The Namibian Map System normally comprises
two projections for either a countrywide (1:1million) or sheet
wise (1:50k, 1:250k) display:

Countrywide
(1:1mio or bigger) 
Sheet
wise
(1:250k, 1:50k) 
Projection
Spheroid
Central Meridian
Latitude of Origin
Standard Parallels
Central Scale Factor
False Easting
False Northing 
Albers Equal Area
Bessel 1841 (Schwarzeck Datum)
E 17
S 22
S 19, S 27
1.0
0.0
0.0 
LO (Gauss Conformal)
Bessel 1841 (Schwarzeck Datum)
variable E 13,15,17,19,21
S 22

1.0
0.0
0.0 
The Schwarzeck
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
meter
(1 GLm = 1.0000135965 SIm).
The transformation
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:
Bessel
Spheroid:
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)
= 0.003342773182
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 Yaxis to
the west. This is a lefthanded Cartesian coordinate system, whilst
the computer thinks in a righthanded system. For the display of
gridded data just use the normal coordinate system but be careful
with the grids origin. Some software allows taking care of this
by giving a negative central scale factor.
There are
some topographical map sheets floating around in UTM (Datum not
specified) and Lambert Conformal (Clarke 1880) projections.
The Geological
Survey of Namibia believes that the research and development undertaken
by Prof. C. Merry provide the most accurate parameters. Therefore
some original notes:
The datum
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 coordinates would need to be converted
to international meters and the (int. meters) coordinates of the
false origin added. Then the standard transverse Mercator equations
can be used to convert the plane coordinates to geographical (int.
meter values for the modified Bessel ellipsoid: a=6377483.m).
The geographical
coordinates would be converted to X, Y, Z Cartesian coordinates
(using a=6377483..m), and the datum shifts added, with appropriate
sign.
These transformed
Cartesian coordinates are converted to WGS84 geographical coordinates,
using the WGS84 ellipsoid parameters.
A few additional
comments:
Prof. Merry
sells Windows software to do these conversions (US$150).
My understanding
is that the military UTM maps use the modified Bessel ellipsoid,
with international meters, not the WGS84 ellipsoid; but I could
be wrong.
The
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
network. 