Nice work !

But something strange occurs when I display maps on Montana :

When all maps are enabled, transition between 50 K and 25 K appears between 300 and 200 m. Up to now, all is normal.

But when only National Park is enabled (Topo Discoverer All UK 50 K and Topo Trails 25 K disabled), I can see the 50 K layer from 500 m to 300 m : or I should not because Topo Discoverer All UK 50 K is disabled and 50 K bitmap from National Park 25 K should have been removed. What's up ?