2

The shapefiles and boundaries I work with are on WGS 84 referenced to SA:

GEOGCS["WGS 84", DATUM["World Geodetic System 1984", SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]], AUTHORITY["EPSG","6326"]], PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]], UNIT["degree", 0.017453292519943295], AXIS["Geodetic longitude", EAST], AXIS["Geodetic latitude", NORTH], AUTHORITY["EPSG","4326"]]

I need to project them onto a plane to make area calculations and intersect shapes. I'm stuck with a few questions:

  1. local official projection is defined by Gauss Conformal Mercator. How do I project files with this map definition in Python? here are a number of definitions: http://spatialreference.org/ref/epsg/2047/, as well as http://epsg.io/?q=south%20africa.

Which package/wrapper/function do I choose to do the heavy-lifiting?

I read that planar projection in SA are done in a variety of ways- Which of the Lo/Wg do I choose for the projection to cover the entire span of the country?

  1. The mapping needs to maintain the areas. Does tha GCM do that?

  2. Is Gauss-Krüger the same as Gauss Conformal?

1

Depending on the context, the Gauss-Krüger and Gauss Conformal are the same or very similar projections (see Gauss–Krüger coordinate system on wiki.

Relevant to your question, there are two important mutually exclusive classes of projections, each having distinctive geometric qualities:

  • Conformal (or orthomorphic) projections preserve angular relationships between great circles.
  • Equal area (or equivalent) projections preserve areal relationships.

Conformal projections definitely do not maintain correct areal scales.

Perhaps for South Africa, choose an Albers Equal Area Conic projection (see, generally wiki/Albers_projection, and specifically, albers-equal-area-conic-south-africa on spatialreference.org

If you do still wish to use the (conformal) Transverse Mercator (EPSG projection 2047) -- or whichever projection you decide you need -- using Python, perhaps you should ask that separately.

  • Very helpful. On the theoretical side, SA is 'wide' and when projecting shapes, the accuracy might be lost 'on the edges' of the projection (from what I understand). Is it advisable to make calculations/project per areas of specific geographic 'widths' (say West, Center, East of the country, or every 2deg) separately or would conic projection be accurate? Excuse lack of formality in the vocabulary, I'm new to working with the geo data and it is very much an intuitive analysis at this stage. – Anna Magdalena Feb 22 '16 at 23:40
  • It's true that with any projection, the larger the area and the further away from the standard lines, the greater the distortion. However, if it is an equal area projection, that quality will not get distorted (shape will). – Martin F Feb 24 '16 at 7:09

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.