103
Commands
Table 4.45 lists the parameters.
Example
To activate the user-defined datum for position computations and measurements,
enter the following command after defining datum parameters with the UDD
command:
$PASHS,DTM,USR
After the user-defined datum is activated, the receiver internally transforms
positions from the reference datum (WGS84) to the user-defined datum. In standard
text books, however, the datum transformations are given from local datums to
WGS84. To simplify the entering of the transformation parameters, the translation,
rotation, and scale parameters are defined from the local datum to WGS84.
The generic formula used to translate and rotate from coordinate system 1 to
coordinate system 2:
e
rx
=
e
x
expressed in radians (this also applies to e
ry
and e
rz
).
Table 4.45. User-defined Datum Parameters
Field Description Range Units Default
d1 Identifier for the geodetic reference
datum; always zero
0N/A0
f2 Semi-major axis 6300000.000 to
6400000.000
meters 6378137.000
f3 Inverse flattening. 1/f3 = f2/(f2-b), where b
is semi-minor axis
290.00000000 to
301.00000000
meters 298.257223563
Dx,Dy,Dz Translation from the local datum to
WGS84
-1000.000 to
+1000.000
meters 0.00
e
x,
e
y,
e
z
Rotations from the local datum to WGS84.
Viewed from the positive end of the axis
about which the rotation takes place, the
positive rotation is counterclockwise; the
negative rotation is clockwise
-10.0000 to
+10.0000
seconds of
arc
0.00
f4 Delta scale factor
(scale factor = 1 + delta scale factor)
-25.000 to
+25.000
ppm 0.00
x
y
z
2
Dx
Dy
Dz
1m
6–
´10+()
1 e
rz
e
ry
–
e
rz
–1e
rx
e
ry
e
rx
–1
x
y
z
1
+=
G12RevD.book Page 103 Tuesday, April 2, 2002 4:33 PM
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