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Signed-off-by: Mattias Andrée <m@maandree.se>

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+.TH LIBTELLURIAN_SEA_LEVEL 3 libtellurian
+.SH NAME
+libtellurian_sea_level \- Calculate a geocentric radius
+
+.SH SYNPOSIS
+.ni
+#include <libtellurian.h>
+
+double libtellurian_sea_level(double \fIlatitude\fP);
+
+double libtellurian_sea_level_radians(double \fIlatitude\fP);
+.fi
+.PP
+Link with
+.I -ltellurian
+.IR -lm .
+
+.SH DESCRIPTION
+The
+.BR libtellurian_sea_level ()
+function calculates the distance from the centre of
+the Earth the ellipsoid, for any point on a the given
+.IR latitude ,
+which givens the nominal sea level at this position,
+which is close to the actual sea level, and is the
+reference datum for the altitude (ellipsoidal height)
+reported in raw GPS measurements.
+.PP
+The
+.I latitude
+shall be specified according to GPS and in degrees.
+.PP
+The
+.BR libtellurian_sea_level_radians ()
+function is identical to the
+.BR libtellurian_sea_level ()
+function except that
+.I latitude
+shall be specified in radians.
+
+.SH RETURN VALUE
+The
+.BR libtellurian_sea_level ()
+and
+.BR libtellurian_sea_level_radians ()
+functions return the distance from the centre of
+the Earth to the ellipsoid, measured in meters.
+
+.SH ERRORS
+None.
+
+.SH NOTES
+Some GPS systems, espcially those for hiking, surveying,
+and aviation, report orthometric height (height above
+sea level), rather than ellipsoidal height to make it
+easier for the user to interpret.
+
+.SH SEE ALSO
+.BR libtellurian (7)