National Geodetic Survey (NGS)
Height Conversion Algebraic Sign Convention
This process is designed to provide the datum shift between the NAVD 88 and NGVD 29 vertical datums at a specified geographic position. The correct use of algebraic signs to convert NGVD 29 heights to NAVD 88 heights, or NAVD 88 heights to NGVD 29 heights, is illustrated with examples.
Rudolf J. Fury M.S.,M.Eng.
05/12/1999
Data grids of (NAVD 88 - NGVD 29) height differences represent the datum shift model.
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| from NGVD 29 ----> NAVD 88 |
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If a NAVD 88 height is desired when a NGVD 29 height is given,
ADD the model value ALGEBRAICALLY to the NGVD 29 height.
FORMULA: height (NAVD 88) = height (NGVD 29) + datum shift (correction) value
Examples:
1. the NGVD 29 height is 500 meters (1640.420 feet) at
36 10 35.0 latitude
098 40 10.0 longitude
After keying this position into VERTCON, the returned
(NAVD 88 - NGVD 29) datum shift (correction) value is
+ 0.202 meter (+0.663 ft)
--------
| ADD | this value ALGEBRAICALLY [ keep the sign ] to the NGVD 29 height:
--------
500.000
+ 0.202
---------
the NAVD 88 height is 500.202 meters (1641.083 feet).
2. the NGVD 29 height is 120 meters (393.701 feet) at
36 10 35.0 latitude
078 40 10.0 longitude
After keying this position into VERTCON, the returned
(NAVD 88 - NGVD 29) datum shift (correction) value is
- 0.267 meter (-0.876 ft)
--------
| ADD | this value ALGEBRAICALLY [ keep the sign ] to the NGVD 29 height:
--------
120.000
- 0.267
---------
the NAVD 88 height is 119.733 meters (392.825 feet).
- - - - - - - - - - - - - - -
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| from NAVD 88 ----> NGVD 29 |
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If a NGVD 29 height is desired when a NAVD 88 height is given,
SUBTRACT the model value ALGEBRAICALLY from the NAVD 88 height.
FORMULA: height (NGVD 29) = height (NAVD 88) - datum shift (correction) value
Examples:
1. the NAVD 88 height is 500.202 meters (1641.083 feet) at
36 10 35.0 latitude
098 40 10.0 longitude
After keying this position into VERTCON, the returned
(NAVD 88 - NGVD 29) datum shift (correction) value is
+ 0.202 meter (+0.663 ft)
-----------------
|SUBTRACT| this ALGEBRAICALLY [ flip the sign ] from the NGVD 29 height:
-----------------
500.202
- 0.202
---------
the NGVD 29 height is 500.000 meters (1640.420 feet).
2. the NAVD 88 height is 119.733 meters (392.825 feet) at
36 10 35.0 latitude
078 40 10.0 longitude
After keying this position into VERTCON, the returned
(NAVD 88 - NGVD 29) datum shift (correction) value is
- 0.267 meter (-0.876 ft)
-----------------
|SUBTRACT| this ALGEBRAICALLY [ flip the sign ] from the NGVD 29 height:
-----------------
119.733
+ 0.267
---------
the NGVD 29 height is 120.000 meters (393.701 feet).
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