[talk-au] Aus tagging guidelines on highway surface tags

[Little Maps]

Hi folks, wondering if I can promote some discussion about the section of
the Aus tagging guidelines on adding surface tags on roads. The text
currently reads,

“For most types of highway=* tags you don't need to specify the
surface=paved key/value pair as this is assumed, however make sure you tag
the road surface when it isn't a paved road.”

This assumption is fine in large cities but is problematic in rural and
regional Australia. Can I suggest that it is replaced by something like the
following...

"Surface tags should be added to roads wherever possible, especially in
regional areas. This advice differs from that on the international
key:surface wiki page, which states that, 'there is normally an assumption
that the surface is surface=paved unless otherwise stated.' However this
assumption is not valid across regional Australia as: (1) most roads,
including many major roads, are unpaved, and (2) mapping intensity varies
greatly among regions. Many roads that do not have a surface tag may not
have been examined by mappers. Adding a surface tag will assist data users
and help mappers to further refine the regional road network."

Long rationale (not for posting on the oz tagging guidelines page)...

Surface tags have been added to relatively few rural roads in many regions.
Hence, the most prudent assumption is that the absence of a surface tag
means that the road surface has not received attention from mappers. A
default assumption that any road without a surface tag is actually paved is
most likely wrong.

Efficiency of mapping. Even if one has no interest in adding tags to paved
roads, the most efficient way to refine surface tags is to interrogate
untagged roads and tag them (e.g. by using an overpass query that
distinguishes untagged, paved and unpaved roads, and variants thereof.
Untagged roads can be inspected and tagged as appropriate.) However, if
mappers are advised to not tag paved roads, then every paved road that is
untagged needs to be re-examined each time this is attempted. This wastes a
lot of effort.

Some apps — especially routing and cycling apps (e.g. Osmand and Komoot) —
allow users to request paved or unpaved routes. Regardless of the (unknown)
assumptions that routers make about road surfaces when creating routes,
apps like Osmand present the data back to users. The suggested route may be
X% paved, Y% unpaved and Z% unknown. In many regions, Unknown is the
largest category. This doesn’t inspire confidence in the route or
underlying data.

Some assumptions about road surfaces can obviously be made. For example, a
primary road is more likely to be paved than an unclassified road. However,
most roads in rural areas are tertiary or unclassified. Some are paved,
many not; the ratio varies unpredictably across regions and it is
impossible to predict which roads are paved unless they are tagged.

Perhaps not surprisingly, the OSM wiki on key:surface gives conflicting
advice, beginning with the (European?) position that “there is normally an
assumption that the surface is surface=paved unless otherwise stated” and
later adding an (American?) view that, “There are no default values for
surface, it is generally considered as OK and desirable to tag it
explicitly for all roads.” The latter approach seems most appropriate in
regional Australia.

Adding surface tags to both paved and unpaved ways is the most efficient
method to: (1) allow data users to accurately predict road conditions (this
benefits users) and (2) improve the rate at which unpaved roads can be
reliably distinguished from paved roads (this helps future mappers). They
may be redundant on motorways, trunk and primary roads, but these make up a
tiny proportion of roads in regional Australia and can all be coded with a
minimum of effort.

Advising mappers to not add a meaningful tag would appear to be counter to
the goals of accurate tagging. Can we change our advice to encourage
mappers to add a surface tag wherever possible?

Thanks for your time, I'm keen to hear your thoughts. Best wishes, Ian
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