Here in Portugal we (and Spain) have landmarks that define the border, and there's a treaty signed in 1846 that states these landmarks should be maintained in cooperation between the two countries.

You can see some examples here, with the numbers and type of landmarks (apparently there are 1010 stone marks, other types of marks are wet marks for example, like rivers and lakes): https://pt.wikipedia.org/wiki/Lista_de_tro%C3%A7os_da_raia_(...

Although we don't officially agree what those actual borders are everywhere:

https://en.wikipedia.org/wiki/Olivenza#Claims_of_sovereignty

We both agree on the border there, see the treaty of vienna. The problem is that spain doesn't do what they agreed to do.

I wonder when this we be brought to light, specially when we have a pressing matter with Spain that's also being avoided, yet it could be extremely damaging to Portugal... I'm talking about the management of Tejo river.

The re routing Spain is doing is contributing to the destruction of a massive portion of Portugal ecosystems.

In the UK (technically Great Britain in this context) at least, there is an open dataset available from the national mapping authority (Ordnance Survey), which is called boundary lines. It contains all of the different boundary lines and borders of local authorities, parliamentary constituencies, regions, nations etc.

This is all geo referenced against the OSGB coordinate system.

I could see interesting challenges in doing this for international borders where both countries use different coordinate systems and projections for their mapping, and the potential for slithers of no-mans-land. I guess at a certain point, you need to refer back to what's on the ground, as over decades the land erodes from the sea, and rock formations collapse and shift as the earth moves etc. The definition of location gets far easier when you have satellites offering precision navigation and timing!

The OS maps the borders of local authorities. However, it doesn't hold the authoritative map of the borders of the counties. I don't mean the modern local authorities that call themselves counties - I mean the traditional counties, which haven't been abolished or changed for centuries. The exact location of these is defined almost tautologically - a place is part of a certain county because it has always been. The exact location of the border is more a matter of folklore than anything else.

Likewise, the exact location of borders between properties is usually ill-defined. Only borders that have gone through a painstaking process of surveying and registering (usually after a dispute or on a new build) are exactly defined. Usually, when you move into a new house, you have a fence around your property, and the boundary is generally assumed to be inside the fence (with conventions for whose land the fence sits on that aren't always followed).

Adverse possession makes it more complicated. A neighbour decided to put up a new fence, and weren't nice about it. I stipulated (in line with the law) that no part of the new fence could be placed on my property (except parts under the ground surface). They decided to put the fence in the "wrong" way round, with the vertical posts on their property, and the fence attached on their side of the posts, effectively giving me sole access to the area of land in-between their fence posts. If I were to sell my house, the new owner would be quite reasonable in assuming that that land was theirs. After the requisite time, I could apply to have the land legally registered as mine, although the new laws mean that the neighbour would be informed and given a chance to "evict" me from the land - they would have to tear down their fence and rebuild it the right way round so that I don't have sole access to it any more. In this way, adverse possession effectively resets the boundary to where the actual fence is in practice every now and again, which means that the land registry doesn't need to store exact boundary locations.

My parents' house is a good example of this. The rear boundary of the property is defined as being some distance from the road. It's not totally clear (although I assume that there is a standard) whether this means the curtilage or something else. When the land behind the property was redeveloped (from a run down witches cottage to a block of flats) it was important to my parents that the treeline at the rear of the property not be cut down. The deciding factor was the rusted remains of an old fence on the far side of the trees. I had been under instructions as a kid to not mess with that or pull it up because my parents knew that it was important to preserve this kind of evidence

In the UK, most freehold boundaries are available as a gml file. I'm looking at moving house and have one open right now in fact, here's a screenshot of it

https://imgur.com/cX2Seow.png

The white area being roads and non-registered land (mainly stuff that hasn't sold for decades and roads).

It's great, I can find a unique owner for any bit of field I have a question about, the size of the plot, etc.

The downside is that to get a copy of the actual entry in the register (to see who owns it and any covenants on the property there are), you have to pay £3. The entire process is digital and automated, IMO it should be free, but I suppose it's not a terrible price.

it has a small price so it becomes difficult/cost-prohibitive to use for nastier purposes.

ex, in my country forest owners get spammed and targeted by forest-management companies that want to cut it down.

Are those boundary lines the actual legal international definition of the borders? Or is it just some convenient representation close enough for ordinary uses?

You'd think location via GPS or similar framework was easy but it's not nearly as easy as you'd think. Among other things, the land is moving. Both continental plates and then also local features like river beds. See e.g. https://www.youtube.com/watch?v=_TX--Fku9NQ

If this was in Germany, there would be a fairly high precision map somewhere that records the intended position of each marker stone. This is usually the case for demarcations between private properties. If one of them gets moved, surveyors get sent out to measure the proper location from the map data and surrounding markers. And there's usually a fine for whoever moved such a stone.

They're stored in law. Treaties define borders in (seemingly) unambiguous terms. Meridians, river beds and so. Problems arise when river beds move and everything becomes extremely ambiguous. In the worst case scenario you've got borders defined by older borders of poorly documented feudal holdings and it's up in the wind where the actual border ought to be. So states have to negotiate to redefine them in modern terms.

Ok…but I’d assume those are pretty hefty law books then?

“Follow river such and so, then on that one field of farmer Jones turn left until the tree, turn right and walk in a straight line to that pointy rock, then walk back to the river etc. etc.”?

Yes, this is pretty much what actual "metes and bounds" deeds from several centuries ago look like.

The ground itself can move over 5 cm a year due to tectonics.

And river meanders are doing just that, meandering to and fro, changing (albeit slowly) continuously. Modern borders need to be tied geodetic datums, which has been also mentioned are also constantly changing (although probably more slowly than most rivers) and need to be updated continuously. The new datums coming out of the GRAV-D project will address the need for continuous updating (https://www.ngs.noaa.gov/datums/newdatums/index.shtml).

I'm not really sure how would one really mark this with stones... https://goo.gl/maps/Nn8ygXHJKkfHGFTs9

Or this one, also Belgium: https://www.google.com/maps/@51.4408275,4.9469359,14.35z

What exactly is going on here?

History. :)

Here is a fun example: https://en.wikipedia.org/wiki/Alaska_boundary_dispute