you still have a question after scanning these FAQs, please contact the
Communications and Outreach
The Geodetic Glossary (National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, Rockville, MD, September 1986) pp. 54, defines geodetic datum as:
These differing definitions require caution when using the
word "datum." The first definition makes datum synonymous with the
selection of a reference coordinate system (origin and orientation).
The second definition makes datum synonymous with a list of coordinates
of the control points. When the first definition is used, the published
coordinates of control points can change when better measurements allow
better determinations. With the second definition, a change in
coordinates should result in a new datum. NGS has used the first
definition for NAD 1983.
The North American Datum of 1927 (NAD 27) is "The horizontal control datum for the United States that (was) defined by (a) location and azimuth on the Clarke spheroid of 1866, with origin at (the survey station) Meades Ranch." ... The geoidal height at Meades Ranch (was) assumed to be zero. "Geodetic positions on the North American Datum of 1927 were derived from the (coordinates of and an azimuth at Meades Ranch) through a readjustment of the triangulation of the entire network in which Laplace azimuths were introduced, and the Bowie method was used." (Geodetic Glossary, pp. 57)
The North American Datum of 1983 (NAD 83) is "The horizontal control datum for the United States, Canada, Mexico, and Central America, based on a geocentric origin and the Geodetic Reference System 1980.
"This datum, designated as NAD 83, is the new geodetic
reference system. ... NAD 83 is based on the adjustment of 250,000
points including 600 satellite Doppler stations which constrain the
system to a geocentric origin." (Geodetic Glossary, pp 57)
NAD 83 was computed by the geodetic agencies of Canada
(Federal and Provincial) and the National Geodetic Survey for several
reasons. The horizontal control networks had expanded piecemeal since
1933 to cover much more of the countries and it was very difficult to
add new surveys to the network without altering large areas of the
previous network. Field observations had added thousands of accurate
Electronic Distance Measuring Instrument (EDMI) base lines, hundreds of
additional points with astronomic coordinates and azimuths, and
hundreds of Doppler satellite determined positions. It was also
recognized that the Clarke Ellipsoid of 1866 no longer served the needs
of a modern geodetic network. For an in-depth explanation see NOAA
Professional Paper NOS 2 "The North American Datum of 1983", Charles R.
Schwarz, Editor, National Geodetic Survey, Rockville, MD 20852,
NGS develops and maintains the current national geodetic
vertical datum, NAVD 88. In addition, NGS provides the relationships
between past and current geodetic vertical datums, e.g., NGVD 29 and
NAVD 88. However, another part of our parent organization, NOS
(National Ocean Service), is the Center for Operational Oceanographic
Products and Services (CO-OPS). CO-OPS publishes tidal bench mark
information and the relationship between NAVD 88 and various water
level/tidal datums (e.g., Mean Lower Low Water, Mean High Water, Mean
Tide Level, etc.). The relationships to NGVD 29 are not published, but
may be calculated independently from specified tidal bench mark sheet
links to the NGS data base. Tidal bench mark information, water
level/tidal datums, and their relationship to geodetic vertical datums
are available at the CO-OPS web site:
The NAD 27 was based on the Clarke Ellipsoid of 1866 and
the NAD 83 is based on the Geodetic Reference System of 1980. The NAD
27 was computed with a single survey point, MEADES RANCH in Kansas, as
the datum point, while the NAD 83 was computed as a geocentric
reference system with no datum point. NAD 83 has been officially
adopted as the legal horizontal datum for the United States by the
Federal government, and has been recognized as such in legislation in
48 of the 50 states. The computation of the NAD 83 removed significant
local distortions from the network which had accumulated over the
years, using the original observations, and made the NAD 83 much more
compatible with modern survey techniques.
A High Accuracy Reference Network (HARN) and a High
Precision Geodetic Network (HPGN) were two designations used for a
statewide geodetic network upgrade. The generic acronym HARN is now
used for both HARN and HPGN and was adopted to remove the confusion
arising from the use of two acronyms. A HARN is a statewide or regional
upgrade in accuracy of NAD 83 coordinates using Global Positioning
System (GPS) observations. HARNs were observed to support the use of
GPS by Federal, state, and local surveyors, geodesists, and many other
applications. The cooperative network upgrading program began in
Tennessee in 1986. The last field observations were completed in
Indiana in September 1997 after horizontally upgrading some 16,000
survey stations to A-order or B-order status. Horizontal A-order
stations have a relative accuracy of 5 mm +/- 1:10,000,000 relative to
other A-order stations. Horizontal B-order stations have a relative
accuracy of 8 mm +/- 1:1,000,000 relative to other A-order and B-order
"The National Geodetic Vertical Datum of 1929: The name, after May 10, 1973, of (the) Sea Level Datum of 1929." (Geodetic Glossary, pp. 57)
"Sea Level Datum of 1929: A vertical control datum established for vertical control in the United States by the general adjustment of 1929."
"Mean sea level was held fixed at the sites of 26 tide gauges, 21 in the U.S.A. and 5 in Canada. The datum is defined by the observed heights of mean sea level at the 26 tide gauges and by the set of elevations of all bench marks resulting from the adjustment. A total of 106,724 km of leveling was involved, constituting 246 closed circuits and 25 circuits at sea level."
"The datum (was) not mean sea level, the geoid, or any other equipotential surface. Therefore it was renamed, in 1973, the National Geodetic Vertical Datum on 1929." (Geodetic Glossary, pp. 56)
The North American Vertical Datum of 1988 (NAVD 88) is the
vertical control datum established in 1991 by the minimum-constraint
adjustment of the Canadian-Mexican-U.S. leveling observations. It held
fixed the height of the primary tidal bench mark, referenced to the new
International Great Lakes Datum of 1985 local mean sea level height
value, at Father Point/Rimouski, Quebec, Canada. Additional tidal bench
mark elevations were not used due to the demonstrated variations in sea
surface topography, i.e., the fact that mean sea level is not the same
equipotential surface at all tidal bench marks.
NAVD 88 was computed for many of the same reasons as NAD
83. About 625,000 km of leveling had been added to the NGVD since 1929.
Thousands of bench marks had been subsequently destroyed and many
others had been affected by crustal motion, postglacial rebound, and
subsidence due to the withdrawal of underground fluids. Distortions
amounting to as much as 9 meters had been seen due to forcing the new
leveling to fit the NGVD 29 height values.
Horizontal coordinates for most points in NAD 27 exist in NAD 83 and may be obtained relatively inexpensively from NGS via the World Wide Web or NGS' Information Services Branch. If the coordinate data are not held by NGS, two possibilities exist. First, the original field observations can be used to compute new coordinates via least squares adjustment software using NAD 83 control coordinates. If this is not possible, NAD 27 coordinates can be converted to NAD 83 coordinates using available software from NGS called NADCON (v.2.1). There is usually a loss of accuracy using this process, but it often is sufficiently accurate for mapping purposes. The accuracy for the conversion is estimated to be 10-15 cm RMS (one sigma) at the data points used to derive the model, with occasional outliers approaching 50 cm. Original field observations are unaffected by the change in datums. See the directory of PC Software on the NGS Web Site.
Elevations for many points in the NGVD 29 exist in the
NAVD 88 and may be obtained relatively inexpensively from NGS via the
World Wide Web or NGS' Information Services Branch. The conversion
procedures noted for NAD 27 coordinates can be used for NGVD 29
elevations. Program VERTCON (version 2.0) can be used to estimate
elevation changes from NGVD 29 to NAVD 88. The accuracy of the
conversion is estimated to be 2 cm RMS (one sigma) at the data points
used to define the model. As above, the original observations are
unaffected by the change in datums. See the directory of PC Software on the NGS Web Site.
Yes, if you have the approximate coordinates of the site, you can retrieve the data sheet of a nearby survey station with this information on it. The DATA SHEET PAGE will enable you to retrieve a data sheet for the area or point you are seeking.
NGS also offers an interactive service as part of the Geodetic Tool Kit to perform this
function for individual points.
WGS 84 is the World Geodetic System of 1984. It is the
reference frame used by the U.S. Department of Defense (DoD) and is
defined by the National Geospatial-Intelligence Agency(NGA) (formerly the
National Imagry and Mapping Agency) (formerly the Defense Mapping
Agency). WGS 84 is used by DoD for all its mapping, charting,
surveying, and navigation needs, including its GPS "broadcast" and
"precise" orbits. WGS 84 was defined in January 1987 using Doppler
satellite surveying techniques. It was used as the reference frame for
broadcast GPS Ephemerides (orbits) beginning January 23, 1987. At 0000
GMT January 2, 1994, WGS 84 was upgraded in accuracy using GPS
measurements. The formal name then became WGS 84 (G730) since the
upgrade date coincided with the start of GPS Week 730. It became the
reference frame for broadcast orbits on June 28, 1994. At 0000 GMT
September 30, 1996 (the start of GPS Week 873), WGS 84 was redefined
again and was more closely aligned with International Earth Rotation
Service (IERS) Terrestrial Reference Frame (ITRF) 94. It is now
formally called WGS 84 (G1150).
We know of no compendium which describes the transformations for all possible combinations of versions. However, a good reference is the paper, "Maintenance and Enhancement of the World Geodetic System 1984" by Malys and Slater in the proceedings of ION GPS-94 (Salt Lake City, Sept 20-23, 1994) volume 1 pp. 17-24. This paper contains a similarity transformation between WGS84 (730) and ITRF92. If one accounts for plate motion, the parameters are:
That paper has further references that address the formulation and sign conventions.
See also the article "Using the HTDP Software to Transform Spatial Coordinates Across Time and Between Reference Frames," by Richard A Snay, Surveying and Land Informations Systems Vol. 58, No. 4, December 1998, and the HTDP program at this web site.
Other seven parameter transformations are available for
many reference frame combinations (Contact Dave Doyle.)
In addition, program NADCON
is available for conversions between NAD 27 and NAD 83 and VERTCON is available for
conversions between NGVD 29 and NAVD 88.
For more information on this topic please contact Cindy Craig.
Between 1987 and 1997, the National Geodetic Survey, in
cooperation with other Federal, State and local surveying
agencies conducted a resurvey of the United States using
Global Positioning System (GPS) observations often referred to as the
High Accuracy Reference Networks (HARNs). Continued improvements in GPS
technology and requirements from users of spatial data will eventually
require a transition to an improved global reference frame based on the
International Terrestrial Reference Frame (ITRF). Positions relative to
ITRF differ from the existing North American Datum of 1983 (NAD 83) by
approximately 1 meter in horizontal position and 1 meter in ellipsoidal
height. NGS already publishes ITRF coordinates for all Continuously
Operating Reference Stations (CORS), and has recently announced the National Adjustment of
2011 Project. NGS will continue to maintain and improve NAD
83 as the official datum of the United States, until such time as it
will no longer support requirements for surveying, mapping and
navigation. NGS is currently conducting workshops and seminars around
the country to educate data users concerning these and other
improvements to the National Spatial Reference System. For further
information, contact the NGS
Information about the National
CORS network is available at this web site. It includes
Frequently Asked Questions, a description of the CORS network,
specialized software, site coordinates, GPS data, etc.
GPS orbits are computed from data collected by a global network of receivers coordinated by the International GPS Service for Geodynamics (IGS). The accuracy of the GPS orbits depends on many factors, including the accuracy of the coordinates of the data collection sites. The earth's surface is not fixed and rigid like an egg shell. It consists of many sections, or plates, which move slowly over time in various directions and rates in a process called crustal motion. Scientists have been studying this movement for several reasons. This includes wanting to know where land masses are with respect to one another and where they will be in the future. Since IGS sites are located on these crustal plates, we must be able to estimate where the sites are when the data are collected.
The International Earth Rotation Service (IERS) periodically
computes the positions of the sites for a given date. The sites define
the IERS, International Terrestrial Reference Frame (ITRF) and the date
defines the epoch. IERS also computes the movements (or velocities) of
the sites to estimate where the sites will be in the "near" future with
some degree of accuracy. The ITRF is an internationally accepted
standard, and is the most accurate geocentric reference system
currently available. The longer the sites operate, the better the
positions and velocities can be determined and the more accurate the
orbits will be.
Yes, NGS has software available (GPPCGP (v.2.0) for NAD 27 and SPCS83 (v.2.0) for NAD 83) to convert coordinates from latitude and longitude to state plane coordinates and the reverse. Program CORPSCON (v.4.1), written and maintained by the U.S. Army Corps of Engineers and available through NGS, is a useful program which combines NADCON (v.2.1), a program which converts geographic positions from older NGS datums to NAD83, with GPPCGP and SPCS83. There is also software available [UTMS(v.1.1)] to convert NAD 83 latitude and longitude to UTM coordinates. These software are available in the directory PC Software of the NGS Web Site. NGS will advise users about the conversion process.
NGS also offers the capability to perform these
computations interactively for single points as part of the Geodetic Tool Kit.
An explanatory supplement to the USGS topographic maps
explaining the symbols should be available where you purchased the
topographic sheets. The placement of information on the topographic
maps are as accurate as the National Map Accuracy Standards allow and
the physical limitations of plotting data on a flat piece of paper
allow. |The Vertical datum upon which the topographic sheets are based
is defined in the legend on the maps. The statement "DATUM IS MEAN SEA
LEVEL" on topographic sheets prior to 1975 refers to the National
Geodetic Vertical Datum of 1929.
Each software product comes with documentation that explains the platform(s) on which the software will run. Most products will run on an 80x86 based PC. Some are written for unix systems. Some may require a math coprocessor when run on 80386 systems. The documentation for each product will also explain if other software is necessary to support the product or prepare the data. In most cases, the distribution package contains all the needed components.
See also the
NGS Software Download FAQ.
NGS uses Fortran, C, and C++. The source code is part of
the standard distribution package for most products. In many cases,
compiled code for a PC is also included. Compiled code for a unix
platform is given in a few cases. The distribution packages are
available under the directory of PC
Software on the NGS Web Site.
All NGS programs have been tested and found to work on a variety of platforms. Therefore your problem is probably related to your local environment and your first action should be to look for help from your local system administrator or computer support staff. If you determine that help from NGS is needed, check the program documentation; this may give the name and telephone number for the programmer or responsible person. If the documentation does not help, contact the NGS Information Center: call (301) 713-3242, or email info_center. The Information Center may be able to find a knowledgeable programmer or user. However, please be aware that the ability of the National Geodetic Survey to provide user support is severely limited. Many NGS program are orphans (i.e., the original programmer is no longer working for NGS and no replacement programmer has been assigned).
See also the
NGS Software Download FAQ.
[This explaination courtesy of Ed McKay]
First, remember this rule: There is only one meter, BUT, there are
two types of feet.
The two types of feet are:
1. The U.S. Survey Foot
It is defined as: 1 meter = 39.37 inches.
If you divide 39.37 by 12 (12 inches per foot), you get the
conversion factor: 1 meter = 3.280833333... U.S. Survey Feet.
2. The International Foot
It is defined as: 1 inch = 2.54 centimeters.
If you convert this to meters and feet, you get the conversion
factor: 1 International Foot = 0.3048 meters.
These two conversion factors produce results that differ by 2 parts per
million; hence for most practical work it does not make any difference
to the average surveyor which one is used since they usually do not
encounter distances this large. For example, converting a distance of
304,800 meters (about 1,000,000 feet) to feet using the two conversion
factors, these are the results:
304,800 meters = 999,998.000 U.S. Survey Feet
304,800 meters = 1,000,000.000 International Feet
A difference of 2 feet in 1 million feet.
NGS has always used meters in their computations, so this has not been
an issue for us. However, the one place where NGS does use feet, and
the numbers are large enough to make a difference, is in the
publication of rectangular State Plane Coordinates (SPCs).
For most of the years surveying has been undertaken in the U.S.,
surveyors have used the U.S. Survey Foot. (Note: Some surveying
historians will mention that other types of linear measure, mostly of
Spanish origin, was also used in the U.S.) In fact, NGS originally
computed and published SPCs in U.S. Survey Feet for many years when the
reference system was the North American Datum of 1927 (NAD 27). And
the conversion formulas (latitude/longitude to SPCs) were developed to
produce U.S. Survey Foot values. In fact, NGS never published NAD 27
SPCs in meters.
However, most other countries, and the engineering community in the
U.S., began using the International Foot as established by the National
Bureau of Standards (NBS), now NIST (National Institute of Standards
To make the transition in the surveying community, in 1959 NBS
published a Federal Register notice stating that the U.S. surveying
community would convert to the International Foot the next time the
National Coordinate Reference System was updated with revised values.
That revision of coordinate values (i.e., latitudes and longitudes) was
realized when the North American Datum of 1983 (NAD 83) was computed
and published in 1986.
NGS began publishing SPCs in meters because going metric was the
direction the Federal government was heading to be consistent in a
global economy, AND, the change in the size of the SPCs values was a
way to alert users that they were using a new horizontal datum. Also,
the new conversion formula (latitude/longitude to SPCs) produced
meters, not feet.
However, the surveying community in various states still wanted SPCs
in feet. This left NGS in the position of not wanting to mandate which
foot (U.S. Survey or International) a state should use. So, NGS left
that decision to the individual states. Currently, NGS publishes SPCs
for 7 states using the U.S. Survey Foot conversion factor, 1 state
using the International Foot conversion factor, and 42 states using
only meters, not feet, for SPCs. Based on STATE legislation we have or
know about, 24 states have legislated the U.S. Survey Foot, 8 states
have legislated the International Foot, and 18 states have no
legislation on which conversion factor must be used.
So, NGS does NOT have an "official" conversion factor. NGS works in
meters ONLY. NGS only uses feet to publish SPCs, and those are
converted from meters using the conversion factor as defined by the
individual states who have requested that we publish SPCs in feet.
P.S. The only other instance where NGS publishes linear values in feet
is for elevations, i.e., orthometric heights. All computations are
still done in meters, but for publication purposes we convert meters to
feet. That conversion is done using the U.S. Survey Foot conversion
factor. We publish elevations in meters to the nearest millimeter
(3 decimal places) and in feet to hundredths of a foot (2 decimal places).
For elevations above 5,000 feet (1,524 meters), the conversion factor
will change the foot value by one in the second place.
If you have further questions on this subject, the best NGS person to
answer them is:
Spatial Reference System Division, NGS
NGS needs the geographic position (i.e., latitude and longitude) of the site of prediction. The topographic (i.e., mean sea level) height at this position is very useful and improves the accuracy of interpolation, but is not mandatory for interpolation. Contact the NGS Information Center [Call (301) 713-3242, or email info_center] with the positional information and they will respond to the request for an interpolated gravity value.
NGS also provides an interactive capability
to compute predicted gravity at a single point.
Yes. Many of them are available at this web site, and the
materials at this site lead to many other documents. The NGS
home page contains a search tool.
The Geodetic Advisor Program provides liaison between NOS and a host state, with a jointly-funded NOS employee residing in the state to guide and assist the state's charting, geodetic, and surveying programs. The program is designed to fill a need for more accurate geodetic surveys, and is in response to states' desire to improve their surveying techniques to meet Federal Geodetic Control Subcommittee (FGCS) standards and specifications.
The National Geodetic Survey, our Nation's first civilian scientific agency established by President Thomas Jefferson in 1807, was called the Survey of the Coast. Its mission soon included surveys of the interior as the nation grew westward. As additional missions, marine charting, were assigned to the agency a reorganization and a new name was established in 1878. The agency became known as the Coast and Geodetic Survey and maintained the name until 1970.
In 1970 a reorganization created the National Oceanic and
Atmospheric Administration (NOAA) and the National Ocean Service (NOS)
was created as a line office of NOAA. To acknowledge the geodetic
portion of NOAA mission, the part of NOS responsible for geodetic
functions was named the National Geodetic Survey.
If you are in a state that participates in the NGS state
advisor program, contact your local state advisor
. For states that do not participate in this program, you may contact
the NGS Information Center, Telephone: 301-713-3242,
For questions, please contact the Communications and Outreach Branch.