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In QGIS (3.4), when you select the CRS EPSG:21036 Arc 1960 / UTM zone 36S, you default to the general one for Kenya/Tanzania (code 1122) with +proj=utm +zone=36 +south +ellps=clrk80 +towgs84=-160,-6,-302,0,0,0,0 +units=m +no_defs .

However, there are three other codes for this CRS EPSG:21036:

  • Burundi: Code 3998: +proj=utm +zone=36 +south +ellps=clrk80 +towgs84=-153,-5,-292,0,0,0,0 +units=m +no_defs
  • Kenya onshore: Code 1284: +proj=utm +zone=36 +south +ellps=clrk80 +towgs84=-157,-2,-299,0,0,0,0 +units=m +no_defs
  • Tanzania onshore: Code 1285: +proj=utm +zone=36 +south +ellps=clrk80 +towgs84=-175,-23,-303,0,0,0,0 +units=m +no_defs

which of course provide different results (source: https://epsg.io/21036)

Is it possible within QGIS to get to choose between those actually different CRS (an option for those CRS to be shown in the CRS Selector window), or is it necessary to create Custom Projections to use them?

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You can tick on the Ask for datum transformation if several are available option, in Settings > Options > CRS > Default datum transformation, and QGIS will ask you wich of the available datums you want to use when you select the EPSG:21036 CRS.

Also, you can predefine a custom default datum transformation between pairs of coordinate reference systems globally there, or for a project in Project > Properties > CRS .


https://docs.qgis.org/testing/en/docs/user_manual/working_with_projections/working_with_projections.html#default-datum-transformations

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