs. NAD 83 (Original) refers to the 1986 general adjustment, as well as any other coordinates that are adjusted to be consistent with the 1986 general adjustment.
* When the HARN surveys and statewide readjustments are completed and assimilated into the NAD 83 datum (following a full national readjustment of the complete HARN and CORS data sets), the NAD 83 system will be considered finalized and no further adjustment of coordinates will be performed. However, NGS may continue to collect adjusted data from the User Densification Network (UDN) for incorporation into the NAD 83 data base.
* NGS will prepare a report for the user community which documents the accuracy characteristics of the NAD 83 datum and its relationship to other common datums, including the ITRF and WGS84 systems. Meanwhile, preparations should begin for the implementation of a new datum based on ITRF:
* The ITRF-based adjustment should be done when all HARN surveys have been completed. And, if possible, it should wait until the height component in the northern tier of states has improved through additional observations.
* It will be necessary to develop an adjustment strategy that includes a rigorous adjustment of the highest order GPS data only (possibly a segmented adjustment or possibly a Helmert Blocking adjustment). Only those GPS observations should be included which are comparable in precision to the internal precision of the ITRF system, namely a few centimeters in 3D coordinates. It may be advantageous to proceed in stages, incorporating data from the CORS network first, then including other subnetworks.
* Lower-order data should be excluded from the ITRF-based adjustment. New coordinates for these points will instead be generated from NAD 83 coordinates by a general finite-element transformation routine (analogous to NADCON), which must be developed.
* Coordinate values in this new system will be distinguished by whether they result from the rigorous adjustment or by transformation from NAD 83.
* The new datum will be defined by the current solution and epoch of the ITRF, whatever that might be at the time of adjustment. The reference epoch will be reported with all coordinate values.
* The Team recommends the new datum be defined with zero velocities on the stable part of the North American tectonic plate; that is, the new datum will rotate horizontally relative to ITRF by the known motion of the North American plate within a no-net-rotation frame.
* The velocities of all points in the new datum, expressed in a no-net-rotation frame, should be available to users and will include two components: the general rotational difference of a fixed North American plate relative to ITRF and any peculiar local motions (including vertical) that may be known for a given point (if available).
* The Team suggests that the new datum be named "NAD 2000'', provided the appropriate concurrences can be obtained from Canadian and Mexican authorities.
* NGS should prepare a report for the user community which documents the definition, realization method, accuracy, and other characteristics of the new ITRF-based datum and its relationship to other common datums, including NAD 83 (averaged over the full system) and WGS84.
Section 6
Supporting Activities and Policies Widespread support from NGS' traditional user community, as well as from other GPS-user groups, is essential for the success of this plan. Therefore, Team 14 strongly recommends early and vigorous involvement of the surveying community in the ACSM and FGCS committees in order to gain their support. Likewise, it would be advantageous to identify and involve other affected groups.
Specifically, Team 14 recommends the following activities and policies to prepare for implementation of the new ITRF-based datum:
* Designate a Project Manager as a full-time position with a grade appropriate to the task. The Project Manager will lead an Implementation Team consisting of geodesists, programmers, and support staff, representing all components of NGS, including the Information Services Branch and State Advisors. NGS must commit sufficient resources to the work of the Implementation Team.
* Within the Implementation Team a committee will be formed to evaluate which GPS data are suitable for direct inclusion in the new datum adjustment and which should be treated as lower-order.
* The Implementation Team will devise standards and criteria to guide any future updates of coordinates within the new datum.
* Form committees through ACSM and FGCS similar to the NAVD 88 Implementation
committees for advice and consultation.
* Develop a NAD 2000 workshop and other forums to interact with the user community. The NGS WWW site should be expanded to provide accurate, current information concerning the progress of this datum change and its implementation.
* State Advisors should be provided presentation materials and detailed information about the implications of the changes. Advisors must be provided the resources to advocate and explain the rationale and modalities of the adoption of the new datum. Advisors should present or arrange workshops on the new datum as part of their usual outreach duties.
* Develop and update adjustment software and transformation software.
* Create database tables and loading software, and revise the data sheet.
* Work with Canadian and Mexican geodetic authorities regarding coordinated datum plans and general agreement with these recommendations.