I think this is a case of me not finding the correct issue.
The problem I have is that I have a set of survey data in Autocad which is using a local grid system to give true distances in CAD. The way this works is that the data has been recorded in British National Grid and then scaled slightly around a single point so that distances measured in CAD equate exactly to distances on the ground.
What I am not sure of is the best way to handle it when we port this data into the GIS (we also want to use ArcGis Online, collector, City Engine with the data).
1) Create all basemaps and coordinates in our base grid system
2) Convert from our "True Distance Grid" to British National Grid when importing to the GIS - Potentially convert back when exporting to CAD
My feeling is the second option would make most sense as I think the GIS reports true distances using the measure tool and a "standard" grid makes sense.
Following either system I am also not sure how to define this "bespoke" coordinate system which is essentially a tweaked British National Grid (scaled by a factor from a single point) within the system which then can be used as an input for any transformation.
Hope this makes sense - Thanks for any thoughts