The Namibian Map System normally comprises
two projections for either a countrywide (1:1million) or sheet
wise (1:50k, 1:250k) display:
(1:1mio or bigger)
Latitude of Origin
Central Scale Factor
Albers Equal Area
Bessel 1841 (Schwarzeck Datum)
S 19, S 27
LO (Gauss Conformal)
Bessel 1841 (Schwarzeck Datum)
variable E 13,15,17,19,21
Datum consists of the Bessel 1841 Spheroid tied to a point called
Schwarzeck located at 22º 45' 35".820 S and 18º 40' 34".549 E. Furthermore
it comprises the 'German Legal Meter' instead of the international
(1 GLm = 1.0000135965 SIm).
from the Schwarzeck datum into WGS 84 implies X, Y and Z shifts
but no rotation. DMA – NIMA and Prof. Charles Merry from the University
of Cape Town do give shift values, from which the latter ones are
the more accurate:
a = 6377397.1550 &”German legal”; meter
b = 6356078.96325 ”German legal” meter
a = 6377483.865 intern. meter
b = 6356165.383 intern. meter
f = 1/299.1528128 (no changes due to length unit)
X, Y, Z shifts in meter
DMA –; NIMA 616 (± 20) 97 (±
20) -251 (± 20)
C. Merry (UCT) 616.80 103.30 -256.90
On the Maps in the LO system you will
find the positive X axis to the south and the positive Y-axis to
the west. This is a left-handed Cartesian co-ordinate system, whilst
the computer thinks in a right-handed system. For the display of
gridded data just use the normal co-ordinate system but be careful
with the grids origin. Some software allows taking care of this
by giving a negative central scale factor.
some topographical map sheets floating around in UTM (Datum not
specified) and Lambert Conformal (Clarke 1880) projections.
Survey of Namibia believes that the research and development undertaken
by Prof. C. Merry provide the most accurate parameters. Therefore
some original notes:
transformation parameters I have supplied are based upon 13 Doppler
points, of which the DMA used a subset of 3. The RMS fit for these
13 points is 3m, which means there may be some parts of the country
where the discrepancy is 10m (this is an indication of the accuracy
of the original terrestrial survey, carried out some 100 years ago).
The map projection
is a modified transverse mercator, with a false origin and units
of legal meters. The plane co-ordinates would need to be converted
to international meters and the (int. meters) co-ordinates of the
false origin added. Then the standard transverse Mercator equations
can be used to convert the plane co-ordinates to geographical (int.
meter values for the modified Bessel ellipsoid: a=6377483.m).
co-ordinates would be converted to X, Y, Z Cartesian co-ordinates
(using a=6377483..m), and the datum shifts added, with appropriate
Cartesian co-ordinates are converted to WGS84 geographical co-ordinates,
using the WGS84 ellipsoid parameters.
A few additional
sells Windows software to do these conversions (US$150).
is that the military UTM maps use the modified Bessel ellipsoid,
with international meters, not the WGS84 ellipsoid; but I could
difference between international meters and legal meters is fairly
small. As far as I know, all practicing surveyors and engineers
use instruments (tapes, EDM, GPS) calibrated in international meters
and solve for a scale factor (sometimes not even this, when working
in small areas) when tying their surveys to the national control