Get Latitude and Longitude
In order to convert elevations in NGVD 29 to NAVD 88, the latitude and longitude must be found for the site. iTouchMap.com provides the following web page to help find the latitude and longitude of any site:
Find a point on the map as close as possible to the site. Left-click the map to get the decimal latitude and longitude. The iTouchMap.com results will provide the longitude as a negative number. Disregard the negative.
Get the Conversion
Note the decimal latitude and longitude obtained above, and open the National Geodetic Survey's conversion web page:
Enter or paste the decimal latitude and longitude in the respective fields. Make sure to disregard the negative for longitude; the form will not accept negative numbers. Enter FT into the Orthometric Height field in order to get the conversion in feet. Click on the Submit button to get the conversion.
NOTE: The conversion website will output a negative number whether converting from NGVD29 to NAVD88 or vise versa. Disregard the negative and adhere to the following:
When going from NGVD29 to NAVD88, the output number is subtracted from the given elevations.
When going from NAVD88 to NGVD29, the output number is added to the given elevations.