# EPSG:9658

## VERTCON

### Attributes

Data source: OGP

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

Revision date: 2016-12-15

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

#### Export

Definition: OGP XML

```<?xml version="1.0" encoding="UTF-8"?>
<epsg:informationSource>US National Geodetic Survey - http://www.ngs.noaa.gov</epsg:informationSource>
<epsg:revisionDate>2016-12-15</epsg:revisionDate>
<epsg:changes>
</epsg:changes>
<epsg:show>true</epsg:show>
<epsg:isDeprecated>false</epsg:isDeprecated>
<epsg:isOperationReversible>true</epsg:isOperationReversible>
<epsg: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.</epsg:example>