For TS: Devices are used to transmit and receive measurement data.
I.5
E
I.6
F
I.7
G
Description
GNSS operates on the WGS 1984 ellipsoid and all heights obtained by measur-
ing baselines are ellipsoidal heights. Existing heights are orthometric heights,
also called height above the geoid, height above mean sea-level or levelled
height. The mean sea level corresponds to a surface known as the geoid. The
relation between ellipsoidal height and orthometric height is
Orthometric Height = Ellipsoidal Height - Geoid Separation N
GS_043
P0
d1
d2
a
b
a
WGS 1984 ellipsoid
b
Geoid
P0 Measured point
d1 Ellipsoidal height
d2 Geoid separation N, is nega-
tive when the geoid is below
the ellipsoid
N value and geoid model
The geoid separation (N value) is the distance between the geoid and the ref-
erence ellipsoid. It can refer to the WGS 1984 or to the local ellipsoid. It is not
a constant except over maybe small flat areas such as 5 km x 5 km. Therefore
it is necessary to model the N value to obtain accurate orthometric heights.
The modelled N values form a geoid model for an area. With a geoid model
attached to a coordinate system, N values for the measured points can be
determined. Ellipsoidal heights can be converted to orthometric heights and
back.
Geoid models are an approximation of the N value. In terms of accuracy, they
can vary considerably and global models in particular should be used with cau-
tion. If the accuracy of the geoid model is not known, it can be safer to use
local control points with orthometric heights and apply a transformation to
approximate the local geoid.
The geoid separations in a geoid field file can be used in the field to change
between ellipsoidal and orthometric heights.
Type
Description
Creation
In Infinity with export onto a data storage device or the
internal memory of the instrument.
Extension
*.gem
Geoid model
Geoid field file
1046
Glossary
