Functionally, it was a beacon - but not of the type that you put a basket of burning wood at the top of, and certainly predating radio by a considerable time. In OSM, it was originally mapped as a beacon, then changed to an obelisk (because to be fair it does look like one) and then changed to try and be both by describing it as landmark=obelisk.
There are likely lots of others around the coast. A quick search finds this one (with lights, apparently, and mapped as historic=monument; note=beacon), and it’s western twin (also mapped as historic=monument; note=beacon but with no lights). See also this stock photo.
So, what’s the best tagging? I don’t think that any sort of monument is correct, because it isn’t. Functionally, it was a beacon, but the way it is constructed resembles an obelisk. There are only 13 uses of historic=beacon.
** unsuccessfully of course, because these efforts always are - a new non-yes/no value appeared last week.
As an aside, these are described as “Admiralty Navigation Beacons”. The organisation that created them ended up here as “an Executive Agency of the UK Ministry of Defence (MOD). We have trading fund status, meaning we are self-funded through the sales of our products and services and operate at no additional cost to the taxpayer.”. Out of copyright Admiralty charts could no doubt suggest the location of many more of these.
Throwing a disused: or abandoned: prefix on a man_made=beacon tag seems like it would be reasonable to me (depending on whether it’s actually missing anything from when it worked). There are currently 11 disused:man_made=beacon and 25 abandoned:man_made=beacon according to taginfo.
That particular pair appear to be the ones described here and here as once defining a leading line that was presumably safe at the time. Probably not a good thing to add unless someone with local knowledge can confirm that that is still a preferred route.
Someone appears to have added a duplicate node for the west beacon indicating this purpose.
Indeed - it wouldn’t make sense to assume that any “go this way” information that was valid 100 or so years ago is still valid now. However, it is still a big solid lump of something, so it makes sense for it to be in OSM in some way, with regular (apart from any seamark) tags.
I’ve added historic=beacon to the Chaldon Hill one, even though that’s a fairly rare combination (and has been used on radio beacons).
Actually, it looks like the seamark was first but because it no standard OSM tags on it a duplicate was created. I guess that they could be merged.
To be sure it’s still used for marine navigation, you’d have to look on a current chart. I’d go as far as assuming it is still current if it can be seen from the sea. If solid , not moving objects can be seen from sea, they get used. This has even included trees for ad-hoc transits. I’ve even seen people select cows! When conditions change that affect navigation, markers, buoys etc. do get moved and information issued via ‘Notices to Mariners’ for charts to be updated manually.
One of my local marine approaches is a white pillar in front of a white wall with an unpainted vertical strip. When the strip and pillar are aligned to see all white, you’re on the deeper channel. A few metres either way and you’re aground at low tide.