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Popular FAQ Pages
Aeronautical Survey Program
Continuously Operating Reference Systems (CORS)
New Datums
Online Positioning User Service (OPUS)
Survey Marks, Bench Marks and Datasheets
What is a Geoid?
Questions about...
Datums
What are NAD 27 and NAD 83?
Why did NGS change from NAD 27 to NAD 83?
How do the horizontal datums differ? Which should I use?
What is HARN or HPGN?
What are NGVD 29 and NAVD 88?
Why did NGS change from NGVD 29 to NAVD 88?
What is the relationship between the geodetic vertical datums (NGVD 29 and/or NAVD 88) and the various water level/tidal datums?
What is WGS 84? Does it change?
Floodplain Maps
How can I find out more about my flood insurance rate map (FIRM) or flood zones?
How do I find more information about a bench mark on my flood insurance rate map (FIRM)?
Can I use NGS tools like OPUS or geoid models to complete an elevation certificate or otherwise determine the elevation of my home/property?
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?
Why does the reference frame change for GPS orbits?
Positions
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?
What do the different symbols on the USGS topographic maps mean? How accurate are these maps? What datum are these maps on?
Can NGS give me the UTM zone or state plane coordinate zone name of a specific site?
Software
What languages are NGS software products written in? How do I get the code for a software product?
I have a question about NGS Software Downloads
What & Why
What are the official conversions used by NGS to convert 1) meters to inches, and 2) meters to feet?
What information is needed for NGS to provide a gravity prediction? For a Laplace correction?
Are good reference documents or papers available for introductory/ intermediate/ advanced Geodetic/GPS/ etc. subjects?
What is the Geodetic Advisor Program? Where are the Geodetic Advisors located?
If NGS conducts land surveys, why is it part of the National Ocean Service?
What should I do if I find a disturbed survey marker?
How can I find out about job opportunities in NOAA and NGS?
Answers...
Datums
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, NAD 83, is the current 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.
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.
The latter HARN was the Nation's first attempt at a truly 3D system. Up until that time we only were interested in latitude and longitude, even after we began using GPS. The HARN ushered us into an age where we were concerned with latitude, longitude, and focused us on precise ellipsoid heights and the advent of fixed height poles.
What are NGVD 29 and NAVD 88? How do the horizontal datums differ? Which should I use?The Sea Level Datum of 1929 was named the National Geodetic Vertical Datum of 1929 on May 10, 1973. (Geodetic Glossary, pp. 57) The Sea Level Datum of 1929 is a vertical control datum 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 United States 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 kilometers 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-United States 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.
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 website.
What is WGS 84? Does it change?Information related to the WGS 84 is given here.
Floodplain Maps
How can I find out more about my flood insurance rate map (FIRM) or flood zones?
The Federal Emergency Management Agency (FEMA) issues FIRMS and defines flood zones. NGS does not participate in the publication of these maps or flood zone determinations. To learn more, you can visit FEMA’s Flood Map Service Center website; contact your Regional FEMA office; or contact a map specialist or 1-877-336-2627.
How do I find more information about a bench mark on my flood insurance rate map (FIRM)?FIRMs are created by Federal Emergency Management Agency (FEMA), but the citing of a bench mark on a FIRM does not necessarily imply the bench mark was used in related flood studies or the creation of the map. Additional information about the control used to create a FIRM may be available in the Flood Insurance Study (FIS), a detailed compilation and presentation of flood risk and elevation data for specific watercourses, lakes, and coastal flood hazard areas within a community. Specific questions about a FIRM should be directed to FEMA.
If you want to learn more about any bench mark cited on a map or just in the area, NGS maintains publically available databases with latitude, longitude and/or elevation information about many geodetic survey marks. You can use NGS tools to search for your mark of interest. Searches may be conducted in an interactive map interface (NGS Data Explorer; DSWorld) or Datasheet Retrieval can be completed using mark identification numbers (i.e. PIDs), or by state/county, latitude/longitude, and other criteria.
NGS no longer maintains or replaces geodetic survey marks, but instead relies on the user community for those functions. You are encouraged to submit mark recovery information to NGS if a mark has been disturbed or destroyed. If survey marks were installed by other organizations and not submitted to NGS, then their corresponding information may not be available in the NGS databases.
Can I use NGS tools like OPUS or geoid models to complete an elevation certificate or otherwise determine the elevation of my home/property?The Federal Emergency Management Agency (FEMA) determines all requirements related to the National Flood Insurance Program (NFIP), Flood Insurance Rate Maps (FIRM), and the related Elevation Certificates. Therefore, NGS does not provide guidance or recommendations regarding which tools or models meet FEMA requirements. Professional surveyors must determine what information and tools to use when completing elevation certificates. NGS makes sure that tools like OPUS, along with associated guidelines and information about geodesy and geodetic control, are available for use by professional surveyors.
GPS
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.
Why does the reference frame change for GPS orbits?The reference frame listed in a precise ephemeris (e.g., in the SP3-c format) reflects the coordinates of the global set of tracking stations that were used to create the orbit. The accuracy of the coordinates (and velocities) of these stations has been steadily improving since the International GNSS Service (IGS) began creating precise ephemerides back in 1994; they are based on the latest International Terrestrial Reference Frame (ITRF) which is created by combining positioning information from the various space geodetic techniques (such as GNSS, SLR, DORIS, and VLBI). These reference frames are computed every three to five years or so, and the "shifts" to the coordinates are "converging", i.e., the shifts are decreasing with each successive frame. However, when large earthquakes occur in a region, the coordinates and velocities for a subset of these tracking stations can be rendered useless - since the sites can move by several centimeters, or even several meters! When this occurs, other stations outside the area of deformation have to take the place of the disrupted stations, new velocities have to be determined for sites operating three to four years or longer, and eventually a new reference frame needs to be realized to improve the availability of sites in that region. In other words, because of earthquakes, tsunamis, hurricanes, subsidence, equipment changes, and other factors that adversely affect geodetic tracking sites, the reference frame has to be continuously updated and maintained.
Another reason why the ITRF changes is because the modeling in the geodetic software improves. In the case of GNSS, all of the GPS and GLONASS data available since 1994 are periodically reprocessed using the latest scientific models. The results are then used (together with results from SLR, VLBI, and DORIS) to create a new ITRF.
Positions
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 here. 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.
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.
Software
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 Website.
What & Why
What are the official conversions used by NGS to convert 1) meters to inches, and 2) meters to feet?
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 United States, surveyors have used the U.S. Survey Foot. (Note: Some surveying historians will mention that other types of linear measure, mostly of Spanish origin, were also used in the United States) 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 United States, began using the International Foot as established by the National Bureau of Standards (NBS), now the National Institute of Standards and Technology (NIST).
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. NGS did not not want to mandate which foot (U.S. Survey or International) a state should use. So, NGS left that decision to the individual states.
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.
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.
What information is needed for NGS to provide a gravity prediction? For a Laplace correction?
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 with the positional information and they will respond to the request for an interpolated gravity value.
NGS Information Center
Phone: 301-713-3242
Fax: 301-713-4172
email
NGS also provides an interactive capability to compute predicted gravity at a single point.
Laplace correction:
You can obtain an estimate of a Laplace correction from the
DEFLEC12A 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.
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 a liaison between NGS and a host state, with a jointly-funded NGS employee residing in the state to guide and assist with the state's geodetic and geospatial activities. Participating states include Arkansas, Colorado, Florida, Hawaii, Michigan, Minnesota, Mississippi, North Carolina, Ohio, Oregon, Texas, Vermont and Wisconsin.
NGS is currently restructuring the Advisor Program, transitioning to coverage for all states with Regional Geodetic Advisors. This is being done over the next several years with a phased shift from the "state" advisor approach to a "regional" advisor focus. The first region in the program is Southwest (Arizona, Nevada, New Mexico and Utah).
NGS also encourages states to identify a State Geodetic Coordinator, designating an employee to act as a liaison between the state and NGS. Participating states include Indiana, Missouri, North Carolina, North Dakota, South Carolina, Tennessee, Virginia, Wyoming and Puerto Rico.
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 or regional advisor program, contact your geodetic advisor. For states that do not participate in this program, you may contact the NGS Info Center.
NGS Information Center
Phone: 301-713-3242
Fax: 301-713-4172
email
Federal positions are announced by the Office of Personnel Management. Visit USAJOBS for job listings and information.
For
questions, please contact the Communications
and Outreach Branch.
