in some cases like layer of sand over poorly draining clay it can happen, especially with road as low point in an area (in Poland glaciers brought both clay and sand if I remember geology right)
you can also get mix of sand and earth getting worst of both dry and wet states (either dust or mud).
@Mateusz_Konieczny Do you also adapting the Surface wiki-template page?
https://wiki.openstreetmap.org/wiki/Template:Map_Features:surface
not for now, I plan to reach some consensus state matching reality at Tag:surface=fine_gravel - OpenStreetMap Wiki
maybe advertise it in OSM Weekly (I hope that someone will send it) for an extra visibility
then send it for extra nitpicking review round at tagging mailing list
then fight with the surface template and ask people to edit wiki pages in other languages
StreetComplete is showing these images
Here is an example of a cycle trail, where we call it “rock dust” (poussière de pierre) which is very fine gravel and gravel dust.
(The above picture is not mine, but from the cycle path’s website)
Looks to me like a nicely compacted track, allowing a smoth and easy bike ride. The particles of fine gravel on the surface should not disturb.
I did a survey of the 400 square kilometers surrounding my home (in Victoria Australia) with this overpass query
[out:json];
waysurface=fine_gravel;
out body;
;
out skel qt;
It found:
fine_gravel 17
compacted 78
gravel 609
of the 17 fine_gravel I visited 8
one was a badly degraded surface=asphalt
the other 7 were typical Australian “gravel” roads with a compacted base and a thin surface layer of loose gravel, 20mm in size, typically only one stone deep
Though I haven’t done a formal analysis, I expect that the 609 gravel and 78 compacted are also typical Australian “gravel” roads and that the term fine_gravel is very infrequently used in a way which distinguishes itself from compacted or gravel.
Great that you’re working on clarifying this tag, @Mateusz_Konieczny!
The sand/fine_gravel/gravel/compacted distinction caused me confusion previously, but the now deleted wiki description of fine_gravel was meaningful to me. Thus I find it quite unfortunate/premature that you’ve removed it… The new page does not say much, and the image used is not compatible with my understanding of fine_gravel.
I used to simply think fine_gravel was smaller gravel (like you?), and was confused about having two distinct tags (gravel, fine_gravel) for one thing with a continuously varying property (size). Reading the wiki I got the impression that fine_gravel is not just gravel with a different size, but a distinct way of construction (thus two distinct tags make sense).
Some key properties that I, in practice, find distinguish (my understanding of) fine_gravel:
I’ve mostly used fine_gravel on maintained footpaths/cycleways, but occasionally I see it fitting for forest roads (usually when they are part of cycle routes and thus probably maintained for that purpose)
I’ve mainly understood surface=compacted to be any looser surface (dirt, ground, gravel, …) that has been solidified by repeated use.
[Edited: Images now included]
Image of what I understand to be fine_gravel:
Notice that erosion has exposed the subsurface layer of gravel on parts of the path furthest from the point of view:
And this is gravel:
And here compacted (which is sometimes hard to distinguish from fine_gravel…):
The wiki defines compacted as:
A mixture of larger (e.g., gravel) and smaller (e.g., sand) parts, compacted (e.g., with a roller), so the surface is more stable than loose gravel. Used, for example, for park paths, better tracks, some service ways, … Best sort of ways below paving with asphalt, concrete, paving stones.
And that corresponds to the use of “compacted” in road construction. Nevertheless one has to make a difference in between the base layers and the top layer (=surface). There are many roads and tracks with a compacted base layer and a loose gravel/fine_gravel top layer.
All of your sample pics show tracks with a compacted base layer, easily to identify by the plane and smooth surface which you can hardly achieve with a surface on top of a soft base layer like earth, clay, mud, ground, dirt or how you want to call it. Nevertheless I would agree to tag the track in pic 1 and 2 as fine gravel and pic 3 as gravel because these tracks have a reasonable amount of loose gravel as top layer.
Pic 4 = compacted but of poor quality or degraded over the time, potholes repaired with some loose material.
Pic 5 = very well compacted, some loose gravel at the side (maybe another track branching off here?)
but not that much to tag surface=gravel imho
Pic 6 = well compacted
Thanks for reviewing my photos, @Map_HeRo!
My post was mostly an attempt to document one persons (i.e. my) real world usage of these tags. I’m neither a native English speaker nor any kind of road surface expert. My use of compacted was mostly inspired by the picture shown in the StreetComplete app. I interpreted that picture as gravel that had been compacted by usage.
And, I must admit that I never gave the compacted wiki-page a good look. I guess my understanding of compacted corresponds more or less to the rougher versions of compacted shown in the wiki. I’ve never seen rollers on the ways I classify as compacted, though I have seen tractors dragging a heavy non-rolling thingy to smooth the surface…
In real life, the scene of picture 5 very much looked like the loose gravel on the side was the original surface (or refill). I assume the compacted surface came about mostly by usage. I might have classified it as gravel if observed earlier in the wear process…
A reasonable compaction does not require a big vibration roller, can be done by different kinds of machines. Even a hand guided vibrator plate will do if you compact layer by layer. Or a tractor dragged heavy compaction dis ting … 
I am quite sure all of your sample tracks have undergone some kind of treatment like this. Once it’s done it may last for years and will require not much more as some gravel to be evenly distributed on top from time to time (depending on the scope of traffic).
Looks exactly like that, part of the gravel distributed on top brushed aside by regular traffic probably in a bend or branch of the track.
From the constructionist, and also data user point of view, there is a property of compacted roads, that I never seen mentioned in this debate. In German it is called “bombiert”. No idea how to say in English. I’d say, it is very hard to combine with “fine_gravel”.
can you explain what “bombiert” means - never heard from this.
Do you mean that the surface is arched and not flat?
Yes, slightly arched. This business https://www.lu-hemetsberger.at/wegebau/ has some photos. The ones where it says “verdichten” (compacting) show a device. Its done to get fewer potholes and those less quickly.
Yes, slightly arched.
If I understand this correctly this is not something we express with the surface value, it is a description of the shape of the section (higher in the center and lower at the borders for drainage)
Very good link to for everyone who wants to understand how easily a compacted track can be refurbished without seveal different heavy machineries.
A surface profile called “bombiert” should be expected at every high quality compacted road and track, at least in areas where regular rainfall occurs. The picture illustrating “surface=fine_gravel” in the wiki shows a nicely “bombiert” compacted track profile, including the ditches on both sides for effective drainage.
Sounds to me like the English translation of “bombiert” in a road construction context would be the “crown” of a road, or a “crowned” road (in American English at least).
https://www.dirtandgravel.psu.edu/sites/default/files/General%20Resources/Technical%20Bulletins/TB_Crown_and_Cross_Slope.pdf
That’s it, centerline crown is what “bombiert” means, whereas this word is not regularly used for road profiles in Germany, maybe more common in Austria.
Similarly “centerline crown” could probably not be that common in british english due to royal conflic potential … ?? … 
… I’m sure Harry would know … 
… OK, off-topic, forget about it!
The picture up to https://wiki.openstreetmap.org/w/index.php?title=Tag:surface%3Dfine_gravel&oldid=2459132 did show a nicely centre line crowned compacted surface - the current picture shows fine_gravel - an OSM term that is not based on engineering language, something that can be laid on top of compacted. My gut feeling, fine_gravel will not stay for long as nice, if the compaction is in any way crowned.
That is corret talking about the specific page “tag:surface=fine_gravel” but I was talking about the sample pic for “fine_gravel” on the wiki mainpage for “key:surface”. There one can still read the “old” (and apparently wrong) definition for “fine_gravel” and also see the pic of the compacted track with centerline crown profile.
