From the Ground Up
The National Geodetic Survey (NGS) is responsible for the maintenance of the National Spatial Reference System (NSRS). One of the NGS goals under the NSRS is to maintain a network of control stations that is suitable in terms of accuracy for all the work that professional surveyors do.
Perhaps this is something that is easy to take for granted, but it is critically important in our profession. Significant changes have taken place over the last 20 years that have driven the demand for accuracy substantially higher. Better equipment, improved field methods and more comprehensive reduction software allow us to meet these high accuracy demands, but only when we have accurate and consistent control positions to begin from.
NGS began a bold initiative in June 2005 aimed at a national readjustment of the North American Datum of 1983 (NAD83), a datum of the NSRS. When completed this year, this new adjustment will provide consistencies and accuracies never before realized in the existing datum. The new adjustment will be known as NAD83 (NSRS 2007). According to NGS, this convention signifies that the datum is based on a “readjustment of all GPS data in the National Spatial Reference System (as opposed to state or regional adjustments) and 2007 will be the date of publication.”
When asked about the readjustment, David Doyle, chief geodetic surveyor for NGS, stated, “The readjustment will be released on February 10, 2007, which is the 200th anniversary of the creation of what is now the National Geodetic Survey.” For the historians among us, the provision for a systematic coastal survey was enacted under the Department of the Treasury on Feb. 10, 1807. First established under President Thomas Jefferson, the agency was known as the Survey of the Coast. It was reorganized as the Coast Survey in 1836 and then as the Coast and Geodetic Survey (C&GS) in 1878. In 1970, the various functions of C&GS were integrated into the formation of the National Oceanic and Atmospheric Administration, and the geodetic office was renamed the National Geodetic Survey.
The Need for ReadjustmentNAD83 was first published in 1986, and was known then as NAD83 (1986). NAD83 was developed through a comprehensive readjustment of all survey data available at the time to establish new coordinate positions for all points in the network. While NAD83 provided a significant improvement over the North American Datum of 1927 (NAD 27), the basis for this readjustment was conventional surveying measurements; GPS was not yet a fully capable system. NAD83 removed significant local distortions found in NAD 27, most of which had accumulated due to the piecemeal nature of expanding the original control framework. Although this adjustment provided significant improvements, changes were rapidly occurring in the way we established new control positions. Thus, a need for even better accuracies in posi-tional values arose.
GPS use rapidly increased soon after the implementation of the NAD83 (1986) datum because more satellites were added to the GPS constellation, which greatly increased the viability and productivity of GPS-based surveys. Then a new issue with the datum was exposed: the accuracies of the new GPS surveys were significantly better than the positional accuracies of the available control stations in the NSRS. This basically meant that new high-accuracy GPS surveys had to be degraded to fit existing control stations, and this quickly became an issue of importance within NGS.
The solution was relatively simple. NGS, in cooperation with many federal, state, local government and private sector partners, conducted GPS surveys to increase the positional accuracies of the existing and new control stations. These GPS surveys formed the basis for the High Precision GPS Networks (HPGN), later renamed the High Accuracy Reference Networks (HARN). This work was conducted on a state-by-state basis with field observations beginning in Tennessee in 1989 and concluding in Indiana in 1997. These readjustments included all available surveying data, both conventional and GPS, in the determination of new positional values. They also created a need for A-order and B-order control designations for GPS-based high-accuracy control points to supplement the existing First-, Second- and Third-Order designations found in the original NAD83. State HARNs proved to be a significant improvement over the original datum and an important resource for all users of GPS positioning.
More change, however, was on the horizon. The growth in popularity and use of the Continuously Operating Reference Stations (CORS) from 1994 until the present time has created a new issue. Previous readjustments were conducted on a state-by-state basis, leaving the door open for minor inconsistencies between the national-based CORS and state-based HARN systems. That also allowed minor differences in HARN positions from one state to another. With the existing NSRS and datum, it is possible to observe a highly accurate network tied to the HARN and find discrepancies of up to 6 or 7 centimeters when compared to similar positions derived from CORS. While both systems are highly accurate, they are also generally independent. In order to remove the inconsistencies between the passive control HPGN/HARN stations and CORS, NGS conducted a final national re-observation campaign from 1997 to 2004, often referred to as the Federal and Cooperative Base Network (FBN/CBN) surveys. The aim of this final national resurvey was to provide accurate connections with the CORS and to ensure the integrity of the ellipsoid height component of the HPGN/HARN stations to not worse than 2 cm.
Basis and Results of the ReadjustmentAfter the national readjustment is complete, new horizontal positions and ellipsoidal heights will be published for all GPS stations included in the adjustment. The CORS network is serving as the basis of control for the readjustment, as CORS positional values are being held fixed in the solution. And in a break from earlier readjustments, only stations established by GPS methods are being adjusted in this process. Conventional observations are not included. In fact, some of the lesser quality GPS stations did not make the cut based on their overall positional quality.
When asked what the biggest challenge was in the national readjustment, Doyle responded, “Simply the amount of data that had to be considered. There are more than 70,000 GPS stations nationwide that are included. That number caught us by surprise. Remember that all have been added to the national database in the last 20 years.”
Doyle continued, “The biggest surprise is that the really big early projects that started us on this road to high accuracy, such as the original Tennessee HARN and the Eastern Strain Network, aren’t of sufficient quality to be part of the readjustment. This data no longer fits into the category of contemporary data. Technology has changed that much over the last 20 years, resulting in considerable improvement in the quality of GPS positioning.”
The positional differences between the existing NAD83 datum and the readjustment may at first seem minor, but for many of today’s higher accuracy applications, these differences are anything but minor. According to Doyle, “There are only a few stations with positional differences in excess of 5 centimeters. The largest differences in most states appear to be in the 6 to 7 centimeter range in either horizontal position or ellipsoidal height. GPS positions in most states are significantly better than these maximums. For example, the state of Kentucky, which includes a little [more than] 1,000 GPS positions in the readjustment, has a minimum shift of about 2 centimeters and a maximum of 3.5 centimeters.”
For the record, Minnesota has the distinction of being the state with the most GPS stations included in the readjustment at 7,003. New Hampshire has the least at 63.
Geodetic BenefitsNGS serves an incredibly important and capable role to all surveying and mapping professionals. Its members place significant technology at our fingertips every day, thanks to their leadership and foresight. This includes the development and management of the HARN and CORS network, and the creation of the Online Positioning User Service (OPUS). In addition, the quick and easy searches for known control monuments they have enabled in their data sheet queries of their website are well chronicled. These tools and resources have made a tremendous impact on the way we conduct our professional lives. The leadership of NGS in the establishment and improvement of the NSRS and the NAD83 datum that we rely on every day is maybe a little less visible and perhaps sometimes taken for granted, but critically important.
Doyle sums it up well: “The biggest benefit [of the readjustment] to geodetic professionals is this will bring us near perfect consistency between the CORS and the HARN. Under the existing system, it is possible to see absolute positional differences of 6 to 7 centimeters between these two highly accurate systems. With the readjustment, geodetic professionals will have a single datum where the residual differences between the CORS and HARN have been removed.”
To read Mark’s previous articles, click to www.pobonline.com and search the Point of Beginning database for “Mark Meade” or “From the Ground Up.”