BNG Layers offset when posted online

Nicauldron

Hi there

I have noticed that when I post data in the BNG projection online, they are offset slightly. 
I solved this by either using a BNG basemap from AGOL layers or projecting the data to WGS84 or Web Mercator before I post it.
However, I thought that the data was meant to project itself on-the-fly when online? 
Is there a way to fix this or should I keep projecting all my data before I post it online?
In the case of the latter, what projections does everyone suggest I use for UK Data (England, Wales and Scotland)

Here is a snip of one of my boundary line offsets. You can see the line is meant to follow the tree line here. Before posting online, it looks perfect in ArcPro.

 

berniejconnors

Nicauldron,

        I don't publish map services very often any longer.  That job has been passed to one of my co-workers.  But I do recall there were several transformation methods to go between ours CRS (EPSG 2953) and WGS84.  If the wrong method was used there would be a ~ 1 metre shift when our data was combined with other data in a Web map.  To prevent this from happening I had to define the preferred transformation in the properties of the MXD I was using to publish the map service.  I expect there is a similar capability for ArcGIS Pro.

        Here is a discussion of the NAD83 to WGS84 transformations that may help you decide if a similar problem exists for the BNG - https://desktop.arcgis.com/en/arcmap/latest/map/projections/choosing-an-appropriate-transformation.htm

Best regards,

Bernie.