British National Grid in QGIS: EPSG:27700 Setup Guide

The British National Grid (BNG) is the projected coordinate reference system commonly used for mapping data across Great Britain. In QGIS, its standard identifier is EPSG:27700, listed as OSGB36 / British National Grid. It uses easting and northing coordinates in metres and is based on the OSGB36 datum and a Transverse Mercator projection. 1 2

This guide explains how to set up British National Grid in QGIS, how to tell the difference between a project CRS and a layer CRS, how to import CSV coordinates, how to reproject data safely, and how to troubleshoot layers that appear in the wrong place.

Quick answer: Set the QGIS project CRS to EPSG:27700 when you want a British National Grid map. Do not automatically change every layer’s stored CRS to EPSG:27700. A layer should be assigned EPSG:27700 only when its existing coordinate values are already British National Grid coordinates.

British National Grid facts at a glance

Item	Value
QGIS/EPSG code	EPSG:27700
Official CRS name	OSGB36 / British National Grid
Geodetic datum	OSGB36
Projection	British National Grid, a Transverse Mercator conversion
Coordinate order	Easting, then northing
Horizontal units	Metres
Main area of use	Great Britain: England, Scotland and Wales
Common coordinate format	Numeric easting and northing, or an alphanumeric National Grid reference

EPSG:27700 is a horizontal CRS. It does not describe height or elevation. If a dataset also contains heights, those values may use a separate vertical reference system and should not be assumed to be part of EPSG:27700. 1

The EPSG definition uses a central meridian of 2° west, a latitude of natural origin of 49° north, a scale factor of 0.9996012717, a false easting of 400,000 metres, and a false northing of -100,000 metres. These projection parameters explain how longitude and latitude are converted into grid coordinates; most QGIS users should select EPSG:27700 rather than enter the parameters manually. 2

Project CRS, layer CRS and reprojection: the distinction that matters

QGIS uses three related ideas that are often confused.

Project CRS

The project CRS controls how QGIS draws layers together on the map canvas. If the project CRS is EPSG:27700, QGIS can transform layers from other correctly defined CRSs on the fly so that they display in the British National Grid project. The project CRS is shown in the lower-right corner of the QGIS window. 3

Layer CRS

The layer CRS describes the coordinate system of the data as it is stored. A WGS 84 GPS layer may correctly remain EPSG:4326 even when it is displayed inside an EPSG:27700 project.

Assigning a CRS versus reprojecting a layer

Assigning a CRS tells QGIS how to interpret coordinate values that already exist. It does not change the numeric coordinates in the source file. Use this only when the file has no CRS or its CRS label is wrong and you know what the coordinates actually represent. 3

Reprojecting a layer calculates new coordinate values in a different CRS and saves a transformed dataset. Use this when you need a permanent EPSG:27700 copy of correctly defined data in another CRS.

Changing a project CRS is not the same as reprojecting every data source. For many mapping tasks, setting the project CRS and leaving each layer’s correct source CRS unchanged is the safest approach.

How to set the QGIS project CRS to EPSG:27700

Method 1: Project Properties

1. Open QGIS and create a new project, or open an existing project.
2. Select Project → Properties.
3. Open the CRS tab.
4. Search for 27700.
5. Select OSGB36 / British National Grid.
6. Click Apply and then OK.

QGIS’s CRS selector can search by EPSG code or CRS name. Searching for 27700 is usually more reliable than trying to identify the correct entry from a long list. 3

Method 2: Use the project CRS indicator

The current CRS is displayed in the lower-right of the QGIS window. Click the CRS indicator, search for EPSG:27700, select OSGB36 / British National Grid, and apply the change.

Method 3: Set the project CRS from a correctly defined layer

If a layer already has the correct CRS, right-click it in the Layers panel and choose Set Project CRS from Layer. Use this only when the layer’s CRS is known to be correct. QGIS will use that CRS for the map canvas and transform other correctly defined layers to it. 3

Set EPSG:27700 as the default for new QGIS projects

If most of your work uses Great Britain data, you can set EPSG:27700 as the default CRS for new projects:

1. Open Settings → Options.
2. Select the CRS section.
3. Find the setting for the CRS used for new projects.
4. Choose Use a default CRS.
5. Search for and select EPSG:27700 — OSGB36 / British National Grid.
6. Apply the setting.

This changes the default for new projects. It does not repair the CRS definition of existing data, and it does not permanently reproject files that you add later.

You may also see an option related to setting the project CRS from the first layer added. That can be convenient, but it should not replace checking whether the first layer’s CRS is correctly defined.

Add British National Grid vector data

To add a Shapefile, GeoPackage layer, GeoJSON file, or another vector source:

1. Choose Layer → Add Layer → Add Vector Layer.
2. Select the file or data source.
3. Add the layer to the project.
4. Right-click the layer and open Properties → Source.
5. Check the layer’s CRS and extent.
6. Confirm that the layer appears in the expected location.

If the data is genuinely stored in British National Grid coordinates, the layer should normally be identified as EPSG:27700. If the source is in WGS 84, ETRS89, Web Mercator, or another CRS, leave that source CRS correctly defined. QGIS can display it in an EPSG:27700 project through on-the-fly transformation. 3

If the layer has no CRS

A missing CRS is not evidence that the data uses EPSG:27700. First inspect the source documentation, metadata, file sidecar files, coordinate values, and the organization that supplied the data.

If the file contains values such as 400,000 eastings and 300,000 northings and the source documentation confirms British National Grid, assign EPSG:27700 to the layer. If you are not sure, do not guess. A wrong assignment can place the data far from its true location.

Permanently reproject data to EPSG:27700

Use permanent reprojection when you need a new dataset whose stored coordinates are British National Grid values.

Vector data

1. Confirm that the input layer has the correct source CRS.
2. Right-click the layer and choose Export → Save Features As.
3. Select an output format such as GeoPackage.
4. Set the target CRS to EPSG:27700 — OSGB36 / British National Grid.
5. Choose an output file and layer name.
6. Save the new dataset.
7. Add the output to QGIS and verify its CRS and location.

The original file remains in its source CRS. Keep it unchanged and treat the EPSG:27700 file as a derived copy.

Raster data

For raster data, use Raster → Projections → Warp (Reproject) or the equivalent Processing Toolbox algorithm. Confirm the input CRS first, select EPSG:27700 as the target CRS, choose an appropriate resampling method, and check the output resolution and extent.

Nearest-neighbour resampling is often used for categorical rasters such as land-cover classes. Continuous surfaces may require a different method. The appropriate choice depends on the data and the analysis, so document it in the project notes.

Import CSV points into QGIS using British National Grid

A CSV file may contain point coordinates as two numeric columns, often called Easting and Northing. The important question is the CRS of those coordinate values, not the CRS you want the final map to use.

CSV already contains British National Grid coordinates

1. Open Layer → Add Layer → Add Delimited Text Layer.
2. Browse to the CSV file.
3. Select the correct delimiter and confirm that the first row contains field names.
4. Set Geometry Definition to Point coordinates.
5. Select the easting column as X field.
6. Select the northing column as Y field.
7. Set Geometry CRS to EPSG:27700.
8. Click Add.
9. Confirm that the points appear in the correct location.
10. Save the temporary point layer to GeoPackage if you need a permanent spatial dataset.

QGIS treats the X field as the horizontal coordinate and the Y field as the vertical coordinate. For British National Grid point data, X is easting and Y is northing. 4

CSV contains longitude and latitude

If the CSV contains longitude and latitude in decimal degrees, it is usually not EPSG:27700. It may be WGS 84, commonly represented by EPSG:4326, but confirm the source documentation.

Import the CSV using the CRS in which the coordinates were collected. Then let QGIS display the points in the EPSG:27700 project, or export a reprojected copy if you need British National Grid coordinates stored in the output.

Do not set a longitude/latitude CSV to EPSG:27700 merely because the project uses British National Grid. That would tell QGIS to interpret degrees as metres and will place the points incorrectly.

Work with National Grid references such as “SU 12345 67890”

A National Grid reference is not the same thing as a pair of decimal-degree coordinates. It contains a 100-kilometre grid-square code followed by digits that refine the position within that square.

For example, a reference such as SU 12345 67890 needs to be parsed into a numeric easting and northing before it can be used as EPSG:27700 point geometry. The number of digits controls the approximate precision. More digits provide a finer grid reference, but they do not create accuracy that was not present in the original source.

A reliable workflow should:

1. Confirm that the reference uses the Ordnance Survey National Grid format.
2. Preserve the original reference in a text field.
3. Convert the grid-square letters and digits with a documented tool or script.
4. Create point geometry in EPSG:27700.
5. Check a sample against a trusted map or source.
6. Record the conversion method and expected precision.

Do not paste a grid reference into a latitude field or treat its letters as a normal easting value.

Coordinate transformations between WGS 84 and OSGB36

EPSG:27700 is based on OSGB36. GPS receivers and many web datasets use WGS 84 or ETRS89. These systems are related but are not identical, so converting between them is a datum transformation as well as a projection change.

QGIS can transform layers on the fly, but the available transformation and its accuracy depend on the source and target CRSs and on whether the required support files are installed. QGIS documentation notes that it attempts to use the most accurate available transformation and may warn when additional transformation support is needed. 3

For ordinary visualization, the default available transformation may be sufficient. For surveying, engineering, cadastral work, or other accuracy-sensitive projects, check the transformation used and consider the Ordnance Survey guidance and the relevant OSTN15 transformation resources. Do not describe EPSG:27700 as providing “metre-level accuracy” simply because its coordinate units are metres. Units and positional accuracy are different properties.

Why British National Grid layers can look wrong

The layer appears in the wrong country

The layer may have been assigned the wrong CRS. Check the source metadata and coordinate values. Do not repair a wrongly positioned layer by changing the project CRS repeatedly.

The layer appears far from other data

Check the layer CRS, project CRS, axis order, units, and whether the coordinates are degrees or metres. A longitude/latitude dataset incorrectly labelled EPSG:27700 is a common cause.

The layer is invisible

Right-click the layer and choose Zoom to Layer. If the extent is unreasonable, inspect the layer CRS and coordinate values. Also check whether the layer is filtered, hidden, empty, or outside the current scale range.

QGIS asks about a datum transformation

Read the source and target CRS in the warning. If the transformation is between WGS 84 or ETRS89 and OSGB36, choose an appropriate available operation. For precision-critical work, install or enable the required transformation grids and record the selected operation.

The map is correct but measurements are unexpected

A project can display layers together while measurement units remain unsuitable for the task. Check the project ellipsoid, measurement settings, CRS units, and whether the analysis should be performed in a projected CRS.

BNG and common UK CRSs are not interchangeable

The following identifiers are often confused:

• EPSG:27700 — OSGB36 / British National Grid: projected eastings and northings in metres for Great Britain.
• EPSG:4326 — WGS 84: geographic longitude and latitude in degrees.
• EPSG:4258 — ETRS89: geographic coordinates in degrees, commonly used for European terrestrial data.
• EPSG:3857 — WGS 84 / Pseudo-Mercator: a web-mapping CRS used by many online tile services; its metre-like coordinates are intended mainly for visualization.

Use the CRS documented by the data provider. Do not select EPSG:27700 just because the data covers the United Kingdom.

Best practices for a reliable QGIS BNG project

Keep the original data unchanged and save transformed copies separately. Use a GeoPackage for derived vector layers where practical, and use descriptive layer names that record the source or processing stage.

Record the source organization, download date, data version, CRS, transformation choice, processing steps, and any assumptions. This is especially important when combining modern GNSS data with historical maps or datasets collected under different datums.

Set the project CRS to EPSG:27700 when it is the appropriate map CRS, but allow correctly defined layers to retain their original CRSs. Validate the project with a known location or trusted reference layer before calculating distances, areas, or spatial relationships.

Frequently asked questions

What is the EPSG code for British National Grid in QGIS?

The EPSG code is 27700, listed in QGIS as OSGB36 / British National Grid.

Should every QGIS layer be changed to EPSG:27700?

No. Keep each layer’s correct source CRS. Set the project CRS to EPSG:27700 for a BNG map, and permanently reproject only when you need a new dataset stored in that CRS.

Is British National Grid the same as WGS 84?

No. British National Grid uses OSGB36 and projected easting/northing coordinates. WGS 84 normally uses longitude and latitude. QGIS can transform between them, but the transformation is not simply a change of labels.

What CRS should I use for UK latitude and longitude CSV data?

Use the CRS documented by the data source. If the values are WGS 84 longitude and latitude, EPSG:4326 is commonly used. Import the coordinates in their source CRS, then display or reproject them in an EPSG:27700 project as required.

Why are my British National Grid coordinates in metres?

EPSG:27700 is a projected CRS whose coordinate axes are easting and northing in metres. Those metre units are coordinate units; they do not by themselves guarantee a particular positional accuracy.

Can I use British National Grid with web basemaps?

Usually yes. Many web tile services use EPSG:3857, while your project can use EPSG:27700. QGIS can transform layers for display, but visual alignment and measurement accuracy should still be checked.

How do I convert an OS grid reference to QGIS points?

Convert the alphanumeric reference into numeric easting and northing using a documented conversion method, then create point geometry with EPSG:27700. Keep the original grid reference as an attribute and validate sample points.

Do I need to install an OSTN15 transformation grid?

Not for every map. The need depends on the source and target datums and the accuracy required. For high-precision work, inspect QGIS’s transformation warning and follow current Ordnance Survey guidance.

Conclusion

For most QGIS projects covering Great Britain, the essential code to remember is EPSG:27700. Set it as the project CRS when you want a British National Grid map, but do not relabel layers unless you know what their stored coordinates represent.

The safest workflow is to identify the source CRS, set the project CRS deliberately, use on-the-fly transformation for display, and create a permanently reprojected copy only when the analysis or deliverable requires one. That distinction prevents many of the alignment, measurement, and data-quality problems associated with British National Grid projects.

References

[1]Ordnance Survey: Coordinate reference systems

[2] EPSG:27700 — OSGB36 / British National Grid

[3] QGIS User Guide: Working with Projections

[4] QGIS User Guide: Opening and importing data

[5] QGIS: Setting the CRS to British National Grid