Question Is MagArrow's "Altitude" with respect to ellipsoid or geoid?

Altitude refers to Meters Above Mean Sea Level.

For both MagArrow I and II, the altitude is ellipsoidal and the earth model is WGS84.

For additional information:

Overview

Reported elevations from MagArrow (and G-864 and MagEx) are the unedited values from the elevation field in the GNSS's GGA NMEA string.  That value is the GNSS's calculated height above geoid; height above geoid is the standard meaning of the elevation field in the GGA NMEA string.

But what does calculated height above geoid mean?

Definitions

• GNSS - Global Navigation Satellite System
• GPS - The GNSS operated by the United States.  Other systems include GLONASS(Russia), Galileo (Europe), BeiDou (China),  QZSS (Japan), IRNSS (India).
• Ellipsoid  - A comparatively simple or abstract geometric model of Earth's surface.
• Geoid - A more complex model of Earth's surface that takes the place of what was previously called Mean Sea Level.  At any particular latitude or longitude, the geoid's surface may be above or below the ellipsoid's by as much as a few hundred meters, depending on regional and local geography and geology.
• Reference datum - A specific model of Earth's shape (such as WGS84, EGM96...), including references to specific landmarks. 

Calculations

A particular GNSS, for example the GPS system run by the United States, provides timing data to a receiver to calculate the receiver's position above or below a particular latitude and longitude on the surface of the ellipsoid.  

The GNSS receiver first uses that timing data to calculate its height over the ellipsoid, and then subtracts from it the local height of the geoid over the ellipsoid (or HAE), to arrive at the local height of the receiver over the geoid (or in old-fashioned terms, elevation over mean sea level):

• h - calculated height above geoid.  This is the value reported in the GGA elevation field.
• H - height of the receiver over the ellipsoid (calculated from GNSS timing signals)
• N - local height of geoid over the ellipsoid, or HAE, per a lookup table or other local reference.

h = H - N     

Because the local height of the geoid over the ellipsoid is not provided by the GNSS, it must be provided locally, i.e. by the GNSS receiver, which may contain an internal database from which the local geoid height over ellipsoid (or HAE) can be found, based on the receiver's latitude and longitude.  Small GNSS receivers contain small HAE databases,  so the HAE value will not be exact.  Some small receivers contain no HAE table at all; in this case HAE is deemed to be zero, so that the reported elevation is the uncorrected height over ellipsoid.​​​​​​​

A user of elevation data from Geometrics' MagArrow, G-864, and MagEx magnetometers may evaluate or adjust the reported values of the GGA elevation field and the GGA HAE field, by comparing the GGA HAE values to another source of local HAE data; this may particularly be useful for GNSSes that report a HAE equal to zero.  Geometrics magnetometers do not currently record the values of a VDOP calculation, which offers an additional statistical estimate of the accuracy of the GNSS elevation measurement.