National Geodetic Survey - Frequently Asked Questions

National Geodetic Survey:
Frequently Asked Questions

Frequently Asked Questions (FAQ) about the National Geodetic Survey can be found either by entering ctrl-F to search for a word relevant to your question or by reviewing the material under one of the topics listed below.

Question MarkIf you still have a question after scanning these FAQs, please contact the Communications and Outreach Branch.


    GPS
  • What is the CORS network?
    I use or want to use data from the NGS Continuously Operating Reference Station (CORS) network.
    How do I find information about the National CORS network?
  • What is the reference frame for NGS computed GPS orbits? Does it change?
  • Why does the reference frame change for GPS orbits?
    Software
  • What special hardware is needed to use the various NGS software products?
    Is a hard drive required?
    Do any NGS software products require other software to run or to prepare the data?
  • What languages are NGS software products written in?
    How do I get the code for a software product ?
  • What should I do if the software product fails or will not run on my computer platform?
FAQ
  • What is a geodetic datum?

    The Geodetic Glossary (National Geodetic Survey, National Ocean Service, National Oceanic and Atmospheric Administration, Rockville, MD, September 1986) pp. 54, defines geodetic datum as:

    1. "A set of constants specifying the coordinate system used for geodetic control, i.e., for calculating the coordinates of points on the Earth."
    2. "The datum, as defined in (1), together with the coordinate system and the set of all points and lines whose coordinates, lengths, and directions have been determined by measurement or calculation."

    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.
     

  • What are NAD 27 and NAD 83?

    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)
     

  • Why did NGS change from NAD 27 to NAD 83?

    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.
     

  • What is the relationship between the geodetic vertical datums (NGVD 29 and/or NAVD 88) and the various water level/tidal datums?

    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:

    tidesandcurrents.noaa.gov
    .

    For additional information contact: User Services, Products and Services Division, CO-OPS, telephone: 301-713-2815, or email: Tide Predictions  
  • How do the horizontal datums differ? Which should I use?

    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.
     

  • What is HARN or HPGN?

    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.
     

  • What are NGVD 29 and NAVD 88?

    "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.
     

  • Why did NGS change from NGVD 29 to NAVD 88?

    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.
     

  • All my data are in the NAD 27 or NGVD 29 datum.
    Do I have to purchase new data or is software available to convert old data to new data?

    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.
     

  • Can NGS give me the UTM zone or state plane coordinate zone name of a specific 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.
     

  • What is WGS 84? Does it change?

    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).
     

  • Where can I find the transformations between versions of WGS 84 and the versions of NAD 83 and the versions of ITRF?

    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.

    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.
     

  • Are derived transformations available relating successive versions of NAD 83 (199X) to each other?

    For more information on this topic please contact Cindy Craig.
     

  • I have heard rumors of a new reference system/datum. What are NGS's plans?

    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 Center.
     

  • What is the National CORS network? I use or want to use data from the NGS Continuously Operating Reference Station (CORS) network. How do I find information about the National CORS network?

    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.  

  • What is the reference frame for NGS computed GPS orbits? Does it change?

    NGS maintains a section on its web site regarding NGS computed orbits here.  For information contact Jake Griffiths.

     
  • Why does the reference frame change for GPS orbits?

    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.
     


     

  • I have the latitude and longitude for a particular site.
    Can I convert this into a state plane coordinate or UTM grid coordinate?
    Can NGS do this for me?

    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.

  • What do the different symbols on the USGS topographic maps mean? How accurate are these maps? What datum are these maps on?

    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.
     

  • What special hardware is needed to use the various NGS software products?
    Is a hard drive required? Do any NGS software products require other software to run or to prepare the data?

    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.
     

  • What languages are NGS software products written in?
    How do I get the code for a software product ?

    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.
     

  • What should I do if the software product fails or will not run on my computer platform?

    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.
     

  • What are the "official" conversions that are used by NGS to convert 1) meters to inches, and 2) meters to feet?

    [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 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:

     David Doyle
     Spatial Reference System Division, NGS
     E-mail: Dave.Doyle@noaa.gov

  • What information is needed for NGS to provide a gravity prediction? For a Laplace correction?

    1. Gravity prediction:
      NGS provides interpolated gravity values at specified positions based on observed gravity in its Integrated Data Base. These values are referenced to the International Standardization Net 1971, which is an absolute gravity datum.

      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.

    2. Laplace correction:
      You can obtain an estimate of a Laplace correction from the DEFLEC09 deflection model. This page also offers the option of an interactive computation of the deflection of the vertical and the Laplace correction for a single point.

     
  • Are good reference documents or papers available for introductory/ intermediate/ advanced Geodetic/GPS/ etc. subjects?

    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.
     

  • What is the Geodetic Advisor Program? Where are the Geodetic Advisors located?

    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.
     

  • If NGS conducts land surveys, why is it part of the National Ocean Service?

    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.
     

  • What should I do if I find a disturbed survey marker?

    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  Ross Mackay,  Telephone: 301-713-1054  x113 , Fax: 301-713-4176
     

  • How can I find out about job opportunities in NOAA and NGS?
    • The federal government provides a web site with information on current federal job vacancies .

Question MarkFor questions, please contact the Communications and Outreach Branch.

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