The official map of your property will not be exactly the same as the one on Lantmäteriet.se.
In more densely populated areas, there will be a local coordinate system, where each property is defined in terms of the neighbouring ones. This also applies to newly formed properties in old areas.
The property borders on digital maps are machine approximations of the mapping from the local coordinate system onto an absolute global coordinate system. This mapping can never be perfect, and it is often much less perfect than it could have been.
When the physical markers are missing or suspected of having moved from their original location (happens all the time for all sorts of reasons), Lantmäteriet will review the original documents of your and any number of neighbouring properties and deduce where the markers ought to be.
Regarding your fence, 20 years is very far from enough to establish "urminnes hävd". I suggest you wait another 100 years before you start assuming that they could act as facts on the ground in a property disputes! :-) And even then I wouldn't bet on it, unless the national archives are all destroyed...
I had to go back and check regarding "Urminnes hävd" (ancient custom). The creation of new instances of this for property rights was blocked back in 1970.
You can still use it, but then you must prove that the property right was an established ancient custom already before 1970. Anything that started after that will never qualify, no matter how much time passes.
My understanding of that type of "hävd", is that you need to have an official but incorrect deed of some sort, that remains uncontested by a true owner for 10 or 20 years.
Simply making uncontested use of the land is not enough.
I guess it is something that can happen quite easily in rural settings with very old property lines. Farmer 1 and 2 agree to some deal and a while later farmer 3 turns up and says "hey that's my land".
In more densely populated areas, there will be a local coordinate system, where each property is defined in terms of the neighbouring ones. This also applies to newly formed properties in old areas.
The property borders on digital maps are machine approximations of the mapping from the local coordinate system onto an absolute global coordinate system. This mapping can never be perfect, and it is often much less perfect than it could have been.
When the physical markers are missing or suspected of having moved from their original location (happens all the time for all sorts of reasons), Lantmäteriet will review the original documents of your and any number of neighbouring properties and deduce where the markers ought to be.
Regarding your fence, 20 years is very far from enough to establish "urminnes hävd". I suggest you wait another 100 years before you start assuming that they could act as facts on the ground in a property disputes! :-) And even then I wouldn't bet on it, unless the national archives are all destroyed...