Discussing the new North American-Pacific Geopotential Datum of 2022 — Part 3

October 4, 2017  - By
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My last e-newsletter column discussed the basic foundation parameters of the North American-Pacific Geopotential Datum of 2022 (NAPGD2022); that is, a global geopotential model, the GRAV-D project, and the GEOID2022 geoid model. It emphasized that NAPGD2022 will provide a more efficient and cost-effective way to maintain consistent orthometric heights, but evaluating the relative accuracy of the geoid model is critical to a successful implementation of NAPGD2022. Performing GNSS/Leveling evaluation surveys will help in evaluating the relative accuracy of GEOID2022. NGS realizes that users will still have the need to perform leveling to obtain millimeter-level accuracy between closely spaced stations, and to evaluate the relative accuracy of a geoid model. NGS is developing geodetic routines and tools to assist users in transforming heights from NAVD 88 to NAPGD2022, and enabling the incorporation of geodetic leveling data into NAPGD2022 to establish NAPGD2022 orthometric heights. This newsletter will highlight NGS’ current plans for estimating NAPGD2022 GNSS-derived orthometric heights and incorporating geodetic leveling data into NAPGD2022 to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. Dan Gillins and Kandell Fancher did an excellent presentation titled “Leveling after 2022” at the 2017 Geospatial Summit. This e-newsletter will highlight some sections of the presentation.

First, it should be noted that NAVD 88 was realized by leveling and water-level transfer data only. To assist users in performing geodetic leveling surveys, the Federal Geodetic Control Subcommittee (FGCS) documented standards and specifications for performing geodetic leveling surveys (See Standards and Specifications for Geodetic Control Networks and FGCS Specifications and Procedures to Incorporate Electronic Digital/Bar-Code Leveling Systems). To support users to estimate consistent NAVD 88 heights using their leveling data, NGS developed a web tool called LOCUS (Leveling Online Computations User Service). LOCUS applies the appropriate corrections to the leveling data and performs a least-squares adjustment to estimate NAVD 88 heights based on user constraints. (See box “Excerpt from NGS’ LOCUS web tool” below.)

To support users to estimate NAVD 88 GNSS-derived orthometric heights, NGS developed guidelines and procedures for incorporating GNSS-derived orthometric heights into NAVD 88. (See NGS Constrained Adjustment Guidelines and Guidelines for Establishing GPS-derived Ellipsoid Heights.) These guidelines and procedures have been discussed in my previous GPS World Survey Scene e-newsletter series.

As described in my last e-newsletter, NAPGD2022 will not be realized with leveling data. So, how will users access the National Spatial Reference System (NSRS) in 2022? NGS has prepared frequently asked questions about the new datums (https://www.ngs.noaa.gov/datums/newdatums/FAQNewDatums.shtml#CAN ). The following is the answer to the question How will accessing the National Spatial Reference System (NSRS) change with the release of the new datums?

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.

As described in previous newsletters, GNSS-derived Orthometric Heights are computed using the following formula: orthometric height (H) = ellipsoid height (h) minus geoid height (N). (See box titled “Slide 9 from Gillins and Fancher presentation titled ‘Leveling after 2022’ presented at the 2017 Geospatial Summit.”) It will not be necessary to connect to a geodetic monument, i.e., a bench mark, because the NATRF2022 ellipsoid height (hNATRF2022) is determined using the NGS CORS and the geoid model (NGEOID2022) is consistent with NATRF2022. In other words, GNSS observations combined with the geoid model will become the primary means for deriving orthometric heights on marks.

Slide 9 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit

Gillins and Fancher addressed the expected relative accuracy of a 2022 NAPGD2022 GNSS-derived orthometric height difference in slide 11 of their presentation. (See box titled “Slide 11 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”) Their estimation assumes a 1 cm sigma for each ellipsoid height value and 1 cm sigma for the relative geoid height value. This results in an estimated relative accuracy of a NAPGD2022 GNSS-derived height difference of +/- 1.7 cm. Gillins and Fancher also addressed the expected accuracy of leveling-derived heights in their slide 12. (See box titled “Slide 12 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”)

Slide 11 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit

This slide is just meant to give an idea of the error budget of GNSS leveling. Actually, if both stations are observed simultaneously, then there is a correlation term that must be tracked and added to the equation for sigma delta H. Further, the value for sigma delta N is poorly understood over very short distances (which are typical for leveling). However, it is reasonable to assume that differences in orthometric height of approx. 2 cm can be achieved with GNSS and a geoid model. The point is to say differences in height are to around 2 cm when only using GPS+geoid

Slide 12 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit

Comparing slides 11 and 12, it’s obvious that leveling-derived orthometric height differences are more accurate than GNSS-derived orthometric height differences between closely spaced stations. NGS recognizes that some users will require a high level of relative accuracy and will continue to perform leveling; and, therefore, they will want their leveling-derived orthometric heights consistent with NAPGD2022. Gillins and Fancher’s presentation stated that NGS has ongoing research to develop models to combine and adjust GNSS-derived heights and/or observations with leveling, and to develop software applications and tools for incorporating leveling-derived heights into NAPGD2022. NGS has performed some preliminary tests of adjusting GNSS derived heights with leveling data using weighted constraints. Slides 16-18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit” depicts the basic concept.

The basic concept is that the user will first establish NAPGD2022 orthometric heights at two stations using GNSS observations and a geoid model. Then, the user will observe leveling height differences between the two stations (see box titled “Slide 16 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit”), and finally the user will perform a least squares adjustment to estimate NAPGD2022 orthometric heights using appropriated weighted constraints of the NAPGD 2022 GNSS-derived orthometric heights and appropriated weighted leveling observations (See box titled “Slide 18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit.”).

Slide 16 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
(Before Adjustment)

Slide 18 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit
(After Adjustment)

We will address this topic in more detail in another newsletter but the major takeaways are given in slide 22 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit. Basically, the GNSS and a high-accuracy geoid model connects the user to NAPGD2022 and provides the overall network accuracy, and the leveling data improves the accuracy of height differences between marks and provides the local accuracy. The addition of leveling with GNSS increases the overall redundancy in a survey network which increases the ability to detect outliers and improves the relative accuracy of the final adjusted height differences.
To assist users in obtaining accurate relative NAPGD2022 height differences, NGS has plans to develop software applications and tools for incorporating leveling-derived heights into NAPGD2022. They have a project called “OPUS-Projects for GNSS & Leveling.” The box titled “Slide 25 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit” is a mockup of the proposed tool. This tool will apply the appropriate corrections to the leveling data and perform a least-squares adjustment to estimate NAPGD2022 heights based on weighted constraints.

Slide 25 from Gillins and Fancher presentation titled “Leveling after 2022” presented at the 2017 Geospatial Summit

This newsletter focused on NGS’ current plans for estimating NAPGD2022 GNSS-derived orthometric heights and incorporating geodetic leveling data into NAPGD2022 to establish orthometric heights consistent with GNSS-derived NAPGD2022 orthometric heights. It emphasized that after NAPGD2022 is established, the primary means for deriving orthometric heights on monuments will be using GNSS observations combined with the geoid model. Future newsletters will discuss in more detail some of NGS’ ongoing research to develop models and tools to combine and adjust GNSS-derived heights and/or observations with leveling.

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