This website does readability filtering of other pages. All styles, scripts, forms and ads are stripped. If you want your website excluded or have other feedback, use this form.

Frequently Asked Questions FAQ | NGS INFO - About Us | National Geodetic Survey

Frequently Asked Questions


Popular topics and questions are listed below, or use “control-F” (or “command-F” on a Mac) to enter a term in a search window. If you still have a question after scanning these FAQs, please contact the NGS Communications and Outreach Branch.

Popular FAQ Pages



Questions about...


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 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?

How accurate are USGS topographic 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...


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 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 CORS network?

Information about the 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 offers an interactive service as part of the NGS Coordinate Conversion and Transformation Tool (NCAT) to perform these functions.

How accurate are USGS topographic maps? What datum are these maps on?

The placement of information on the topographic maps are as accurate as the National Map Accuracy Standards 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 datasheet of a nearby survey station with this information on it. The Datasheet Page will enable you to retrieve a datasheet for the area or point you are seeking.

NGS also offers an interactive service as part of the NGS Coordinate Conversion and Transformation Tool (NCAT) to perform this function.


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.

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

Review the NGS Publications library, and the NGS home page contains a search tool.

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

The Regional Geodetic Advisor is a federal employee of NOAA’s National Geodetic Survey (NGS). The Advisor serves as a liaison between NGS and its public, academic and private sector constituents within their assigned region. They provide expert guidance and assistance to these constituents who are managing the geodetic component of geospatial activities that are tied to the National Spatial Reference System (NSRS). Recognizing that a Regional Advisor supports an entire region in which individual states may have local and unique geodetic needs or priorities, NGS recommends that each state identify someone to serve as a State Geodetic Coordinator. Learn more about Regional Advisors on the program's webpage.

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.

NGS Information Center
Phone: 301-713-3242
Fax: 301-713-4172
email

How can I find out about job opportunities in NOAA and NGS?

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.

Website Owner: National Geodetic Survey  /  Last modified by ngs.infocenter Oct 12 2018 NOS Home NGS Employees Privacy Policy Disclaimer USA.gov Ready.gov Site Map Contact Webmaster