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Home arrow Archives   The American Surveyor     

Frames for the Future: New Datum Definitions for Modernization of the U.S. NSRS (Part 1 of 4) Print E-mail
Written by David H. Minkel and Michael L. Dennis   
Saturday, 04 February 2012

A 1.275Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

In 2008, the National Geodetic Survey (NGS) released its 10-Year Plan (NGS, 2008). In that plan, NGS describes the replacement of the two national datums currently in use. These are the horizontal datum (the North American Datum of 1983, or NAD 83) and the vertical datum (the North American Vertical Datum of 1988, or NAVD 88). Note that NAD 83 (1986) was originally a two-dimensional datum in that only latitude and longitude were provided (that is, it was a horizontal datum and is still commonly referred to as a horizontal datum). Subsequent realizations of NAD 83, such as NAD 83(HARN) or NAD 83(NSRS2007), also provide ellipsoidal heights; they are three-dimensional and properly called a geometric datum. For the remainder of this article there will be no further use of the term horizontal datum as it is no longer appropriate.

These new geometric and vertical datums will provide more consistent three-dimensional positioning throughout the US, including changes in position with time (velocities), thus adding a fourth dimension to the system. The objectives of this article are to: 1) describe the new datums, 2) compare them with the current horizontal and vertical datums, 3) provide suggestions for easing the transition of geospatial data based on NAD 83 and NAVD 88, and 4) solicit comments from surveyors and other geospatial professionals. Since these new datums are currently under development, a complete description of the new datums and their ramifications is not currently possible. Nonetheless, enough is known to initiate a discussion with the user community, and this is the appropriate stage of the process for seeking user input. Note that NAD 83 and NAVD 88 are both well known by the term "datum", and their replacements, to avoid confusion, will continue to be called "datums" throughout this article. Although NGS recognizes the importance of accurate and consistent terminology, the question as to whether the term "datum" or "reference frame" is more appropriate has not yet been settled and will not be pursued in this article.

Background
The NGS 10-Year Plan acknowledges the utility and efficiency of Global Navigation Satellite Systems (GNSS) technology in virtually all geospatial activities. It is with this acknowledgment, and a desire to improve the utility of GNSS techniques, that NGS is planning a migration of the National Spatial Reference System (NSRS) from its existing datums based on networks of passive survey marks, to a GNSS-based NSRS comprised primarily of active stations. In this article, a passive mark is defined as a conventional survey mark (for example, a brass disk set in a concrete monument) with a position and/or height precisely known at the time it was observed. An active station is defined as a GNSS base station; a network of these stations­ referred to as Continuously Operating Reference Stations (CORS)­comprises the "back bone" of the NSRS. These CORS sites are constantly monitored by NGS for changes in position, and they were used to define the most recent realizations of NAD 83 within the U.S (NGS, 2011). In addition, GNSS data from the CORS are provided free of charge to the user.

A geometric datum, such as NAD 83, is defined by a total of seven parameters that specify the location, orientation, and scale of a Cartesian coordinate system with respect to an Earth-Centered, Earth-Fixed (ECEF) reference frame (which in turn is referenced to Earth's celestial surroundings). Furthermore, in order to compute the more humanfriendly coordinates of latitude, longitude and ellipsoid height, two additional parameters are required, specifying the size and shape of an ellipsoid centered at the origin of the geometric datum.

A "realization" of a datum is the coordinate set for control stations determined from various observations performed at a particular time, such as GNSS observations, or traditional terrestrial observations such as triangulation. As new information becomes available (such as better models, newer observations, etc.) new coordinate sets can be determined which may be used to define a new datum realization, if such a change is warranted (note that coordinate changes, by themselves, do not automatically invoke a new realization; other factors must be considered). The realization of a datum by NGS is typically identified by appending the datum name with the year of the adjustment as the datum tag, such as, NAD 83(1986), NAD 83(1992), or NAD 83(NSRS2007). That is, NAD 83(NSRS2007) is a single, specific realization of NAD 83. The datum remains the same, but the coordinate set improves, one realization to the next.

The so-called HARN (High Accuracy Reference Network) or HPGN (High Precision Geodetic Network) surveys are commonly referred to as NAD 83 (HARN or HPGN). However, the technically correct name replaces HARN and HPGN with a date; for example, in Arizona the HARN realization is NAD 83 (1992), while in Utah it is NAD 83 (1994). NAD 83 (1986) and NAD 83 (NSRS2007) are the only nationwide realizations of NAD 83 passive control; all others are regional realizations. As a related aside, the NGS Datasheet for control stations identifies the NAD 83(NSRS2007) realization simply as NAD 83(2007), due entirely to formatting restrictions on the datasheet.

Looking to the future, the cost involved in maintaining or expanding our existing national network of passive survey control is beyond the resources of NGS. The use and maintenance of active stations and appropriate geophysical models is considerably more cost-effective and will result in generally more accurate and consistent positioning. Improved temporal continuity of the datum can be assured by periodic update of geophysical models at less cost than re-observing the existent network of passive stations comprising the NSRS. Furthermore, a nationwide network of active stations allows local jurisdictions to economically establish and maintain their own passive control stations when required, though adding this information to the NGS-maintained database of passive control allows it to be consistently tied to the national datum.

There are issues with NAD 83 and NAVD 88 with respect to their ability to support modern geospatial technologies and applications (Smith, 2010). These issues require addressing if the NSRS is to adequately support the Nation's future needs. Below is a brief summary of these issues:
• NAD 83 is not truly geocentric, which makes it inconsistent with geocentric-based GNSS positioning (that is, with respect to Earth's center of mass)
• Inconsistencies exist between the NAD 83 coordinates of passive marks and the Continuously Operating Reference Station (CORS) network
• NAD 83 does not report station motion ("velocities") for passive marks
• NAVD 88 exhibits a cross-country "tilt" or "slope" relative to the geopotential surface it was meant to reference
• Passive marks are often in inconvenient locations and are vulnerable to disturbance and destruction
• There exists an approximate 0.5 m bias in the NAVD 88 reference surface relative to the geoid surface that best approximates global mean sea level (as determined by the current NGS gravimetric geoid)
• Passive marks are subject to undetected vertical motion from subsidence, tectonic uplift, freeze/thaw, and other vertical crustal motions which invalidate heights in some areas of the US.
• The gravity model and computational techniques used to determine NAVD 88 are not consistent with those currently used for geoid modeling
• Changes to Earth's gravity field cause changes to the geoid, and thus to orthometric heights, but NAVD 88 does not allow/account for those changes (because it is based on a static gravity model). This limits the usefulness of NAVD 88 passive marks for certain applications (such as, detecting and managing height change at Great Lakes region due to post-glacial isostatic adjustment)

The intent of this article is to present the expected changes that will be introduced by using these new datums as part of modernizing the NSRS. For a more detailed discussion of the issues associated with NAD 83 and NAVD 88, the reader is directed to Smith (2010).

For ease of reading, for Parts 2 through 4 of this article, the new datums will be called the National Geometric Datum (NGD) and the National Vertical Datum (NVD). However, please note that the names of the new datums have not yet been determined.

David Minkel is a geodesist in the Geodetic Services Division of the National Geodetic Survey. Formerly the NGS Geodetic Advisor to Arizona for 12 years, he has worked in NOAA's charting and geodetic services for the last 36 years.

Michael Dennis is a geodesist at NGS and is currently manager of the National Adjustment of 2011 (NA2011) Project. He is also a licensed engineer and surveyor, including ownership of a consulting and surveying firm.

A 1.275Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE

 
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