Improving the wiki definition of surface=clay

julcnx · February 17, 2026, 7:31am

Following the discussion about firmness=*, it became clear that part of the confusion comes from how some surface materials — especially clay — are currently documented on the wiki. The existing description of surface=clay focuses mostly on tennis courts and does not really help mappers recognise clay surfaces in other contexts such as paths or roads, nor does it explain how it relates to surface=dirt.

Since clay was repeatedly mentioned as an example where material alone does not determine usability (dry vs wet behaviour), it seems useful to clarify the definition so that it reflects observable properties while remaining accessible to non-experts. Clay-rich materials are also common on unpaved roads worldwide, particularly in rural areas where local soil is used as the surface, so expanding the documentation beyond sports facilities would better reflect real mapping situations.

It is also worth noting that surface=clay is already referenced in the description of surface=ground as a more specific value (“use a more precise value such as grass, clay, sand, dirt…”), which further supports documenting it more clearly for general mapping use, not only sports facilities.

I would suggest updating the description along these lines:

Surface composed of natural soil or manufactured material with a significant clay content. This is a more specific value than surface=dirt, since clay is a fine, cohesive earth material distinguished by its characteristic behaviour (often hard when dry, but sticky or slippery when wet). Clay surfaces are commonly compacted and can be firm in dry conditions, but may soften, rut, or become difficult for vehicles or bicycles to use when wet.

Identification does not require geological expertise; reasonable field observation or local knowledge is sufficient. When the material cannot be distinguished confidently, surface=dirt or surface=ground should be preferred.

Most common on tennis courts. Sometimes used for other sports: soccer, athletic tracks, boules, etc. May also occur on paths and roads.

A tennis court surface may be made of manufactured clay (crushed stone, brick, shale, or other unbound mineral aggregate).

The goal is mainly to:

clarify that surface=clay is a more specific subtype of surface=dirt
document recognisable behaviour (hard when dry, sticky/slippery when wet)
make it clear that geological certainty is not required
expand beyond sports use cases to real-world unpaved infrastructure

This should also help make the distinction between material (surface=*) and condition (firmness=*) clearer.

Feedback welcome.

rhhs · February 17, 2026, 4:16pm

A very good start!

Some suggestions:

add a link to Clay - Wikipedia where its mechanical properties are also mentioned
be more definite about its mechanical properties (avoid “often”, “may”; “can be” → “are”); mention that vehicles can get stuck in wet clay
mention that a very dry clay road surface that is frequently used will turn to dust
when wet, the smoothness of such a surface is in a state of flux

I hope it will soon be most commonly used on paths and roads, and that tennis courts will become a niche application, so maybe we should anticipate that in the wiki text?

Should we add advice for data consumers, such as that in humid climates, this surface should be avoided esp. for cyclists, but can be suitable surfaces for wheeled vehicles in dry climates?

We could also mention other non-paved surfaces and what the difference is with surface=clay. For instance “For more coarse-grained material that is soft when dry but becomes more firm when humid, consider surface=sand. If the surface is almost always wet, consider surface=mud”

Duja · February 17, 2026, 7:11pm

I’m exactly of the opposite opinion: use the tag only for tennis courts, where the word clay has a precise meaning, and don’t use it for other things at all.

According to taginfo, it is currently used on 35,000 leisure=pitch grounds (vast majority being sport=tennis), and mere 2,275 highways.

If you start promoting its use for highways, it is bound to become vague and skunked sooner than later. It is difficult to distinguish from ground or mud, even by on-the-ground survey, let alone armchair mapping. Google search for “clay path” shows top results for baked-clay pavers, which should be tagged as surface=paving_stones I suppose. I think we would do much better with less possible surface values, since what we currently have is Bloody Mess™.

aighes · February 17, 2026, 11:20pm

Maybe rather google for clay road surface and check the images

Apparently folks also manages to differentiate between ground, earth and sand. Where do you see the additional difficulty to differentiate clay as well. I would be pretty certain folks in areas where clay is more common than earth or sand will be able to do so. If you can’t, stick to ground, that’s fine as well.

ftrebien · February 17, 2026, 11:33pm

Kind of. The wiki says that ground, much like unpaved, is not specific and asks for a more specific value instead. It says that earth is equivalent to dirt, so we end up with just dirt (with organic material) vs sand (without). The difference is most markedly colour, but organic material also tends to bind the particles more, so dirt/earth is not as loose as sand usually is. Near the coast I sometimes have a hard time distinguishing the two in certain mixes of the two types of soil.

I guess I would have the same kind of difficulty distinguishing between clay and dirt / earth, especially when there’s a mix of the two (what our local geologists may refer to a clay-like or “clay-y” soil).

Not saying I approve or reject the proposal, I’m actually undecided.

aighes · February 17, 2026, 11:41pm

dirt or earth contains ~ 10% actual clay. What would be your suggestion if that percentage is much higher? Since the properties of that “soil” is totally different. Especially when it’s getting wet.

Hungerburg · February 17, 2026, 11:49pm

Just happened over this Wikipedia article Locker-, Halbfest- und Festgestein – Wikipedia - it is in German only, but it is very clear that clay belongs to the loose set, like pebblestone, shingle, scree, gravel, fine gravel, sand &c. It is just composed of the finest particles in the set. No idea how to translate that into “firmness®”, which is a registered trademark in OSM, from what I learned, even though nobody really able to tell, what it is about.

ftrebien · February 18, 2026, 1:06am

There seems to be a method to identify how much clay a particular sample of soil is made of. Maybe it would only be used in case of a mapping dispute.

The part that is a bit harder to evaluate for me is this one:

I assumed that when this does not happen, it is usually with surface=compacted or surface=fine_gravel, so maybe part of what I mapped as surface=dirt may actually be clay under the proposed definition. Still, when I see something like this, I’m not sure if that’s clay or mud.

This video seems informative. It points at signs of clay after it has dried out (it forms a characteristic crackled texture), including in roads in wet climates (it creates specific track marks that hold their shape), and then in dry climates (more difficult to assess).

Wikipedia mentions clay on gravel road as characteristic in laterite and murram roads. I’ve never seen these names being used to describe any roads near where I live, so I probably lack the experience to identify them. Could it be that surface=laterite better represents what you guys have in mind? Or maybe laterite is just one type of clay?

I don’t know for sure, but one idea could be to base that threshold on this USDA chart of soil textures (from Wikipedia’s article on loam):

At first glance, it looks like clay could mean a soil composition of at least about 40-50% clay.

Finally, clay may indeed be the default type of soil in some regions. In the USDA taxonomy, for example, in Southeast Asia (where the OP maps if I remember correctly), ultisols are common, which are very high in the clay component. In my area (based on that article), the typical soils are alfisols (that have clay under the topsoil, so clay only surfaces when the subsoil has been exposed) and mollisols (which are not rich in clay). This probably explains why I don’t detect clay roads as easily as some of the other participants here.

osmuser63783 · February 18, 2026, 7:03am

“Clay” on a tennis court is a completely different surface from the picture in the top post (a dirt road in an area of the world with a high clay content in the soil, which is rock solid when dry and turns into mud in the rain).

It would be unfortunate to use the same tag for both surfaces..

aighes · February 18, 2026, 11:32pm

Apparently in English both is called the same. So what would be your suggestion?

In literally translated from German you would call the pitch material ash (Asche)* and the road material clay (Lehm).

*) as in “Aschenbahn” for running track and “Aschenplatz” for soccer/tennis pitch.

julcnx · February 19, 2026, 3:16am

surface=clay is already documented as a valid value for roads, and it’s already in use for highways today. So this isn’t a new proposal to expand scope, it’s mainly about improving the documentation so people understand when to use it.

If the wiki had been clearer, I would already have used it for thousands of tracks and trails I know where clay is the dominant material. This surface is very common in many parts of the world, so having a way to describe it is useful.

Some concerns may come from limited exposure to regions where clay roads are widespread and often recognizable in practice, even if there are mixed cases where distinction is harder. That’s why clearer examples and documentation (e.g., vs ground/dirt/mud) would help.

I understand the concern, but this already happens with many surface=* values on OpenStreetMap (e.g., metal or wood used in different contexts).

Natural clay roads and clay sports surfaces share the same base material, so creating a new tag would only fragment usage and go against the usual approach of tagging the material itself.

julcnx · February 19, 2026, 3:34am

Thanks, this is a thoughtful analysis.

I don’t think a percentage threshold is necessary for mapping on OpenStreetMap. Like most surface=* values, the goal is to describe the dominant observable material and its behavior, not a lab composition.

One key distinction is material vs condition: clay is the substrate, while mud is a temporary wet state. A clay road when wet may look like mud, but the underlying material is still clay.

Laterite can be clay-rich, but it’s a specific geological type; many clay roads wouldn’t be described that way.

Regional familiarity also matters, where clay soils are common, the material and its behavior are often recognizable. Clearer documentation with observable indicators would help, for example:

Color: often red, orange, yellow, or brown (iron-rich soils).
Very fine texture: little to no visible sand or gravel; powdery dust when dry.
Hard when dry: can become very firm and smooth after compaction or grading.
Plastic/sticky when wet: material sticks to shoes/tires and becomes very slippery.
Rutting behavior: ruts form with relatively smooth walls and hold their shape after drying.
Desiccation cracks: polygonal cracking pattern when dried after saturation.
Water behavior: poor drainage; puddles persist longer than on gravel or sandy soils.
Dust when dry: fine, cohesive dust rather than loose grains.
Cut banks/exposed edges: cohesive vertical faces rather than crumbly material.

Mateusz_Konieczny · February 19, 2026, 7:56am

but conflict/difference and low use for highway= can be mentioned at the page

osmuser63783 · February 21, 2026, 10:37pm

surface=clay_soil?

No, one is a highly specialised, engineered surface and one is bare soil. Fragmenting usage would be the goal because the two behave so differently.

julcnx · February 22, 2026, 3:25am

You’re right that the preparation differs, but in OSM we normally tag the material and rely on context for usage. Since both are clay-based and already clearly separated by tags like leisure=pitch vs highway=*, introducing clay_soil would mostly duplicate surface=clay rather than solve a real ambiguity.

Creating a new tag is operationally heavier (proposal + migration). Documenting existing surface=clay usage is pragmatically simpler and delivers value faster.

Hungerburg · February 22, 2026, 11:21pm

If only documentation of tag surface=fine_gravel would honour this! It as of recently documents how it is used on leisure=playground or leisure=outdoor_seating; while most of its mappings are for highway=*, where in the newly documented way, it can be found only occasionally on a few footways perhaps, but certainly not on so many tracks, as where it has been mapped (before documentation switched.)

julcnx · March 2, 2026, 9:06am

Thanks everyone for the input. Based on the discussion, here’s what I’m planning to update on the wiki page, followed by a draft of the proposed content.

Changes from the current version:

Following rhhs’s suggestions: stronger language on mechanical behaviour (vehicles can get stuck, not just “may become difficult”), mention of dust on heavily used dry surfaces, a note that any smoothness=* value reflects conditions at survey time, and cross-references to surface=dirt, surface=mud, and surface=sand.
Following osmuser63783’s concern: an explicit note on the two contexts (manufactured sports surface vs natural road soil) so the naming overlap is visible rather than hidden, without splitting the tag, since leisure=pitch vs highway=* already provides the disambiguation.
Following Mateusz_Konieczny’s suggestion: current highway usage (over 2,000 ways as of early 2026) is mentioned, and the text recommends falling back to surface=dirt or surface=ground when identification is uncertain.

The goal, as Hungerburg’s fine_gravel example reminds us, is to document actual usage, not prescribe new behaviour. The highway use already exists; the current wiki just ignores it.

If there are no objections I’ll edit the page in the next few days.

Proposed wiki content:

surface=clay describes a surface where clay is the dominant material, either natural soil with a high clay content or a manufactured sports surface made of crushed stone, brick, shale or similar unbound mineral aggregate.

Note on terminology: On sports pitches, “clay” is a conventional name for a manufactured crushed-stone surface. On roads and paths, it refers to natural clay-rich soil. Both are mapped as surface=clay; the feature type (leisure=pitch vs highway=*) provides the context.

A key characteristic distinguishing surface=clay from surface=dirt is wet-weather behaviour: clay becomes sticky and plastic when wet, making roads difficult or impassable for vehicles.

Sports use

Most common on tennis courts (leisure=pitch + sport=tennis). Also used on soccer pitches, boules courts and athletic tracks.

Roads and paths

Currently used on over 2,000 highway=* features globally (as of early 2026), compared to over 35,000 leisure=pitch features.

Clay-rich soils, including laterite, are the dominant unpaved road surface in many tropical and subtropical regions (Southeast Asia, Sub-Saharan Africa, South America). Use surface=clay when clay is clearly the dominant material and the surface behaves accordingly. When uncertain, prefer surface=dirt.

Mechanical behaviour

Clay does not drain quickly, the same road can be firm when dry, impassable when wet. Routers can combine this tag with weather data to assess passability.

When dry: firm to very firm; can become hard and smooth after compaction or grading; produces fine dust on heavily used surfaces; generally passable by most vehicles and bicycles.
When wet: sticky and plastic; vehicles can get stuck; bicycles very difficult or impossible to ride; extremely slippery. Note that any smoothness=* value reflects conditions at survey time, dry-season and wet-season conditions can differ dramatically.

Seasonal note: in climates with a distinct dry season (e.g. monsoon regions), clay roads may be reliably firm for months at a time. For coarser-grained material that drains more quickly, consider surface=sand. If the surface is almost always wet, consider surface=mud.

Field identification

Geological expertise is not required. Useful indicators:

Colour: often red, orange, yellow or brown (iron oxide); pale grey or white in some regions
Texture: very fine and smooth; no visible sand grains or gravel; powdery dust when dry
Wet behaviour: sticky and plastic; sticks to shoes and tyres; smooth-walled ruts that hold their shape after drying
Drying pattern: characteristic polygonal cracking after saturation
Drainage: puddles persist longer than on sand or gravel
Cut banks: cohesive vertical faces rather than crumbling material

When the material cannot be confidently identified, use surface=dirt instead.