When using the "OS Open Background" service in ArcGIS 10.3 alongside some OS Local vector data, I noticed that when zoomed in they align perfectly (see first image). However, when zoomed further out they appear to be out of alignment (see second image - the dark grey line should be under the red line). Can anyone explain to my what is the cause for this?

I had a response from ESRI (via Twitter) but I don't understand: "This is due to the reference scale of the different products at the different scales. Strategi data is very generalised (as a product to display at that scale needs to be) and so it causes these discrepancies."

I could understand if it was the other way around, i.e. looking OK at smaller scales, but zooming in showing a small difference. But it's the other way around.




The answer to this turned out to be quite simple and ESRI's response does make sense. At the larger scale the OS Open Background uses OS Local Map. At a smaller scale it switches to using OS Strategi. These two datasets don't totally agree with each other, which is where the discrepancy comes in.

This is described here: https://www.arcgis.com/home/item.html?id=a4ee9f2805754f4b9004103f67688bf1

The sources of data are Ordnance Survey Strategi data for small and mid-scales and Vector Map District and Open Map Local for the largest scales.

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