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I am using QGIS v3.12 and projection EPSG:27700 - British National Grid.

I am trying to add a label to my map layout which will give the grid reference of the centre of the map as a UK 8 digit grid reference ( e.g. "AB123456" ).

So far I have managed to get the x and y coordinates using the expression: 

Grid reference at map centre: [%round(x (map_get (item_variables('Map1'), 'map_extent_center')),0)%] , [%round(y (map_get (item_variables('Map1'), 'map_extent_center')),0)%]

Is there any function available in QGIS that can translate these coordinates into the required format?

Sorry my coding skills are not that great!

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You need a custom function which I have translated from my Java translation of the OS Visual Basic to python. Insert the following into a new Function in the Expression Function editor (see this class for more details of how to do this). 

from qgis.core import *
from qgis.gui import *

gridLetters = [["SV", "SQ", "SL", "SF", "SA", "NV", "NQ", "NL", "NF", "NA", "HV", "HQ", "HL"],                          
               ["SW", "SR", "SM", "SG", "SB", "NW", "NR", "NM", "NG", "NB", "HW", "HR", "HM"],                          
               ["SX", "SS", "SN", "SH", "SC", "NX", "NS", "NN", "NH", "NC", "HX", "HS", "HN"],                          
               ["SY", "ST", "SO", "SJ", "SD", "NY", "NT", "NO", "NJ", "ND", "HY", "HT", "HO"],                          
               ["SZ", "SU", "SP", "SK", "SE", "NZ", "NU", "NP", "NK", "NE", "HZ", "HU", "HP"],                          
               ["TV", "TQ", "TL", "TF", "TA", "OV", "OQ", "OL", "OF", "OA", "JV", "JQ", "JL"],                          
               ["TW", "TR", "TM", "TG", "TB", "OW", "OR", "OM", "OG", "OB", "JW", "JR", "JM"]]                          

@qgsfunction(args='auto' , group='Custom', usesgeometry=True)                                                                                                                        
def gridSquare(size, geometry, feature, parent): 
    centroid = geometry.centroid() 
    easting = centroid.asPoint().x()
    northing = centroid.asPoint().y()    
    xText = "%06d" % easting                                                                                            
    yText = "%07d" % northing                                                                                           
    x = int(xText[:1])                                                                                                  
    y = int(yText[:2])                                                                                                  

    gl = gridLetters[x][y]                                                                                              

    result = gl                                                                                                         

    if size == 100:                                                                                                     
        return result                                                                                                   
    elif size == 10:                                                                                                    
        return gl + xText[1:2] + yText[2:3]                                                                             
    elif size == 1:                                                                                                     
        return gl + xText[1:3] + yText[2:4]                                                                             
    elif size == .1:                                                                                                    
        return gl + xText[1:4] + yText[2:5]                                                                             
    elif size == .01:                                                                                                   
        return gl + xText[1:5] + yText[2:6]                                                                             
    elif size == .001:                                                                                                     
        return gl + xText[1:6] + yText[2:7]

You can then use expressions like:

 gridSquare(.1, $geometry)

to get output like TF392033 or gridSquare(1, $geometry) to get TF3903. In a layout you should be able to do something like:

[%gridSquare(.1, map_get(item_variables('Map 1'), 'map_extent_center'))%]

but I can only get NULL as a response to any of the item_variables function calls - may be my map isn't called Map 1?

Update - if I rename my map to map then this works:

Map Centre is at [%gridSquare(.1, map_get(item_variables('map'), 'map_extent_center'))%]

enter image description here

Update

I've packaged this into a QGIS plugin (experimental) which should show up in the repository soonish. In the meantime you can get it from https://github.com/ianturton/osgridref

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  • Ian, thabnk you very much for taking the time to help me with this. I have added the custom function as suggested and it works splendidly.
    – Jools
    Commented Apr 20, 2020 at 19:54
  • please accept the answer if it solves your problem
    – Ian Turton
    Commented Apr 21, 2020 at 7:49
  • This solution is brilliant. I'm also trying to use it to label a grid in a composer with OS grid squares around the edge, does anyone know what function I'd use to create that?
    – geobar
    Commented Sep 8, 2022 at 11:12

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