Note that OSM Wiki summary descriptions are quite suspicious as for years they claimed that neither surface=gravel nor surface=fine_gravel is used for such surface which seems clearly mismatching actual tagging.
There was a lengthy discussion about surface of unpaved roads in the German community some time ago, including the difference in betweeen “fine_gravel” and “compacted”: Verwendung von Surface. As you surely do not want to read the whole stuff, here is my comment to your topic based on my experience in a construction company:
The photo for “fine_gravel” shows a forest road with a well-compacted gravel surface, i.e. surface=compacted. Nearly all roads based on water-bound materials (forest roads, farm roads) belong to this category and normally consist of several layers of gravel in different mixing ratios so that they can be compacted well (common are 0-16mm, 0-32mm and 0-56mm).
The texts in the Wiki on “compacted” and “fine_gravel” both describe this type of surface, I cannot see any significant difference. It is possible that the difference is meant to be that the surface layer of fine_gravel consists of a finer mixture than that of compacted, but who wants to determine this difference precisely on site? In my opinion, it is all “compacted” and that applies to all unpaved roads regularly used by motor vehicles.
Of course, there are also paths that actually have a surface of fine gravel or chippings (without containing sand/dust from 0-2 mm), but these tend to be footpaths or cycle paths in parks, driveways on private property and the like. Pure gravel or grit surfaces without fines part are not well suitable for motor vehicles, as the loose material would be swept away by traffic in a very short time. In my opinion, “fine_gravel” would be applicable to these ways although they also do have well compacted base layers normally.
If you cannot point to individual stones anymore, you are looking at sand or dust of grit sizes in between 0 and 2mm. Fine gravel, also known as grit or chips usually are called the sizes from 3-8mm. If the grit or grain size is larger than 8mm the common description is just gravel or crushed stone.
The difference between those is the material source: broken material with sharp edges produced in a quarry is correctly named crushed stone whereas naturally rounded material sieved out of a gravel pit is named gravel. In common language it is usual to call all the stuff just gravel.
I agree. fine_gravel is just likegravel, but it is smaller / finer particles. It is equally loose as less specific surface=gravel.
If the fine gravel is compacted with even smaller particles (like sand) so it becomes firm and not loose, then it becomes much more solid surface=compacted, on which bicycle wheels do not fall into and is quite good for cycling.
this picture (from surface=gravel wiki page) shows to me what fine_gravel (e.g. uncompacted fine gravel) often looks like (although often it is even deeper / more loose, i.e. more than one layer of it, so bicycle easily falls into it as is hard to cycle over, like this):
It would make no sense to me if surface=fine_gravel was basically exact duplicate of surface=compacted, as that wiki change implies.
hard to say based on this photo, that could be well-compacted or loose pile of moving fine gravel (visible tracks hint that it is relatively loose, but even with good compacted surface passage of many vehicles or heavy machinery can result in this)
this picture from surface=gravel page shows to me what fine_gravel (e.g. uncompacted fine gravel) often looks like (although often it is even deeper / more loose, i.e. more than one layer of it, so bicycle easily falls into it as is hard to cycle over):
to me this looks like gravel, although many of it looks like fine gravel there are also many pieces that seem to be bigger (than 8mm), 8mm are already at the upper end of what can be called “fine”
That is correct, but when this happens the surface is no longer compacted but loose gravel. The road may well have a compacted base layer which is essential for the sustainability, but this one definitely has a loose surface layer. At the front left side you can see some corrugations (washboarding) which is a clear indication for a loose surface (notwithstanding if it had been compacted earlier and just become loose due to heavy traffic).
I would also call this a gravel road - there will be some fine gravel and even sand for sure but the majority is gravel up to some 50mm estimated.
Edit: Sorry, got the wrong pic, my comments were referring to the picture of Tamsa Road (and I believe @Mateusz_Konieczny did as well). Nevertheless the other pic shows a regular gravel surface as well - most of the stones are too big for fine gravel.
Yeah I was referring more to looseness part of it; if it is indeed bigger than 8mm (I can’t really tell) then it would no longer be fine_gravel but indeed “regular” gravel. The other picture File:Tamsa.JPG - Wikimedia Commons is probably better example of looseness that both gravel and fine_gravel share (as opposed to compacted which is firm, not loose)
FYI: Gravel and pebbles describe the same type of stone, naturally rounded by erosive processes whereas the correct term for sharp edges (artificially broken) material is “crushed stone”. Nevertheless "gravel’ is the common term used for all kinds of small stones of a size roughly in between 2 - 80 mm and any road or track with a surface of such stone, may it be rounded or sharp edged, compacted or loose is called a “gravel road” in common english. For more details have a look here.
A sample pic for pebbles:
A sample pic for gravel:
And here is a pic of a nicely compacted road:
which is called “gravel road” nevertheless.
And another pic of a road with loose gravel surface (as seen above)
Curiously, what we here call “Kies” (fine-gravel in OSM speak), in construction means pebbles according to Kies – Wikipedia and what we here call “Schotter” (pebbles in OSM speak) in construction means crushed stone Schotter – Wikipedia