Some may recognize that this post is similar to another topic I posted several months back also on the South African National Grid (SAGRID). At the time, I was having issues with the -1 scaling factor settings in Franson CoordTrans and the format of the input SAGRID coordinates (should they be positive or negative). It turned out that despite the SAGRID being a negative scaling factor coordinate system, the scaling factor settings in the Franson grid must be set at +1 (I have a feeling this is because the multiplication of two negatives make a positive value).
I am now working at another location in South Africa. This time the coordinate values are located to the west of the central meridian which, by all accounts, should mean that they are positive easting values. The northing values are still negative. The problem is that when I try to convert a point from this area with +easting and -northing values, the position is off by about 120km to the east. I am using the same +1 scaling factor settings that I have now tested repeatedly in other locations of South Africa (all to the east of the central meridian, of course) so I do not think the problem is in the scaling factor.
Additionally, when I input independently acquired lat/long coordinates of the same point to back calculate what the SAGRID (Lo 19) equivalent is, Franson returns coordinate values which are negative on both the easting and northing!
What my testing has shown is that if I enter negative easting and northing values into franson (set to a +1 scaling factor) then the point is placed correctly and the lat long conversion is accurate for the southern hemisphere (i.e. negative lat). I just don't understand how this is correct, however, because I am working west of the central meridian (Lo 19) and the easting should be positive.
Has anyone else run into a similar problem with their easting values in a south-oriented coordinate system? I would appreciate any advice on how to fix this error.