Certain tile map providers (WMTS) use a different zoomlevel definition.
To make them compatible with slippy map format, an offset can be specified for downloading.
I'm against adding specific code here to account for different map formats. We work with Slippy Map tiles as a single format - converting into this format is the user's responsibility.
Certain tile map providers (WMTS) use a different zoomlevel definition. To make them compatible with slippy map format, an offset can be specified for downloading.