Technical Note 5: South African data with coordinate reference systems with negative y-values
Summary
Users in South Africa and Namibia use a unique type of coordinate reference system called Transverse Mercator South Orientated (TMSO). Many commercial and open source software do not support this particular coordinate reference system. Instead these software use the Transverse Mercator projection with a North Orientated coordinate system. This results in projected data coordinates containing negative coordinate values.
What are the issues?
Software packages such as AutoCAD, ESRI, Hypack, QINSy, and many more utilize Transverse Mercator projection with a North Orientated coordinate system as their only or default method to project data in this area.
Fledermaus utilizes a list of default coordinate systems which is sourced from the ESPG (version 7.9) identified coordinate systems. EPSG coordinate systems currently contain only the South Orientated coordinate frames of reference for the South African region.
When Fledermaus is handed data from software such as AutoCAD or Hypack with North Orientated data, if someone assigned it a South Oriented coordinate system this would result in an incorrect CRS. It is critical to assign this type of data a North oriented coordinate reference system (CRS).
If you have North Orientated data you will need to properly identify it by one of the following methods:
a) Download the set of custom North Orientated coordinate reference systems that QPS have created for you and load them in to Fledermaus so you can use them. These are available in this South Africa Custom CRS zip file. There is an Howto for import here - Howto Import custom North Orientated coordinate reference systems to handle South African region data with negative y values
OR
b) Create a custom coordinate system from one of the south oriented ones available and then use it. This method is described in Howto Create custom North Orientated coordinate reference systems to handle South African region data with negative y values
Who does this impact?
If you have projected data from the South African region that you wish to import into Fledermaus it is important to know if it was projected by a North oriented coordinate frame of reference or a South oriented frame of reference. The give away is to look at the Y coordinate. If it is negative you have a north oriented frame of reference, if it is positive you have South Oriented Transverse Mercator data. If you see a CRS that don't explicitly tell you the frame of reference it will very likely be a basic TM projection that is North oriented.
A solution that some sofware has opted for is to 'go along' with the simplification and use Tranverse Mercator (North Oriented) instead of Transverse Mercator South Oriented (TMSO). However, as soon as this data is handed to another software that properly implements TMSO the problem is perpetuated and the client is still facing the same problem..
QPS developers after some discussion, decided the best solution is to properly assign a custom coordinate reference system that describes the data.
Additional Information.
Datum's and Coordinate Systems - a document produced by National Geo-spatial Information (NGI), South Africa's national mapping organisation. This document is about the Hartebeesthoek94 Datum which is the official geodetic datum for South Africa. The datum was implemented in 1999 when it replaced the Cape Datum.
Durban City, Surveying and Land Information Department produced a useful document summarizing coordinate systems used in South Africa - An Introduction to South African Coordinate Systems