South-African Grid System

[+Jors]

Im sure i'm not the only one who wanted to do this, so I put this question to Garmin:

 

I fail to get the USER UTM grid to display the grid co-ordinates. In South Africa, using the official grid co-ordinate system for either one of the two datums currently in use, (WGS84 or Cape datum) I am setting the USER parameters to:

Longitude origin = E29°00.000'

Scale = +1.0000

False Easting = 0.0000 m

False Northing = 0.00000 m

 

The unit (GIII+) only displays "--------" where I would want to see co-ordinates. What would be the purpose of activating the USER UTM settings if co-ordinates cannot be displayed? Assistance in order to display grid-co-ordinates will be greatly appreciated.

 

Tony Broad from Garmin replied:

 

Sorry, the South African grid works backwards and upside down compared to all other grids - our user UTM grid increases to the North and East, the South African grid increases to the South and West so our current software cannot model this.

 

We are looking into how this can be changed, but I do not know if this will be done to our older GPS's as and when it is done.

[+Jors]

Setting the scale factor to -1,000000(negative) does the trick!

 

[This message was edited by Jors on June 23, 2002 at 12:27 PM.]

[+GlobalRat]

Jors, this may be of interest

 

From the director of surveys site:

Points are allocated unique geographical co-ordinates. These are latitude, longitude and height. The height could be either orthometric (above the geoid (defined as mean sea level)) or above the surface of the reference spheroid (ellipsoidal height). In order to map these co-ordinates, they need to be projected onto the plane. In South Africa we use the Gauss-conform projection. The projection effectively projects the latitude and longitude coordinates onto a surface referenced by a particular longitude and the equator. The South African system is defined in belts of two degrees in longitude. Every odd meridian is the central meridian of reference (simply referred to as that "Lo" or Longitude of origin). The Lo.s are 17, 19, 21, 23, 25, 27, 29 and 31. Each two degree belt is therefore flattened with reference to that Lo. (ie. Lo.19 has a flat surface representing the area between 18 degrees east and 20 degrees east)

 

The projected co-ordinates are referred to two axes being Y, east/west, and X, north/south. The Y co-ordinate is positive west of the Lo., zero on that Lo. and negative east of that Lo. The X co-ordinate is positive south of the equator and zero on the equator. It is therefore obvious that projected co-ordinates will represent a degree of distortion relative to the distance that co-ordinate is away from its Lo. as they are plane representations of a curved surface. There is no distortion on the Lo. For this reason, the South African system is restricted to two degree belts.

 

Note: The South African system closely resembles the UTM system. The UTM system in the Southern Hemisphere has a false northing at the equator being 10,000,000 metres and decreases southwards, and a false easting of 500,000 metres on the Lo. increasing eastwards. This system consists of six degree belts centred on a Lo. and these belts are referenced with zone numbers. Zone 33 covers Longitude 12 to Longitude 18 and is centred on Lo.15.

 

The following simple conversion is applicable only where the Lo. of the South African system and the central meridian of the UTM system coincide ie. 15ºE 21ºE 27ºE and 33ºE

 

Y(Lo.)=(500 000-E(UTM))/0.9996

X(Lo.)=(10 000 000-N(UTM))/0.9996

 

The UTM system incorporates a scale distortion of 0.9996 at the Lo. to reduce distortion at the edges of the belt.

 

GlobalRat