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As advised in the wiki page documenting this tag, paths should always be tagged with surface.

However I don't see any specific values to descrive paths with medium-sized rocks and stones. There is the value pebblestone, but I don't think it is appropriate for a surface like this:

alt text

These stones are neither small nor rounded, so I woudn't call them pebbles! In my opinion, gravel wouldn't be appropriate, either.

Also, it seems we miss a value for a path full of boulders (large rocks), like this:

alt text

Finally, how would you tag a path with a rocky surface? For example:

alt text

What values would be appropriate for these 3 surface types?

asked 23 Jan '12, 17:15

solitone's gravatar image

accept rate: 0%

For the third type, I suggest access=crazy_biker ;-)

(24 Jan '12, 08:42) NicolasDumoulin

Well, you can use any tags you like, but obviously it makes sense to use the same values as other mappers, so taginfo can be useful. From looking at the popular values for surface, I'd say "rocky" is the most popular option that fits the bill. If that isn't granular enough perhaps you might find Mtb:scale helpful?

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answered 23 Jan '12, 19:12

SomeoneElse's gravatar image

SomeoneElse ♦
accept rate: 16%

I'd say rocky is appropriate for the 3rd and perhaps the 2nd example. However it's not for the first. Maybe I can propose a new value?

As for mtb:scale I think it describes different features of the pah, not necessarily related to its surface.

(23 Jan '12, 20:11) solitone

I don't think that it's necessarily helpful to "propose" a new value - I'd have a look at what other people are using for the tag, and if nothing fits, pick something better. Although the any tags you like page says "please document" in this case I suspect that added lots more entries to key:surface isn't going to help as that already has "User Defined" and a link to tagwatch at the bottom.

(23 Jan '12, 20:20) SomeoneElse ♦

The problem is I'm not a native speaker and so I'm unsure about the right terms.

I see that fragments of rock are classified, according to size, in the following categories:

  • Boulder: a detached rock mass, somewhat rounded or otherwise modified by abrasion in transport and larger than a cobble with a minimum size of 256 mm;
  • Cobble: a rock fragment larger than a pebble and smaller than a boulder, rounded by the action of water wind or glacial ice. It is between 64 and 256 mm;
  • Pebble: a rock fragment larger than a sand grain or granule and smaller than a cobble, which has been rounded by the action of water wind or glacial ice. It is between 4 mm and 64 mm.

But they all have in common a rounded shape caused by erosion, while in the mountains you may well have rock fragments which are much younger, and so not rounded at all!

Do you think these terms could determine confusion? See for instance my original first picture--would you call those fragments "cobbles"?

Are there more generic terms, not necessarily related to fragments modified by abrasion?

Otherwise, values like "pebbles", "cobbles", and "boulders" (in plural) would be ok for key "surface"?

Thanks again!

(24 Jan '12, 16:19) solitone

I guess that you're talking about something like this scale? The "Wentworth Scale" words are borrowed from common English words, but when used in a geological context refer to particular sizes only. In everyday English, "Boulder" might be interpreted as something larger than the Wentworth Scale value. Also, "cobble" is most frequently used in English to describe stones of about that size used to surface roads, and might be confusing if used to describe a rock-strewn path.

After looking at the taginfo values I'd probably use "mud", "sand", "gravel" and "rocky" in order of grain size. Historically (before looking at taginfo) I've used "rocks" for "larger than gravel" but taginfo definitely suggests "rocky" as the more popular options. I wouldn't try and subdivide surface grain size too much as people won't necessarily understand what you're trying to say. You can of course still used other words like maybe "boulders" if you think that's the best way of getting across what you need to.

In answer to the second part of your question, although the phi scale / Wentworth Class are often used to describe the size of grains carried by rivers etc., I don't think that they exclusively refer to stuff modified by abrasion (from what I can remember from geology classes at college a VERY long time ago, anyway).

(24 Jan '12, 18:20) SomeoneElse ♦

Yes, thas is the scale I was referring to.

So you'd use rocky also for the path in the first picture? My feeling is that there is too difference between gravel and rocky, and I wonder whether cobbles would be fair.

(25 Jan '12, 06:48) solitone

Personnally, I wouldn't use "cobbles" because that's often used to describe a road surface (e.g. here)

(25 Jan '12, 14:43) SomeoneElse ♦

Yes, I do agree. BTW, surface=cobblestone is used for paved ways.

But still, I'd like to have 2 categories for rocky paths:

  • One for paths with large rocks (picture 2 in my 1st post) or for paths with a surface of compact (not fragmented) rock (picture 3). Perhaps "rocky" would be the most suitable choice for this category?
  • A second one for paths with smaller rocks (around 5-25 cm), where "gravel" doesn't seem a satisfactory option. But "boulders" wouldn't be appropriate either, since it would imply very big rocks.

What might be used, instead of "cobbles"?

(25 Jan '12, 16:53) solitone

Eventually I made up my mind and I'll go with your suggestion--use just "gravel" and "rocky". Therefore, all the 3 examples above would be tagged with "rocky".

I think I can simpy add key "smoothness" to give an idea about grain size.

Thanks for the input!

(27 Jan '12, 08:42) solitone
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question asked: 23 Jan '12, 17:15

question was seen: 8,281 times

last updated: 27 Jan '12, 08:42

NOTICE: is no longer in use from 1st March 2024. Please use the OpenStreetMap Community Forum