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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.

zoomed_in

zoomed_out

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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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