[Tagging] surface=gravel problems

Bert -Araali- Van Opstal bert.araali.afritastic at gmail.com
Tue Apr 27 00:31:08 UTC 2021

Sorry if I jump into this discussion with some delay, I don't want to 
ignore what previously has been discussed but as being from a region 
(Africa), where most (up to 90%) of roads are unpaved I do think are 
local experience can provide some valuable input.

In this regard I would ask several questions:

1. Does a simple term as "gravel" or "fine-gravel" or any of the other 
values accurately reflects the top surface (as in the surface=* tag) of 
a road or track ?

2. Does this surface tag adds any valuable information for the road 
user, to determine if it's traversable or usable in all seasons ?

3.  Does adding a smoothness key improve the issue raised in question 2 ?

Answering these questions is very complex, no short answer covers the 
whole scope of issues related to unpaved roads, to my opinion both on a 
global as a local African scale, but I will do my best.

1. Does a simple term as "gravel" or "fine-gravel" or any of the other 
values accurately reflects the top surface (as in the surface=* tag) of 
a road or track ?

Most of the gravel roads are NOT pure gravel or crushed rock (no matter 
if it's fine, coarse or other grades).  Civil road engineers always make 
a mixture of gravel with another material, taking into consideration the 
availability of the different materials, the stability of the applied 
top surface when used in dry, wet and other (like icing in winter) 
conditions and it's hazardous or less favourable impacts on the 
surroundings and the users.
So have to evaluate how much gravel is mixed with ground, sand or 
clay-like material (like the murram we use locally). Gravel can be from 
river deposits, having mostly smooth round shapes and poor 
interlocking.  SO for the road user not interesting as not providing a 
stable traversable surface.  Crushed rock or rubble has mostly better 
interlocking purposes, so can provide a better stable and traversable 
surface.  Both gravel shape types are mixed with fine materials like 
ground, and or clay to decrease run-off of the fine aterial during rain. 
Negative is the noise and in combination with clay like materials, 
increased dust, which can have severe impacts on the environment and 
other road users.
A lower amount of gravel mixed with clay-like materials, deceases the 
amount of dust and noise but makes the road more slippery, up to the 
level that you can't even traverse it by foot.
Roads or tracks without high frequency traffic grow grass on the fine 
material in-between the gravel.  So the top surface becomes grass, no 
longer gravel.  It decreases run-off, washboarding and other effects and 
improves even the interlocking of the gravel and thus the stability.

This problematic and careful balance is true for most gravel roads 
across the world.  So saying a road surface is gravel or fine-gravel is 
essentially wrong, because it's a mixture.  Up to what percentage of 
rock or stone content or coarse sand should be called gravel ?
It varies across different engineering standards and even local 
standards across the world.  So does it make sense to add more specific 
information as just paved or unpaved ? In my opinion not, so I stopped, 
and many others did in Africa.  A road or track without a long term 
consistent surface is just tagged as unpaved and we would even deprecate 
the use of any other values. Because in many cases it's not consistent 
in the long term (grass growth, erosion, dust).

2. Does this surface tag adds any valuable information for the road 
user, to determine if it's traversable or usable in all seasons ?

No as already explained above.  Even if we take seasonal variances into 
account the surface and usability of the road changes a lot during wet, 
dry or icy conditions. One might argue and say adding a seasonal tag may 
improve this.  Yes it can, but not in conjunction with the surface tag. 
Like a low gravel content murram road is perfectly usable for all 
vehicles and on foot in the dry season, however becomes so slippery and 
a mud pool during the wet season that it's not usable at all.  A higher 
gravel content might make it usable and stable enough to use during the 
wet season, however again this depends on the gravel amount, it's shape 
and how it's mixed (just the top or also the sub layers).  So saying 
surface=gravel doesn't add any value in regard to this question.

3.  Does adding a smoothness key improve the issue raised in question 2 ?

No. An unpaved road might be perfectly smooth during the dry season.  
During the wet season, all the water runs off over the unpaved road, it 
turns into a small river with detrimental erosion effects. SO after a 
heavy rain shower and drying up it turns into a surface full of grooves 
and craters, when dried up hard as burned clay, no longer smooth at all. 
In some cases not even traversable any more by a normal high 4x4 
vehicle. Often repairs take place manually after a few months.

So to me, discussion about surface values other then unpaved or paved 
doesn't make a lot of sense as just to describe visual appearance, not 
forgetting that most gravel roads are mixtures.
To add information to a map as to describe if unpaved roads are 
traversable in different seasons needs a different tagging scheme.  As 
was and is done on many old maps.  We have all-season roads, we have 
seasonal roads where you should describe the major hazards, being, dust, 
noise, smoothness and sliding in dry, wet and winter conditions.  A 
complete new tagging scheme. It might need further detailing as to wide 
and narrow tire vehicles and on foot.
All efforts which we use today for smoothness and surface is just short 
term, non-conditional information, lots of information that can't be 
used to provide conclusive routing or road conditions.

The same might be said for paved roads. We have dense closed and open 
asphalt, concrete with surface impregnation or smooth, ribbed concrete 
... all with the purpose to improve road conditions during wet , dry and 
or icy conditions.  Reduce the hazards described.  Adding a surface tag 
as asphalt or concrete doesn't provide the required detail to be 
conclusive.  So deprecate the values, just leave it as paved. Invent a 
new tagging scheme, which might be similar as the one for unpaved roads.


Bert Araali

On 15/04/2021 10:56, Mateusz Konieczny via Tagging wrote:
> For long time this tag was described at Key:surface page as
> "Broken/crushed rock with sharp edges, known as ballast on railways. 
> Usually loosely arranged.
> Typical size for road and surface construction range between 4 and 8 cm".
> ( https://wiki.openstreetmap.org/wiki/Key:surface 
> <https://wiki.openstreetmap.org/wiki/Key:surface> )
> But it turns out that surface=gravel is primarily used for roads where 
> far smaller
> pieces of rocks are used.
> ---------------------------------------------------------------------------------
> It is result of
> - "gravel roads" term is often used in many countries for something 
> where far smaller pieces
>   of rock are used, some of such roads would qualify for 
> surface=compacted.
>   See https://wiki.openstreetmap.org/wiki/File:Gravelsign2.jpg 
> <https://wiki.openstreetmap.org/wiki/File:Gravelsign2.jpg>
> - "gravel" and equivalent terms in other languages is often used by 
> people to refer to pieces
>   of rock far smaller than 4cm
> - there was not good tag for roads with surface of small loose gravel
>   (surface=fine_gravel at 
> https://wiki.openstreetmap.org/wiki/Key:surface 
> <https://wiki.openstreetmap.org/wiki/Key:surface> has
>   "A multilayer pavement with a stone or gravel basis and a topmost 
> surface of firm, granular grit,
>   basalt or quartz," what anyway excluded cases of some gravel thrown 
> on earth/mud/drt )
> - surfaces matching old Wiki definitions are existing but really rare
> https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg 
> <https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg>
> ---------------------------------------------------------------------------------
> How one may know that Wiki definition mismatches actual use?
> assumption:
> Surfacing roads/paths/etc with track ballast-sized pieces of gravel is 
> extremely rare
> (though it happens)
> but in actual use surface=gravel is extremely common, far more common that
> old Wiki definition.
> So actual use clearly mismatches wiki definition.
> In addition, many people using surface=gravel were unaware of this 
> definition and unaware
> that such surfaces described in Wiki actually exist!
> See 
> https://lists.openstreetmap.org/pipermail/talk-au/2021-March/014445.html 
> <https://lists.openstreetmap.org/pipermail/talk-au/2021-March/014445.html>
> ---------------------------------------------------------------------------------
> So:
> It turns out that surface=gravel is for anything from "basically 
> compacted" to
> "basically track ballast".
> I recommend checking https://wiki.openstreetmap.org/wiki/Key:surface 
> <https://wiki.openstreetmap.org/wiki/Key:surface> in your language
> and updating definition to match actual tag use.
> It would be nice to find some good tagging for surfaces like
> https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg 
> <https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg>
> surface=track_ballast? surface=huge_gravel? surface=ballast?
> It would be also nice to find some more specific values for
> cases of small gravel thrown on mud/dirt/grass - as surface=gravel
> turns out to be very unspecific ability to tag this in detail would be 
> great
> (useful especially for cyclists)
> surface=small_gravel? Something else?
> ---------------------------------------------------------------------------------
> PS AAAARAGAGAHHA. Tag used over 1.5 million times had actual use and 
> OSM Wiki
> definition completely and utterly mismatched.
> And we need new tags and resurvey of 1 500 000 elements to have useful 
> information about surface.
> Right now surface=gravel may be both
> https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg 
> <https://wiki.openstreetmap.org/wiki/File:%C5%A0t%C4%9Brkov%C3%A1_cesta_u_Pl%C3%A1%C5%A1t%C3%ADku.jpg>
> and
> https://wiki.openstreetmap.org/wiki/File:Gravelsign2.jpg 
> <https://wiki.openstreetmap.org/wiki/File:Gravelsign2.jpg>
> that for many purposes are worth distinguishing.
> Arghhh.
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