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What & Why
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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, December 1989.
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 44 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 stations Of these 16,000 stations, NGS has
committed to maintaining about 1,400 survey stations, named the Federal
Base Network, and the various states will maintain the remainder.
"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. ("Results of the General
Adjustment of the North American Datum of 1988,"
Surveying and Land Information Systems Vol. 52, No. 3, 1992 pp.
133-149)
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.
("Results of the General Adjustment of the North American Datum of 1988,"
Surveying and
Land Information Systems Vol. 52, No. 3, 1992 pp. 133-149)
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
Imagery and Mapping Agency (NIMA) (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 (G873). WGS 84 (G873) was adopted as the reference
frame for broadcast orbits on January 29, 1997.
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:
| dx | -0.9 cm |
| dy | 0.8 cm |
| dz | -2.3 cm |
| Rx | -3.6 mas |
| Ry | 0.6 mas |
| Rz | 3.1 mas |
| scale | -7.7 ppBillion |
That paper has further references that address the formulation and sign conventions.
For more information, check the NIMA satellite page at http://164.214.2.59/GandG/sathtml/home.html.
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.
Program NADCON has been updated after each statewide readjustment to account
for the shifts due to the HARN upgrade. (Contact Cindy
Craig.)
Between 1987 and 1997, the National Geodetic Survey, in cooperation with
other Federal, State and local surveying agencies has conducted a resurvey
of the United States using Global Positioning System (GPS) observations
often referred to as the High Accuracy Reference Networks (HARNs). All 50
states, American Samoa, Guam, Puerto Rico and the Virgin Islands have now
been connected with a network of A-order and B-order horizontal control
points. Continued improvements in GPS technology and requirements from users
of spatial data will eventually require a transition to an improve 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 will over the next 3-5 years implement
an adjustment to include the HARNs and other GPS data that have been submitted
to NGS for adjustment and publication. 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 see the
New Reference page at this site.
Two informative pages are the Federal Aviation Administration Satellite Navigation Product Teams web page and the Canadian Geodetic Survey Division web page.
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. (Contact Miranda Chin or Don Haw.)
There have been many terrestrial reference frames used for NGS computed GPS orbits (post-fit ephemerides). A complete list is available, including dates and times of implementation, upon request. Recent reference frames are as follows:
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.
These are defined in the Federal Geodetic Control Subcommittee publications "Standards and Specifications
for Geodetic Control Networks," September 1984, and "Geometric Geodetic
Standards and Specifications for Using GPS Relative Positioning Techniques,"
version 5, May 11, 1988, reprinted with corrections August 1, 1989, both
available from the National Geodetic Survey.
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.
NGS provides geodetic data products and services. In the course of developing these products NGS also develops computer programs. Software developed by the government is generally considered to be in the public domain and is made available to the general public for the cost of reproduction and distribution (usually free if downloaded from an ftp or web site). It is distributed with the disclaimer that it is not warranted to be suitable for any particular purpose. This means that we cannot formally certify it to be Y2K compliant.
As a practical matter, the government also has an interest in ensuring that the programs it uses will continue to operate. None of the programs currently in the NGS catalog and distributed on the NGS ftp and WWW site are known to contain features that will cause them to fail on the Y2K rollover.
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 e-mail 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
and Technology).
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 hundreds of feet (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. But, we don't
have many places in the U.S. where elevations exceed 5,000 feet!
If you have further questions on this subject, the best NGS person to
answer them is:
David Doyle
Senior Geodesist
Spatial Reference System Division, NGS
E-mail: Dave.Doyle@noaa.gov
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 e-mail 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.
For additional information on the program, contact: Mr. Gilbert
J. Mitchell, Chief, Geodetic Services Division, N/NGS1,
telephone (301)713-3228.
A list of state
geodetic advisors is available on the NGS
home page.
The image in the left portion of the banner graphic shows conventional
line-of-sight triangulation observations used in geodetic surveying before
the advent of satellite-based positioning systems. The observer shown here
is measuring angles between control point locations in a survey along the
Alaska - Canada Boundary. The bandana he is wearing is for protection from
mosquitoes.
Photo Credit: National Geographic Magazine July 1912
[ large image: 256kb ]
The image to its right shows a Global Positioning System (GPS) receiver,
the instrument primarily used today in precise positioning surveys. The
observer is Phillip Johnson Tuwaletstiwa, NOAA Corps retired, operating
a GPS receiver on a mesa in Chaco Culture National Historical Park (NHP),
in northwestern New Mexico. Chaco Culture NHP was set aside to preserve
and interpret the remains of several large structures associated with the
cultural center of the prehistoric Anasazi people.
Photo Credit: William Stone [ large
image: 295kb ]
The NGS emblem on the leftmost portion of the home page shows the North
American continent covered by a triangle symbolizing triangulation - the
traditional method used in precise positioning surveys.
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 Mr.
Ronnie Taylor, c/o FLDEP, Bureau of Survey and Mapping, MS 105 ,
3900 Commonwealth Blvd., Tallahassee, FL 32399 , Telephone: 850-245-2606, Fax: 850-245-2645
Do you have a scientific question?
Ask one of the NGS scientists!
| If your question is about... | Then ask... | |
|---|---|---|
| The geoid | Dr. Dan Roman | Dr. Dan Roman |
| Crustal motion | Dr. Richard Snay | Richard Snay |
| Vertical Datum | Dave Zilkoski | Dave Zilkoski |
| Horizontal Datum | David Doyle | Dave Doyle |
| The National CORS | Miranda Chin
or Don Haw |
Miranda Chin
Don Haw |
| The Cooperative CORS | Julie Prusky | Julie Prusky |
| GeodeticAdvisors | Gilbert Mitchell | Gilbert Mitchell |
| See the NGS Personnel Directory for current phone numbers. | ||
Email comments, suggestions, or corrections to
Charlie Schwarz
This file was last modified on August 20,2004.
http://www.ngs.noaa.gov/faq.shtml
For information, contact: NGS Information Services Branch
Telephone: 301-713-3242; Fax: 301-713-4172
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| Last updated by NGS.webmaster on August 20, 2004 |
