Map eastings in QGIS 2:18 Print Composer alternate between correct values, then negative values

Question

Using CRS OSGB 1936 (ESPG 4277) at various map scales in Composer, I find that the northings are all correct, but the eastings alternate between two or three correct values, then two negative ones.

I upgraded to version 3.2.2. The problem still exists.

JimT · Answer

Summarized from the comments (which appear to have solved the issue):

EPSG 4277 is the geographic coordinate system OSGB 1936.  In reality this is little used except as an intermediary when converting ETRF89 latitude/longitude coordinates to/from the Ordnance Survey National Grid (EPSG 27700), which is the projected coordinate system for Great Britain (also commonly known as British National Grid, and also confusingly known as 'OSGB 36').

EPSG 4277 has units degrees.

EPSG 27700 has units metres and is the CRS code to use for the OS Grid.

Further information may be found in A Guide to Coordinate
Systems in Great Britain.

Map eastings in QGIS 2:18 Print Composer alternate between correct values, then negative values

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