What I mean is, if I have a map which uses the OSGB36 (or EPSG:27700) projection, and I draw two areas (let's say polygons) on my map which cover the same area on the map, does this mean that the real life polygons they correspond to will have the same area in hectares, regardless of the location in Great Britain?

I'm wanting to know this because I'm working with GeoTIFF files using OSGB36, and I want to know if I can calculate the area of a polygon simply by counting pixels within the polygon and multiplying by the real-life area of one pixel.

As an aside, if OSGB36 does preserve area, presumably WGS84 does not - in which case am I right in thinking that it's easier to use OSGB36 if I need area calculations to be accurate?

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    I'm unfamiliar with OSGB36. Could ypu provide more info, such as an EPSG number? – Stu Smith Aug 27 '20 at 14:50
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    I guess OP is talking about EPSG 27700, the (infamous) British Ordonance Survery Grid. – Erik Aug 27 '20 at 15:00
  • digimap.edina.ac.uk/webhelp/os/data_information/os_data_issues/… ~ The following are used interchangeably, although in reality they mean slightly different things: British National Grid = BNG = OSGB36 = EPSG:27700 – nmtoken Aug 27 '20 at 15:52
  • That's correct - I do mean EPSG:27700. I'll add to the question. – Paul Pearson Aug 27 '20 at 18:22

Technically, OSGB36 is the geographic coordinate reference system that is used as the basis for British National Grid, EPSG::27700. British National Grid uses transverse Mercator as its map projection algorithm. TM is conformal so no, it doesn't maintain areas. It's designed to maintain local angles (shapes).

On the other hand, depending on how accurate the area calculations need to be, it may be 'good enough' for your purposes.

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