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In late 2012 and early 2013, the Federal Emergency Management Agency (FEMA) released Advisory Base Flood Elevation maps for many communities in coastal New Jersey and New York. These maps provide advisory elevations with a reference to the North American Vertical Datum of 1988 (NAVD 88). Here's a look at what vertical datums are and how to use them with the advisory maps, in question and answer form.
If you have additional questions regarding vertical datums and the Advisory Base Flood Elevation maps not answered on this page or require additional assistance, please call the National Flood Insurance Program Help Center at 1(800) 427-4661 or contact us through our online form today.
What is a Vertical Datum? A vertical datum is a base measurement point (or set of points) from which elevations are determined. Historically, the standard datum used by the federal government was the National Geodetic Vertical Datum of 1929 (NGVD 29). However, the North American Vertical Datum of 1988 (NAVD 88) is now the national standard. There are also many other different vertical datums available. For example, local vertical datums have been established for each of the five boroughs of New York City: • Bronx Datum; • Brooklyn Datum; • Manhattan Datum; • Queens Datum; and • Richmond Datum. Why are there so many vertical datums? Over the years, different datums have been established based on the technology and information on hand and the need for a vertical reference point in a given area. The use of Global Position Systems (GPS) satellites to measure elevations and observations from coastal gages have helped government agencies develop new, more accurate datums, such as NAVD 88, which allow for the better comparison of elevations. Why does it matter which vertical datum an elevation is referenced to? Elevation values based on different vertical datums cannot be used together directly since they are based on a different vertical reference point (See graphic below). When comparing elevations on the Advisory Base Flood Elevation maps with elevation information on Elevation Certificates and other documents from different sources, care must be taken to ensure all elevations are in the same datum. If they are not the same, a conversion factor must be applied so that the values are referenced to the same datum before they are used. Failure to do so can result in improper structure design (e.g., building at the wrong elevation) which can have serious implications in terms of complying with community and state building requirements. Flood insurance rates can also be impacted.  Why is FEMA using NAVD88 for the Advisory Base Flood Elevations maps? NAVD88 is more compatible with modern surveying and mapping technologies like Global Positioning Systems (GPS). It also is more accurate than the previous national vertical datum, NGVD29. What do I do if I have elevation information for my house but the information is not in the NAVD88 datum? When you have elevations referenced to different datums, you will need to apply a conversion factor in order to properly compare the two. The conversion factor needed will depend on the original datum an elevation is referenced to, and the datum you are converting the elevation to. Is there a table of conversion factors for the various New York City datums available? Yes. Please see the table below. New York City Datum Conversion Table
Find the datum conversion factor value in the table below by locating the current datum of your elevation data in the top row and the datum you'd like to convert to in the left column. Then find the conversion factor value at the intersection of those fields. For further assistance with datum conversions, please contact the FEMA Map Information eXchange at 1(877) FEMA-MAP (1-877-336-2627).
How does one use the New York City datum conversion table?
As an example, using the table above, if you have a first floor elevation of 6.0 ft (Richmond Datum) for a property in Staten Island (Richmond County) then the elevation in NAVD88 would be: 6.0 ft + 2.092 feet = 8.092 ft. Similarly, if you had an ABFE elevation of 8.0 feet (NAVD88) for the same property in Staten Island the elevation in the Richmond Datum would be: 8.0 ft – 2.092 ft = 5.908 feet. Is there a table of conversion factors for New Jersey available? Conversion factors (please see table below) are needed only for Atlantic, Bergen, Burlington, Cape May, and Union Counties in New Jersey as the effective Base Flood Elevations (BFEs) shown on these Flood Insurance Rate Maps (FIRMs) are in NGVD29. For other counties in New Jersey where ABFEs have been developed, the effective BFEs shown on the FIRMs are in NAVD88. Thus no conversion factor is needed to compare the elevations on the ABFE maps and FIRMs for those counties. I have elevation data in datum NGVD29 for:
OR I have elevation data in datum NAVD88 for:
How does one use the New Jersey datum conversion table? As an example, using the table above, if you have a first floor elevation of 6.0 ft (NGVD29) for a property in Burlington County then the elevation in NAVD88 would be: 6.0 feet – 1.3 feet = 4.7 feet. Similarly, if you had an ABFE elevation of 3.0 ft (NAVD88) for the same property in Burlington County the elevation in NGVD29 would be: 3.0 feet + 1.3 feet = 4.3 feet. Are there any tools on the web to help convert between datums? Yes, the National Geodetic Survey has developed a tool for advanced users to convert between NGVD29 and NAVD88. This tool, called VERTCON, can be found at http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl. How does one use the VERTCON datum conversion tool? The National Geodetic Survey provides an online tool to help users convert between the datums for any location. A step-by-step guide is provided below. 1) Access the datum conversion online tool at http://www.ngs.noaa.gov/cgi-bin/VERTCON/vert_con.prl.  2) Enter the North Latitude and West Longitude positions. List both positions as positive numbers. Positions may be entered in any of the following three formats: o degrees, minutes and decimal seconds ( xxx xx xx.xxx) o degrees and decimal minutes ( xxx xx.xxx) o decimal degrees ( xxx.xxxxx) Note: There MUST be one or more blanks between entry fields. Decimals can be keyed commensurate with the field's precision, but are not required.  3) Enter the height to convert. Note: The default unit is meters. Enter “ft” after the number to show the elevations and conversion factor in feet.  4) Select the vertical datum of the height entered (i.e., this example is converting 10 ft NGVD29 into NAVD88; therefore, NGVD29 was chosen). 5) Click Submit. The result reveals both the conversion factor (-1.115 ft in this example) and the what the original elevation height is once converted to the new datum (8.885 ft NAVD88 in this example).  Where can I find more information about vertical datums? If additional details are needed, the following resources may be helpful. • For FEMA s guidelines regarding vertical datum conversions, visit: • FEMA's NAVD88 policy is presented in Procedure Memorandum 41 (March 2006). • Additional information on vertical datums, including the conversion from NGVD29 to NAVD88 at any location, can be found by visiting the National Geodetic Survey website. Back to Top Get Plug-Ins |
Vertical Datums and Advisory Base Flood Elevation Maps |