EPSG:9658

Vertical Offset by Grid Interpolation (VERTCON)

Attributes

Data source: EPSG

Information source: US National Geodetic Survey - http://www.ngs.noaa.gov

Revision date: 2020-01-16

Remarks: Any NAD realization may be used as the Interpolation CRS; bi-linear interpolation is used. Input expects longitudes to be positive west.

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Formula

The relationship between NGVD29 height and NAVD88 height vertical coordinate reference systems for the coterminous US is available through three gridded data files of offsets (sometimes called height differences). The vertical offset at a point is first interpolated within the grid of values using bi-linear interpolation. The interpolated offset is then applied as an offset:
•	If a NAVD88 height is desired when a NGVD29 height is given, add the interpolated offset to the NGVD29 height.
•	If a NGVD29 height is desired when a NAVD88 height is given, subtract the interpolated offset from the NAVD88 height.

Most horizontal positions of the bench marks used to generate the VERTCON grids were scaled from USGS topographic maps.  The estimated uncertainty of the scaled positions, 6 arc-seconds, is greater than the differences between NAD27 and NAD83 coordinates. Therefore the latitude and longitude used for interpolation of the grids can be referenced to either NAD27 or to NAD83(1986) or to any of the NAD83(NSRS) realisations (HARN, NSRS2007 or 2011).

Example

For a point in the Conus central grid area at 29°28'00.443"N, 98°28'49.346"W (NAD83) with NGVD29 height of 247.47 meters (811.9 feet).

First reduce the sexagesimal degree values to decimal degrees:
29°28'00.443"N = 29.4667897°N
98°28'49.346"W = 98.4803739°W

Then obtain the offsets at each of the surrounding grid nodes:
NW corner 29.50°N, 98.50°W, offset = 0.144m
NE corner 29.50°N, 98.45°W, offset = 0.132m
SE corner 29.45°N, 98.45°W, offset = 0.121m
SW corner 29.45°N, 98.50°W, offset = 0.125m

Then using bi-linear interpolation for 29.4667897°N, 98.4803739°W, offset = 0.129m

Then NAVD88 height = 247.47 + 0.129 = 247.599m.

For the reverse calculation of a NAVD88  height of 247.599m to NGVD29 height:

First obtain the offsets at each of the surrounding grid nodes:
NW corner 29.50°N, 98.50°W, offset = 0.144m
NE corner 29.50°N, 98.45°W, offset = 0.132m
SE corner 29.45°N, 98.45°W, offset = 0.121m
SW corner 29.45°N, 98.50°W, offset = 0.125m

Then using bi-linear interpolation for 29.4667897°N, 98.4803739°W, offset = +0.129m

Then NGVD29 height = 247.599 - 0.129 = 247.470m.
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