What are these "obelisks"

[hungerdunger]

I have been holidaying in El Cotillo for over 15 years.  Every year I wonder what the purpose is of the little white "obelisks" are which are scattered along the coast between El Caleton and the lighthouse (and apparently along the track from the lighthouse to Corralejo). I've heard a couple of opinions from other people, but I wonder whether anyone can give me a DEFINITIVE answer.

https://drive.google.com/file/d/1ozHb-wmr_fAP4CIRKt86l0RQBvPJbC_V/view?usp=sharing
https://drive.google.com/file/d/1hKNhj9CMGuVjwNYhhVkKNRWGioGBAgw9/view?usp=sharing

[calculator]

DOMINIO PÚBLICO MARÍTIMO TERRESTE - DPMT

Land water boundary defined by Ley de Costas.

[suendrob]

DOMINIO PÚBLICO MARÍTIMO TERRESTE - DPMT

Land water boundary defined by Ley de Costas.

That's what I've been told, too.       
Apparently you must not be seen to park beyond the line of them, with harsh consequences.

[MLA]

http://ezinearticles.com/?Coastline-Law-Leaves-Thousands-Of-Spanish-Property-Investors-Without-Right-Of-Title&id=1175666