Geodetic Leveling

Precise geodetic leveling remains an important surveying technique used to extend vertical control from the established network of official orthometric heights established on survey marks across the Nation. The orthometric height is the vertical distance H along the plumb line from a point of interest to a reference surface which is defined as the zero point for the Nation's official Orthometric Vertical Datum (currently NAVD 88), as shown in the figure below. Note that the plumb line is not straight due to the undulation of the geoid, the surface of constant gravity (resulting in a deflection of the vertical on the Earth surface).

To establish vertical control via geodetic leveling, a leveled connection ("tie") needs to be made to previously-determined vertical control with published, adjusted orthometric heights on NGS Datasheets. In the Continental United States, published orthometric heights are expressed with respect to the North American Vertical Datum of 1988, or NAVD 88. See the Vertical Datums page for more information on the official orthometric vertical datums outside the Continental U.S.

A geodetic level line begins by establishing a valid level tie between existing bench marks with published orthometric heights of the appropriate Order and Class (the number of ties depending on Order and Class as well). The level line extends across new or existing marks, and then concludes with another valid tie at the opposite end of the level line. A “section” of leveling is a level line run between two (permanent) survey marks, so a typical level line is the accumulation of a number of connected sections. In geodetic leveling, sections are required to be run twice as a means to check and validate the measured height difference: once in a forward direction (“forward run”), and a second time in the reverse direction (“backwards run”). The disagreement between the forwards and backwards runnings is called “misclosure,” which is one of the variables used to classify the Order and Class of leveling (see heading below). For more information on geodetic leveling, please refer to NOAA Manual NOS NGS 3 Geodetic Leveling.

Geodetic leveling is classified according to Order and Class, which refers to a number of differences in the specific procedures followed and the allowable error (see FGCS Specifications for Digital Barcode Leveling). For example, the Orders and Classes of leveling have different maximum allowable section misclosure error (presented in 2. Geodetic Leveling, above), typically expressed as millimeters accumulated error per square-root of kilometers run, or, as shown in the table below. Note that there are a number of additional requirements for each Order and Class, presented in the FGCS specifications and procedures (see 5. Specifications and Support, below).

A geodetic level line will extend vertical control from an existing set of bench marks with published, adjusted orthometric heights of a certain Order and Class. When the leveling project is submitted to NGS, the level line will be adjusted to the existing vertical control network, and new heights of the same Order and Class will be published.

Survey marks can often be damaged or destroyed due to natural occurrences or human activity. When vertical control marks with published, adjusted heights will be destroyed, they can either be replaced by a Single Mark Level Tie (3rd Order). However, that will typically downgrade the Order and Class of the original vertical control. To maintain the same Order and Class of the original mark(s), a geodetic level line will have to be run from an existing set of vertical control, through the replacement mark(s), and finishing on another set of existing vertical control. Both options for preserving a vertical control mark are described in the NGS Bench Mark Reset Procedures.

Geodetic leveling supported by the National Geodetic Survey requires the use of high precision digital level instruments and one-piece invar rods. Please contact ngs.rods.instruments@noaa.gov for more information or to submit your instrument and rod calibration files.

Specifications for modern geodetic leveling using digital level instruments are given in the FGCS specifications and procedures, and include such things as minimum bench mark ties, maximum spacing between bench marks, instrument specifications, calibration procedures, and a large number of field procedures, including misclosure.

Adherence to FGCS specifications and procedures are required but not entirely sufficient for successful submission of the project to NGS. NGS additionally requires the following:

• 500-meter rule for level ties: the different bench marks used as ties need to either be located in different geological units, or be spaced at least 500m apart (linear distance). This can be found on page 2-19 of NOAA Manual NOS NGS 3.


• Temporal discontinuity: A surveyor can continue along a level line, returning to a previously closed section without needing to create a new level tie so long as the closure was made no more than two weeks prior. If more than two weeks have passed, the surveyor will have to run another two-mark tie regardless of the order and class of the level line. If a longer-than-two-week interruption is foreseen, it is advisable to leave the last section open, so that, upon resumption of leveling, the last section closure will serve as the necessary tie.

NGS provides support for geodetic leveling via training opportunities (see the NGS Training Calendar), your Regional Geodetic Advisor, and product support for the required processing software.

NGS also provides a Leveling Online Computations User Service (LOCUS) for users to process and perform least squares adjustment of geodetic leveling data. This can allow users to create their own adjustments, and also get a preliminary result prior to submitting a leveling project to NGS for bluebooking.

To submit your geodetic leveling (or Third Order Reset) project to NGS, please follow the instructions below:

1. The first step in submitting your project to NGS is to submit a leveling project proposal using the Survey Project Proposal Form. If the proposal is accepted, an official leveling accession number ("L-number") will be assigned to the project.
2. Please refer to the Translev Help menu for a list of all of the Translev files required for submission. All file names should start with the appropriate L-number.
3. A full set of WinDesc description files are required, including the following: *.des, *.dsc, *.dis, *.nbr, *.err (note that there should be no errors present in the *.err file, although warnings may be acceptable, depending on the situation). All file names should start with the corresponding L-number (e.g. L1234.des).
4. Three photos are required per mark observed. See the NGS Digital Photo Requirements.
5. Scanned field logs, raw data files, and a project report explaining the procedures used, and any deviations from the original proposal. Information on the content of the project report can be found in the help menu of Translev.
6. All of these required files should be compressed to a single ZIP file (e.g. *.zip) and submitted via the button below. Note that file sizes are limited to about 50 MB. If your project is larger, make sure your photos are sized correctly, and if the size is still too large, consider submitting photos separately (compress photos into another ZIP file and submit separately). Each submission should clearly identify the corresponding L-number and submission contents. Please contact your Regional Geodetic Advisor if you have any questions or concerns.

• Bench Mark Reset Procedures
• FGCS Specs for digital barcode leveling
• NOAA Manual NOS NGS 3 Geodetic Leveling