Guidance on Use and Documentation of Horizontal and Vertical Datums in USGS Publication Series Information Products

By Office of Science Quality and Integrity

March 2019

This document provides guidance to U.S. Geological Survey (USGS) authors on using and documenting geodetic datums for horizontal (location) and vertical (elevation) coordinates in USGS publication series information products (refer to Survey Manual (SM) chapter SM 1100.3).

Background

Scientists frequently use geospatial data obtained from multiple sources, and datums commonly must be standardized before datasets that are referenced to different or unknown datums can be used. The North American Datum of 1927 (NAD 27) and the National Geodetic Vertical Datum of 1929 (NGVD 29) are considered superseded as the national standards. The North American Datum of 1983 (NAD 83) and the North American Vertical Datum of 1988 (NAVD 88) provide a more accurate representation of the Earth's shape and a more accurate depiction of the location of objects in North America than previous datums. Use of these datums for Federal geospatial products is standard practice, and it is important to record and publish datum information for any document that contains horizontal or vertical coordinates.

As part of its mission to define, maintain, and provide access to the National Spatial Reference System (NSRS), the National Geodetic Survey (NGS) continues to refine this system on the basis of advances in global positioning system technology. In 2012, the NGS completed a nationwide adjustment of all passive control that had been positioned using Global Navigation Satellite System (GNSS) technology. The result was a new realization of the NAD 83 horizontal datum NAD 83 (2011). Information about how coordinates are referenced to NAD 83 (2011) can be found at the National Oceanic and Atmospheric Administration (NOAA), Continuously Operating Reference Station (CORS) Coordinates webpage. General information about horizontal datums can be found at the NGS website.

When ground-based or remotely sensed GNSS survey coordinates are used in creating or controlling the position of geospatial data, the horizontal datum reference should include information about the realization date and epoch of that datum.

In 1993, NAVD 88 was affirmed as the official vertical datum in the NSRS for the Conterminous United States and Alaska (refer to the Federal Register Notice referenced below). For vertical datums applicable to U.S. territories, refer to the NGS vertical datums web page.

If ellipsoid heights are established by using a GNSS survey and converted to elevations (orthometric heights) for publication, reference information should include the geoid model used in the conversion in addition to the vertical datum. For more information about geoid models and how to use them, refer to the NGS GEOID models web page.

Use of the term "sea level" as a synonym for NGVD 29 in USGS publication series information products is discontinued. However, Mean Sea Level (MSL), a tidal datum that pertains to local mean sea level, should not be confused with or substituted for the fixed datums of NGVD 29 or NAVD 88. The Center for Operational Oceanographic Products and Services (CO-OPS), in the NOAA's National Ocean Service, publishes tidal bench mark information and information on the relation between NAVD 88 and various water level/tidal datums (such as Mean Lower Low Water, Mean High Water, Mean Tide Level, and others). For more detailed information about the CO-OPS, refer to the NOAA CO-OPS web page.

Recommended Datums

The NAD 83 and the NAVD 88 are the recommended datums to use in USGS publication series information products. The use of these datums establishes a common reference for all horizontal and vertical data in the NSRS for the conterminous United States and Alaska. The NAD 83 and the NAVD 88 datums also provide the necessary linkages for Global Positioning System (GPS) data and are supported by GPS continuously operating reference stations. Refer to the above information about including the datum realization date and epoch and geoid model information when using coordinates developed from GNSS surveys.

The Federal Geodetic Control Subcommittee of the Federal Geographic Data Committee (FGDC) has affirmed NAD 83 and NAVD 88 for official use for civilian surveying and mapping, and every effort should be made by USGS scientists to collect and publish new data using these datums. The FGDC is an interagency committee that promotes the coordinated development, use, sharing, and dissemination of geospatial data on a national basis.

Datum Documentation

Include datum documentation in a USGS publication series information product according to the following criteria.

The product refers to an altitude or an elevation (orthometric height).
- State the vertical datum name and year in that document.
- Example: "Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88)."
- If a GNSS survey is used to establish elevations (orthometric heights), the geoid model used to convert ellipsoid heights to orthometric heights should be referenced.
The product expresses or provides relative location.
- State the horizontal datum name and year in that document.
- Example: "Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83)."
- In some cases it may be necessary to state the adjustment (realization date and epoch) used for the horizontal coordinates (refer to National Adjustment of 2011 Project, National Geodetic Survey). Include expanded documentation if the series information product has the following characteristics.
The product contains data that were converted into a uniform datum for the information product.
- State the datum name and year to which the data were originally referenced and the datum name and year to which the data were converted and appear in the product.
- Example: "Historical data collected and stored as National Geodetic Vertical Datum of 1929 (NGVD 29) have been converted to North American Vertical Datum of 1988 (NAVD 88) for this publication."
- The methodology or software, including version number, used for the conversion also should be provided.
The product contains multiple datums within one document.
- State the datum names and years.
- Examples: "Vertical coordinate information is referenced to the North American Vertical Datum of 1988 (NAVD 88) and to the Marcus Hook, Delaware River, Pennsylvania Mean Lower Low Water tidal datum (Mean Sea Level)."
  "Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83), unless otherwise noted."
- When the wording "unless otherwise noted" is included (as shown in the note above regarding adjustments for horizontal coordinates), each exception must contain a full datum reference. If the exception is on or in the same geospatial element as the standard datum, both shall be recorded.

Placement of Documentation

Place the documentation in a USGS publication series information product according to the following criteria.

The product has a traditional book-report layout with front matter.
- State the datum information (as described in "Datum Documentation" above) immediately following the conversion factors. If a vertical datum reference is needed within an illustration, the abbreviation for the standard datum can be used.
- Example: "NAVD 88"
The product is a fact sheet or has an otherwise nontraditional layout.
- State the datum information (as described in "Datum Documentation" above) where an elevation or location is given, for example in (1) the body of the text, (2) a footnote within the text, (3) a table headnote or footnote, or (4) an illustration explanation.
- If a vertical datum reference is needed within an illustration, the abbreviation for the standard datum can be used if the full datum information is given elsewhere in the illustration.
- Example: "NAVD 88"

Additional Information

If the datum used for existing data is not apparent:

Refer to the required metadata that accompanies the dataset.
Query the dataset by using your geospatial software package.
Refer to the header file for the dataset.
Visually compare the dataset in question to a dataset of known datum.
Inspect information on the map or illustration source file.

References for additional information on datums:

Frequently Asked Questions About Datums

Refer to the following frequently asked questions (FAQs) about using and documenting horizontal and vertical datums in USGS publication series information products. Additional FAQs about datums are available at NOAA National Adjustment of 2011 (NA2011) Project Frequently Asked Questions (FAQs) web page.

Is the use of NAD 83 and NAVD 88 required? No, NAD 83 and NAVD 88 are the recommended horizontal and vertical datums, and every effort should be made to collect and publish new data using these datums. However, data collected using other horizontal and vertical datums are acceptable.
When is the statement of a datum required in the text body? If data collection location, bench marks, or height above a given elevation is referred to in the text body, the datum is required. General physical descriptions of an area, which include elevation, do not require a datum.
Does "sea level" serve as a substitute for a specified vertical datum? No, sea level is used only for references to very general elevations. For example, "The base of the model approximates the bottom of the high conductivity unconsolidated sediments (about 800 ft. above sea level)." If the specific vertical datum is NAVD 88, areas outside North America can reference the sea level vertical datum or another local vertical datum. If the specific vertical datum is NGVD 29, areas outside the conterminous United States and Alaska can reference the sea level vertical datum.
Can the term "mean sea level" be used? Yes, for specific local, coastal datums "mean sea level" can be used but it must be defined in the specific datum statement on the conversion factors page.
When should elevations or locations be converted to a different datum for the purpose of the report? If data presented in a report are meant to be compared but were collected using several datums, it is recommended that a uniform datum be presented. For example, native NGVD 29 or NAD 27 data would be transformed to match data more recently collected using NAVD 88 or NAD 83, and the original datum should be documented in the report. (The archival data should remain in the native datum because transformations are approximations and not reversible.)
At what stage of product development does datum documentation need to be incorporated? Effective February 24, 2016, any manuscript intended for released in a USGS publication series is required to include datum documentation when submitted for Bureau approval.
What is new about the datum documentation policy? Specification of the datums for horizontal and vertical Earth coordinates is now required in all instances.
Should the term altitude or elevation be used? Both terms may refer to height above sea level but elevation may also mean uplift in a geologic sense. To avoid ambiguity, use "altitude" in geologic reports to indicate height above sea level and use "elevation" to mean uplift. However, because the term elevation is widely used by engineers and topographers to mean altitude, USGS reports directed to such readers may follow that usage. Note that the term elevation when used in this context is equivalent to the term “orthometric height.” Consistency is essential; do not use altitude and elevation interchangeably within a report, and do not use elevation for uplift if you also use it to mean altitude (refer to "Suggestions to Authors of the Reports of the United States Geological Survey," 7th edition).

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