By default, Alloy uses a simple Helmert transformation when converting geometry from the British National Grid coordinate system (EPSG:27700) to WGS 84 (EPSG:4326). This works well most of the time and requires no user input.

However, it can result in slight misalignment in a handful of cases, and we do like to give you choices where we can!

Therefore, as of the October 2023 update, you can now specify a grid shift file for a particular transformation (e.g. OSTN02, OSTN15) when importing or exporting geometry data via the API.

To use a different transformation, first upload its grid shift file to Alloy. The max size is 100MB and the supported formats are:  GSB, LLA, DAT.

Using the API

When using the Alloy API to import or export data, just add +nadgrids=<fileid> to the proj4 string (where <fileid>is the Item ID of your uploaded grid shift file).

When importing data using the ImportSettingsDataWebModel, you can specify the proj4 string as part of its geometry property.

When importing NSG/NLPG data using one of the two ImportSettingsStandardFormatWebModels, no proj4j string is specified. Instead, you can populate its gridFileItemID property with the Item ID of the grid shift file.

Using Alloy Web

When configuring a custom import in Alloy Web, you can configure the Geometry in step 3.

After choosing the Type, select the magnifier icon in the Coordinate system field and choose one of those listed. This populates the field with a proj4 string, which you can then suffix with +nadgrids=<fileid>.