Understand the differences between the North American Vertical Datum of 1988 and the new 2022 Vertical Reference Datum
My Survey Scene columns have focused on procedures and routines for establishing GNSS-derived orthometric heights. My last column focused on analyzing NGS’ GPS on BM data set that is used to make National Geodetic Survey’s (NGS) hybrid geoid models. It provided procedures that users could employ when analyzing the differences between the modeled geoid values and the computed geoid values using GPS/Leveling data. This GPSBM data set or one similar will be used to make the next hybrid geoid model, as well as provide input to the transformation model between the North American Vertical Datum of 1988 (NAVD 88) and the new 2022 Vertical Reference Datum. As I emphasized, all geospatial users should help develop this GPS on BMS data set to help improve the National Spatial Reference System and future hybrid geoid models.
Large relative differences in residuals between neighboring stations provided examples of stations that should investigated based on different reasons: No. 1, a weak NAVD 88 is leveling network design in the region; No. 2, the station’s published height attribute code implies that the station was not rigorously adjusted into the NAVD 88; and No. 3, station pairs have different adjustment dates indicating a possible adjustment distribution correction issue or movement.
It was mentioned that NGS has a program called “GPS on Bench Mark” to support users that occupy bench marks with GNSS equipment. It was also mentioned that in addition to participating in the NGS’ GPS on Bench Mark program, all geospatial users should participate in the NGS 2017 Geospatial Summit, which will be held in April 2017 in Silver Spring, Maryland. This summit is an opportunity for all users of the National Spatial Reference System (NSRS) to obtain a better understanding of NGS’ plans to modernize the NSRS. Users will be able to provide feedback directly to NGS leadership.
This column will briefly address NGS’ plans to replace the North American Vertical Datum of 1988 in 2022; and, in my opinion, why GNSS users need to obtain a better understanding of the differences between the NAVD 88 and the new 2022 Vertical Reference Datum.
First, NGS has a very nice website that discusses their new datums of 2022.
The frequently asked question section provides information on the expected changes in coordinates.
I have highlighted three FAQs that I believe users should learn more about. (See box titled “Excerpts from NGS’ New Datums FAQs, below.) Under the FAQ “Why is NGS replacing the North America of 1983 and the North America Vertical Datum of 1988 (NAVD 88)” it states that the NAVD 88 is both biased (by about one-half meter) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today.
|Excerpts from NGS’ New Datums FAQs Web Page
Why is NGS replacing the North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88)? NAD 83 and NAVD 88, although still the official horizontal and vertical datums of the National Spatial Reference System (NSRS), have been identified as having shortcomings that are best addressed through defining new horizontal and vertical datums. Specifically, NAD 83 is non-geocentric by about 2.2 meters. Secondly, NAVD 88 is both biased (by about 0.5 meters) and tilted (about 1 meter coast to coast) relative to the best global geoid models available today. Both of these issues derive from the fact that both datums were defined primarily using terrestrial surveying techniques at passive geodetic survey marks. This network of survey marks deteriorates over time (both through unchecked physical movement and simple removal), and resources are not available to maintain them. The new reference frames (geometric and geopotential) will rely primarily Global Navigation Satellite Systems (GNSS) such as the Global Positioning System (GPS) as well as an updated and time-tracked geoid model. This paradigm will be easier and more cost-effective to maintain.
How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums? The NSRS will be accessed using Global Positioning System (GPS) technology that references Continuously Operating Reference Stations (CORS) and relies on a time-dependent gravimetric geoid model. This method of accessing the NSRS is a paradigm shift from accessing NAD 83 and NAVD 88 through the use of geodetic survey marks.
How can I learn more about the new reference frames? NGS will participate in and host meetings to discuss the transition from NAD 83 and NAVD 88 to the new reference frames. We will continue to update our website with events, announcements and new outreach materials, as they become available.
The “Get Prepared” section on the New Datum website explains how users can get a ready for the new datums. In this section, NGS provides a figure that depicts the approximate orthometric height change between the NAVD 88 and the 2022 Vertical Reference Datum (Figure 1).This figure may look familiar to many of you. The difference between the NAVD 88 and the National Geodetic Vertical Datum of 1929 (NGVD 29) was more than a meter from the east coast to the west coast. This difference was documented in a 1992 report titled “Special Report – Results of the General Adjustment of the North American Vertical Datum of 1988.” Figure 2 shows a plot of the differences between NAVD 88 and NGVD 29. A similar difference was detected in 1929 when the NGVD 29 general adjustment was performed. Figures 3 and 4 depict the adjusted height differences between the NGVD 29 fully-constrained adjustment and a minimally-constrained adjustment. The heights of 26 tide stations were constrained in the fully-constrained NGVD 29 general adjustment. Four of the constrained tide stations have been labeled to show the differences between the east coast and the west coast of the U.S. As indicated in the plots depicting the results of the NGVD 29 General Adjustment, the difference between stations at St. Augustine, Florida, and Fort Stevens, Oregon, was 86.3 centimeters. This is similar to the trend between the NAVD 88 and NGVD 29. It should be noted that most of the original NGVD 29 leveling data were revealed, so the leveling data observed prior to 1929 were not included in the general adjustment of the NAVD 88. In 1929, the Coast and Geodetic Survey (the predecessor of the NGS) decided to constrain the heights of the 26 tide gauges and force the differences between the constraints. Prior to performing the NAVD 88 general adjustment a special project was performed to evaluate possible constraints including constraining heights of various tide gauges. Regardless of which datum definition scenario was chosen, i.e., the height of one tide gauge or heights of multiple tide gauges on the east and west coasts of the U.S., the results showed that large differences between NAVD 88 and NGVD 29 heights would exist. These differences were due to many factors, such as large distribution corrections (residuals) from the NGVD 29 adjustment, better estimates of corrections applied to account for systematic errors, crustal movement, and estimating geopotential differences using real gravity values instead of normal orthometric height differences. Based on the results from the special study, NGS decided to constrain the height of one tide gauge and not force the differences between constraints. This was mainly because of the uncertainty of the heights of the tide gauges representing the same equipotential surface. The new 1988 heights are much better estimates of orthometric heights than are the NGVD 29 heights.
As part of the NAVD 88 datum definition study, NGS also compared Satellite-Derived Orthometric heights computed using the best available geoid model and ellipsoid heights with NAVD 88 leveling-derived heights. See box titled “Excerpt from 1992 report titled “Special Report – Results of the General Adjustment of the North American Vertical Datum of 1988.” As stated in the report, based on the comparison, combining VLBI-derived orthometric height difference data with leveling data in NAVD 88 would not have helped to control remaining errors in the leveling data, or significantly improved the estimates of adjusted heights in the network adjustment. The results were consistent with the accuracy statements of GEOID90. VLBI-derived orthometric heights did not show the same 1.5 m difference indicated by the LMSL (epoch 1960-78) tidal heights. Therefore, if the coast-to-coast leveling does indeed contain long-wave-length systematic errors, the errors probably are not as large as 1.5 m.
Corrections to account for known systematic errors were applied to the leveling data involved in the NAVD 88 but it is recognized that the leveling data could still have a small systematic error remaining in the data. The leveling distance from the east coast to the west coast is over 5,000 kilometers. If we assume that the leveling crew performed a setup every 150 meters, then the number of setup across the country would be over 30,000 setups. If we assume a systematic error of 0.02 millimeters, then the accumulated error could be at least 600 millimeters (60 centimeters). Although, there also could be a small systematic error in the scientific geoid model due to an undetected and/or unmodeled long wavelength error. (Of course, this statement is from the NAVD 88 Project Manager, me, and may be a little bias). At this moment, it really doesn’t matter why the systematic difference exists, just that it does and that the new 2022 Vertical Reference Datum will be established using the same process used to generate the scientific geoid models. Therefore, it is important for all users of geospatial data to prepare for the changes.
|Excerpt from 1992 report titled “Special Report – Results of the General Adjustment of the North American Vertical Datum of 1988”
Comparison of NAVD 88 Adjusted Heights from the General Adjustment with Satellite-Derived Orthometric Heights
In a report by Despotakis (1987), discussed in the datum definition study by Zilkoski, Balazs, and Bengston (1989), numerical computations of geoid heights using several methods were compared with satellite-derived geoid heights (ellipsoid heights minus orthometric heights) at laser tracking stations around the world. Despotakis’s report states:
The numerical computations of the geoid undulations using all the four methods resulted in agreement with the “ellipsoidal minus orthometric” value of the undulations on the order of 60 cm or better for most of the laser stations in the eastern United States, Australia, Japan, Bermuda, and Europe. A systematic discrepancy of about 2 m for most of the western United States stations was detected and verified by using two relatively independent data sets. The cause of this discrepancy was not found.
The results of the 1989 datum definition report provided a possible explanation for this systematic discrepancy of 2 m in the western U.S. stations (i.e., the difference between NGVD 29 and NAVD 88 in western United States was about 1.5 m). Applying NAVD 88 heights to Despotakis’s study reduced the 2 m bias to 60 cm.
The problem with adjusting space-derived orthometric height data with leveling data is similar to the problem of using LMSL tidal heights as weighted observations in a leveling network adjustment: The uncertainties in space-derived orthometric height differences are too large to help control remaining errors in the leveling data. Space-derived ellipsoid height differences over long lines are probably more precise than leveling-derived orthometric height differences over the same distance. The uncertainties of geoid height differences used to convert ellipsoid height differences to orthometric height differences are large compared with the formal errors of leveling height differences. Several Very Long Baseline Interferometry (VLBI) stations, which were tied into NAVD 88, were included as special junction stations. The results of the final adjustment comparing NAVD 88 adjusted heights with VLBI-derived orthometric heights derived using the best available estimates of ellipsoid heights (Strange 1991) and geoid heights (Milbert 1991) are given in Figure 11.
Figure 11 indicates that the results are consistent with the accuracy statements of GEOID90. In coast-to-coast geoid height differences, the accuracy of the underlying geopotential model OSU89B (Rapp and Pavlis 1990) dictates the accuracy of GEOID90. OSU89B is believed to have a standard error of approximately 60 cm. VLBI-derived orthometric heights do not show the same 1.5 m difference indicated by the LMSL (epoch 1960-78) tidal heights. Therefore, if the coast-to-coast leveling does indeed contain long-wave-length systematic errors, the errors probably are not as large as 1.5 m. Combining VLBI-derived orthometric height difference data with leveling data in NAVD 88 would not have helped to control remaining errors in the leveling data, or significantly improved the estimates of adjusted heights in the network adjustment. As the accuracies of geoid models continue to improve, space-derived orthometric height data will be incorporated into NAVD 88 and future adjustments.
The “Get Prepared” section on the New Datum website has a “GPS on Bench Marks” option. This is where NGS recommends that you obtain accurate GNSS-derived ellipsoid heights on NAVD 88 bench marks. We discussed this program in my last column. In Addition to improving the transformation model from NAVD 88 to the new 2022 Vertical Reference Datum, occupying more NAVD 88 bench marks with GNSS will help to identify regions of the country where the GNSS-derived orthometric heights obtained using the 2022 Vertical Reference Datum will be more accurate than the current NAVD 88 leveling-derived orthometric heights.
My last column used the GPS on BMs dataset to identify potential issues in the published NAVD 88 heights. Figure 5, below, shows two stations (FA1337 and FA1560) are about 20 kilometers apart, and the difference in residuals is -18.6 centimeters (-12.4 centimeters minus 6.2 centimeters). This is a large difference for only 20 kilometers. What is even more significant is that the group of stations near FA1337 are all negative residuals (around -10 centimeters) and the group of stations near FA1560 are all positive residuals (around 6 centimeters). When the two stations are only 13 kilometers apart the GPS on BMs residual is 13.6 centimeters (Figure 6). These are two examples where the 2022 Vertical Reference Datum will provide a more accurate orthometric height difference between stations less than 20 kilometers apart. These differences are significant and could easily effect the results of many construction, transportation and flood plain mapping projects.In this column, we highlighted NGS plans for the 2022 Vertical Reference Datum and provided approximate height differences that users can expect to see. We also provided a little history behind the differences between the NGVD 29 and NAVD 88, and how each replacement of the vertical reference datum is improving the user’s ability to obtain the most accurate orthometric height.