Vision slits shall not be singled out as a specific kind of landmarks because this landscape value is subordinated to other landmarks.
Vision slits are places where opens an impressive view on distant, visually interesting landmarks. This landscape value is related to observation points but contrary to vision slits in observation points there opens a wide view where the main value is not constituted by a single landmark but rather by a landscape as a whole. Vision slit may have a much narrover angle of view and it has a dominant - single landmark. Valuable vision slit requires more than just a possibility to see this landmark - it is necessary to have a harmonious, expressive landscape around it.
This value is accentuated not just for dry classification. It is important to stress that through overgrowth and degradation of landscape numerous outstanding and potentially beautiful looking landmarks are hardly seen and look unattractive. After input of certain effort it could be possible to regain semi-forgotten "postcard views" of existing landmarks.
Characteristic example is Kuksas Palace (in image to the right) and the view on it from the peninsula in opposite bank of mill pond. This building in less prominent location would not be especially interesting. It would be seldom photographed also if the building and the landscape around it would be less spruce. And finally - there would be no such picture if there was no suitable location to take it.