# [time-nuts] TBolt self survey

bg at lysator.liu.se bg at lysator.liu.se
Thu Feb 11 07:21:30 UTC 2010

```Charles, Raj,

> Raj wrote:
>
>>My GPS reading is about 850 +- 1
>>The "official" height ASL = 920
>>This calculator gives me ~936
>>So what should I use ?
>
> Use the self-survey result.  The GPS isn't wrong, it's just reporting
> your altitude in a different system than you are used to.  The
> calculator allows you to check the GPS-reported altitude in the
> system that you are familiar with.
>
> Specifically, the calculator translates the GPS altitude (which is
> specified not with respect to actual mean sea level, but rather to an
> abstract, calculated "geoid") to one more familiar to you (specified
> with respect to mean sea level).  In your case, 936 is in pretty good
> agreement with 920, telling you that the GPS-reported value is plausible.
>
> Best regards,
>
> Charles

The wording is somewhat dangerous. GPGGA - the most common GPS message ! -
gives altitude reltive to Mean Sea Level. MSL height/altitude is the same
as Geoid height/altitude. [1]

The most basic GPS altitude is indeed height above the (WGS84) ellipsoid
(HAE). This is the "best" ellipsoid approximation of the geoid on a global
scale, as chosen in the WGS84 datum.

The difference between MSL (real world) and HAE is called geoid separation
or height of the geoid. This difference will be in the range of ca +-100m.
It can be calculated for each (horizontal) position on earth with the help
of a geoid model (ie EGM96 [2]). A full model to reach cm or dm level
accuracy for the whole earth requires a large model. A coarse lockup table
or a local model is easier to implement inside a small GPS receiver. The
GGA message will also give you the receiver used geoid separation. [1]

[1] http://www.gpsinformation.org/dale/nmea.htm#GGA
[2]  http://cddis.nasa.gov/926/egm96/egm96.html

--

Björn

```

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