Why users will need to perform GNSS occupations as part of a leveling project after 2022

This column will address why users will be required to perform GNSS occupations when submitting a leveling project to the National Geodetic Survey (NGS) after 2022. It will highlight a section of NGS Blueprint for 2022, Part 3, “Working in the Modernized NSRS,” that discusses the process of performing leveling projects after 2022. My October 2017 column briefly discussed NGS’ preliminary plans for incorporating geodetic leveling data into the North American-Pacific Geopotential Datum of 2022 (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.

As a side note, NGS just released NOAA Technical Report NOS NGS 72–GEOID18, a report that provides a comprehensive explanation of the data and methods used to create the latest NGS hybrid geoid model. My February 2020 column provided an analysis of the differences between the latest published hybrid Geoid18 values provided on NGS’ Datasheet and the computed geoid height value using the published NAD 83 (2011) ellipsoid height and NAVD 88 orthometric height.

In support of the modernization of the National Spatial Reference System (NSRS), NGS has published three documents denoted as Blueprints for 2022 that describe the modernization of the NSRS (see the box titled “NSRS Modernization NGS Blueprint Documents”).

There are several sections in NGS Blueprint for 2022, Part 3, “Working in the Modernized NSRS,” that discuss the process of performing leveling projects after 2022. Something that will be new after 2022 is that NGS will require users to perform GNSS occupations in order to incorporate their leveling results into the new modernized NSRS.

NGS realizes that in the immediate future GNSS will not replace geodetic leveling for determining the most accurate local orthometric height differences. NGS’ plans include preparing a new leveling manual that will explicitly explain how to work in the modernized NSRS. Some of the new surveying procedures are described in Section 2.10 of Blueprint part 3. In section 2.10, NGS states that there will be substantial changes in how they process and serve up survey data, and that there will be some new ways of executing surveys. This column will focus on sections “2.10.2 Leveling” and “2.11.5 Leveling on Passive Marks” that discuss the new procedures for executing leveling surveys in the modernized NSRS. One major change is that leveling surveys will require Global Navigation Satellite System (GNSS) occupations to ensure orthometric heights computed in leveling surveys are up-to-date and are connected to the NSRS through the NOAA CORS Network. After the modernization of the NSRS in 2022, the NOAA CORS Network will be the primary access to the NSRS. This means leveling and classical surveys will require GNSS surveys to be part of the project. NGS’ plans include creating an OPUS option for processing all types of surveys. Users will be able, within OPUS, to adjust their projects using any mix of CORS data and passive control. Saying that, these same projects, on submission, will be deconstructed at NGS and reduced to the raw observations, then adjusted solely to the NOAA CORS Network to determine Final Discrete coordinates every GPS Month. The GPS Month concept may be new to some users. Blueprint Part 3 describes the concept in section “2.11.3 GNSS on Passive Marks.” The basic concept of a GPS Month is that it is four consecutive GPS weeks, with the first week in the GPS month having a GPS week number that is a multiple of four (see box titled “Definition of a GPS Month”).

Definition of a GPS Month

GPS month: Four consecutive GPS weeks, with the first week in the GPS month having a GPS week number that is a multiple of 4.

In this fashion, NGS defines:

• GPS month 0 = GPS weeks 0, 1, 2, and 3 (1/6/1980 through 2/2/1980)
• GPS month 1 = GPS weeks 4, 5, 6, and 7 (2/3/1980 through 3/1/1980)
• GPS month 2 = GPS weeks 8, 9, 10, and 11 (3/2/1980 through 3/29/1980)
• …
• GPS month 513 = GPS weeks 2052, 2053, 2054, and 2055 (5/5/2019 through 6/1/2019)
• etc.

So, what does this really mean to the user when performing a leveling project in 2022. For a leveling project to be processed using NGS software and/or submitted to NGS for inclusion into the NSRS database, the user must follow specific rules.

The following is from Blueprint, Part 3, section “2.10.2 Leveling:”

“As GNSS occupations are required for geodetic leveling, the rules for how many and how frequently will be:

• For a leveling project to be processed using NGS software and/or submitted to NGS for inclusion into the NSRS database, its field observations should not span more than one year. Longer projects should be broken into sub-projects of less than one year.
• A minimum of three “primary control marks” must be in the level network for every project.
• More primary control marks should be added so there is never more than a 30-kilometer linear distance between marks in the entire network.
• Each primary control mark must have the following GNSS occupations (details on using GNSS occupations to work in the NSRS will be found in the update to NGS 58):
  • A minimum of two occupations within +/- 14 days of the beginning of leveling, but also falling within the same GPS month and whose local start times are separated by between 3 and 21 hours.
    • It is preferable, but not required, that all occupations on any primary control mark occur within the same GPS month as those of all other primary control marks.
  • A minimum of two occupations within +/- 14 days of the end of leveling, but also falling within the same GPS month and whose local start times are separated by between 3 and 21 hours.
    • It is preferable, but not required, that all occupations on any primary control mark occur within the same GPS month as those of all other primary control marks.
• All projects exceeding six months must have a third set of GNSS occupations on all primary control marks some time near the middle of the project, without a rigorous rule as to when. They must follow the “minimum of two occupations” rule as per above, and each mark’s occupation is required to fall in the same GPS month, with a preference that all primary control marks are occupied in the same GPS month.”

The box titled “GNSS Procedures for Leveling Projects” highlights the GNSS rules that need to be adhered to when performing leveling projects in 2022.

GNSS procedures for leveling projects

The boxes titled “GNSS + Leveling 2022 Procedures at the Start of the Leveling Project” and “GNSS + Leveling 2022 Procedures at the End of the Leveling Project” provide conceptual diagrams that illustrate what this means to a typical leveling project.

So, why is NGS requiring users to perform GNSS observations in support of leveling project. Leveling is a differential measurement technique; it generates relative height differences not absolute heights. In NGS’ modernized, time-dependent 2022 NSRS, the absolute height will be provided by up-to-date GNSS data; and the accurate relative height differences between leveling marks will be provided by the leveling data. (See box titled “Why NGS Requires GNSS Occupations on Primary Marks.”)

Why NGS requires GNSS occupations on primary marks

• The Connection to NAPGD2022 is Obtained Through GNSS and a High-Accuracy Geoid Model
• Network Accuracy
• The Accuracy of the Height Differences are Provided Through the Leveling Data

• Local Accuracy
• Combining the leveling and GNSS increases the redundancy in a survey network

NGS is developing models and tools to facilitate the incorporation of leveling survey data and adjustment results into the new modernized NSRS in 2022. Blueprint, Part 3, section “2.13.3 OPUS for Leveling,” describes NGS plans to support leveling surveys through the use of the OPUS web tool. The box titled “OPUS for Leveling” outlines how NGS will modify the OPUS web tool to support leveling surveys.

OPUS for leveling

• Support for leveling surveys will follow many of the best aspects of OPUS
  • Uploading and processing digital data files
  • Using a web-based graphical interface
  • Submitting data to NGS
• Leveling is a differential measurement technique
  • It generates relative height differences not absolute heights
• For users who need absolute heights in the NSRS
  • OPUS will support a mix of GNSS and leveling in a single project
• NOTE: NGS will require a GNSS survey to be performed at specific times before and after leveling surveys in order for the data to be submitted for inclusion in the modernized NSRS after 2022.
• NOTE: Leveling surveys longer than one year must be broken up into multiple projects. Leveling surveys between 6 and 12 months in duration require a third, intermediary GNSS data collection.

This column highlighted that in the modernized NSRS the only way to get “into the datum” will be through a GNSS survey. It noted that leveling projects generate relative height differences not absolute heights. In NGS’ new modernized, time-dependent NSRS, the absolute height will be provided by up-to-date GNSS data; and the relative height differences between leveling marks will be provided by the leveling data. A major requirement will be that users must collect GNSS data both at the beginning and at the end of a leveling survey project. Leveling survey projects that take longer than one year to complete must be broken up into multiple projects. NGS is developing model and tools to facilitate incorporating all types of survey data into the new NSRS. I would encourage all readers to read NGS’ Blueprint for 2022 documents to obtain a better understanding of the new, modernized NSRS.