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I've read plenty of posts on this site regarding coordinate systems but none of them have helped solve this issue. I'm using Ordinance Survey Lat/Long coordinates for postcodes but a small number are being placed out at sea above Northern Wales (see image).

Surely if the coordinates were completely wrong, it would be more than a fraction of the total points that were incorrectly geolocated.

All layers are in OSGB 1936 / British National Grid (EPSG: 27700). Changing this to WGS 84 doesn't resolve the issue.

enter image description here

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I think these are Northern Irish postcodes. Confirm this by checking the code of those in the sea. There's probably also other NI postcodes overlapping England and Wales, and wrongly located. Do you have Belfast (BT) postcodes?

If you are using OS CodePoint data, then there's a country code field. Any locations coded as Northern Ireland have coordinates in the Northern Irish grid system. I'm not sure what the coordinate reference system is for this - it might be epsg:2157.

To combine these on the same map, you'll need to split the data, reproject the NI ones with epsg:2157 and the EWS ones with epsg:27700 to get lat-long for both. Then combine.

There's some information about this, specifically about Belfast (BT) postcodes, on the OS site:

https://www.ordnancesurvey.co.uk/business-and-government/help-and-support/products/code-point.html

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  • Fantastic, this solved the issue! When I was working with the ONS postcode directory in R, I simply removed the Northern Ireland postcodes and remapped the remaining points in QGIS.
    – Thirst for Knowledge
    May 10, 2016 at 9:33
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    It may be worth mentioning that all postcodes in Northern Ireland are BT. It's not just for Belfast City itself.
    – TRiG
    Jul 11, 2017 at 1:42

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