Using your own rasters with OS Open basemaps
/With the release of the OS Open basemaps in ArcGIS Online, customers may wish to overlay their own raster tile layers on the basemaps. In some instances the following error might be seen:
In order for your raster tile layer to be compatible with the OS Open basemaps, you need to ensure two things. Firstly, that your data is in British National Grid projection and secondly, that you are using the OS Open basemaps tiling scheme.
To check if your data is in the correct projection refer to this technical article. If your data is not in British National Grid you can use the Project tool to re-project your data.
Your tile layer needs be cached in the same tiling scheme as OS Open basemaps so the zoom levels are consistent. You can download the OS Open basemaps tiling scheme here. The tiling scheme contains a text file and an XML file which will be used to define the scale extents for your data so that it matches the OS Open basemaps. To cache your data you can use either ArcMap or ArcGIS Pro.
Caching using ArcMap
If you are using ArcMap to cache your tile layers, you can import the tiling scheme by going to File > Share As > Tile Package. In the Tiling Scheme drop down list select 'A tiling scheme file' to add the OS Open basemaps tiling scheme.
Caching using ArcGIS Pro
In ArcGIS Pro, you can use the geoprocessing tool "Create Map Tile Package". Ensure you have unchecked the “Package for ArcGIS Online | Bing Maps | Google Maps” box. You can then import the tiling scheme in the 'Service' dialogue box by dragging and dropping the XML file (note that it is not possible to browse to an XML file in the dialog below).
Publish your tile package (.tpk file) into ArcGIS Online (refer to step 4 in the following article). You will now have a tile layer which is compatible with the OS Open basemaps.