Why not to print OS Maps API basemaps in High DPI

Although enabling the High DPI option for an OS Maps API base map ensures that overlays are rendered at 300 DPI, the scale of the base map in the generated PDF does not correspond accurately to the scale displayed on the screen.

As a result, we ask you to keep the High DPI option in the Studio disabled.

The scale shown in iShare differs from the published scales for my WMTS base map, why is that?

The difference in scales in iShare vs those published for a WMTS base map such as OS Maps API is due to a difference in the DPI used to calculate the scales. iShare uses the OpenLayers library which has a DPI of 72 while WMTS scales are commonly calculated at 96 DPI.

Scale is the ratio of a distance on a map and the corresponding distance on the ground. This implies that the map distance and the ground distance are measured in the same unit. A web map displays a collection of pixels that correspond to a ground size but the size on the map is depended of the physical output DPI. This makes scale an imprecise measure for web mapping.