© Valeport Limited 2003
MIDAS Surveyor Operating Manual
Page 20
0420805d.doc
4.2.7
GEODETIC SETUP
The MIDAS Surveyor will display and log position data in either Latitude/Longitude or Eastings/Northings format. This
may be set under the Display Setup sub-menu (section 4.2.7.2).
To display data in Lat/Long format, the desired spheroid must be defined – as default this is WGS84, but the user may
alter this under the Geodetic Setup menu. Each spheroid is defined by 9 parameters, two of which define the shape of
the spheroid, and 7 of which define the datum of the spheroid. The MIDAS Surveyor allows all of these 9 parameters to
be set as required.
Spheroid Shape
WGS84 Default Value
Semi-major axis
6378137.0000
Flattening (actually entered as 1/Flattening)
298.25722300
Spheroid Datum
WGS84 Default Value
X axis linear position
0.000
Y axis linear position
0.000
Z axis linear position
0.000
X axis rotational position
0.000000
Y axis rotational position
0.000000
Z axis rotational position
0.000000
XYZ Datum Scale Factor
0.0000000
Note that some spheroid references give an “E²” figure instead of Flattening. E² stands for Eccentricity ², which is:
E² = 2 x Flattening – Flattening ²
Solving this equation gives:
Flattening = 1 -
√
(1-E²)
The value that the MIDAS Surveyor requires is 1 / Flattening, which should be in the region of 300.
Note also that some spheroid definitions (particularly in the USA) only use the Linear Position Datum values. The
remainder (rotational positions and scale factor) should be set to zero.
To display data in Eastings/Northings format, the local grid must be defined – as default this UTM (Universal Transverse
Mercator), but again, the user may alter this and define their own grid under the Geodetic Setup menu. The MIDAS
Surveyor will only allow a Transverse Mercator Projection to be used. A total of 5 parameters must be set to define the
local grid:
Grid Parameters
UTM Default Value
Origin Longitude
3.000000
Origin Longitude
0.000000
False Easting
500000.000
False Northing
0.000 (North Hemi)
10000000.000 (South Hemi)
Scale Factor
0.9996
