# Relation between Geoidal separation and antenna altitude in GGA sentence?

I have searched about definitions and relations about geoidal separation and antenna altitude reported in GGA sentence. The more I read the more I am confused.So in a simple manner can anyone explain me relation between altitude and geoidal separation ?

I am assumming that my gps source is using WG84 datum.

The geoidal separation just reports the height difference between the ellipsoidal surface and the geoid model's surface. Natively, GNSS calculates ellipsoidal height (height above ellipsoid surface) but it's usually more useful to have a geoidal height, approximately a height above mean sea level.

The geoidal height is often called an altitude or elevation.

The relationship is

``````h = N + H

where

h = height above ellipsoid
H = elevation, orthometric height
N = geoidal separation (some books call this the geoidal height)
``````

The GPS/GNSS device will have some geoid model that it's using to calculate the N value which is the 2nd "z" value given in a GGA NMEA sentence. The first is the calculated orthometric height value, H. Here's the layout of the GGA sentence:

``````eg3. \$GPGGA,hhmmss.ss,llll.ll,a,yyyyy.yy,a,x,xx,x.x,x.x,M,x.x,M,x.x,xxxx*hh
1    = UTC of Position
2    = Latitude
3    = N or S
4    = Longitude
5    = E or W
6    = GPS quality indicator (0=invalid; 1=GPS fix; 2=Diff. GPS fix)
7    = Number of satellites in use [not those in view]
8    = Horizontal dilution of position
9    = Antenna altitude above/below mean sea level (geoid)
10   = Meters  (Antenna height unit)
11   = Geoidal separation (Diff. between WGS-84 earth ellipsoid and
mean sea level.  -=geoid is below WGS-84 ellipsoid)
12   = Meters  (Units of geoidal separation)
13   = Age in seconds since last update from diff. reference station
14   = Diff. reference station ID#
15   = Checksum
``````
• thanks . One more question what is meant by [not those in view] . Dec 17 '15 at 7:49
• There might be 7 satellites in view of the receiver but only 5 are actually being used to calculated the receiver's position, perhaps due to the geometry of the satellites. Dec 17 '15 at 8:27

`-=geoid is below WGS-84 ellipsoid` would suggest that the relationship is

``````H = h + N
``````

See also The geoid undulation. Note that in the article the definitions of H and h are reversed.

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– PolyGeo
Jan 17 '18 at 11:38

Looking at NMEA Revealed,

We have for description of the field altitude:

Antenna Altitude above/below mean-sea-level (geoid) (in meters)

And for description of the field geoidal separation:

Geoidal separation, the difference between the WGS-84 earth ellipsoid and mean-sea-level (geoid), "-" means mean-sea-level below ellipsoid

Let's note (as in answer by @mkennedy):

``````h = height above ellipsoid
H = elevation, orthometric height (i.e. antenna altitude in GPGGA)
N = geoidal separation (some books call this the geoidal height)
``````

And consider N to be negative. Then according to catdb.org, the mean-see-level is below ellipsoid ("-"), so `H > h` (H is height from mean see level, h is height from ellipsoid). So `H = h + D` with D positive. As N is negative we must take D = - N.

So the correct formula is:

``````h = N + H
``````

See this scholarly discussion at from ESRI Understanding Our World