[talk-au] Tracks flagged as missing from government data

Andrew Harvey andrew at alantgeo.com.au
Thu Aug 19 14:17:41 UTC 2021


TL;DR;
1. no access tags don't mean access=yes (public) it just means access hasn't been set.
2. access=unknown is probably better than access=private + fixme:access=actually unknown access but I don't want routers to use this for now

On Thu, 19 Aug 2021, at 9:58 PM, Joseph Guillaume wrote:
> I'd suggest the problem is with routers assuming that a missing access tag should be interpreted as access=yes.
> 
> If a common understanding was reached that a missing access tag on a highway=track is *assumed* to imply access=no until proven otherwise, then it seems the problem goes away?
> 
> When armchair mapping, I don't want to have to tag access=private, fixme="access to be checked" just for the router to have that behaviour.

Exactly.

Replying to both your points Ian and Joseph, put it this way, if I was driving and asked for directions, I'd expect the router to ignore highway=track ways completely (regardless of access restrictions set) unless there was no other way to get there except take the track.

If the only way to route there is via the track, then I'd expect the directions to include it even if access=private, but with a warning note. If no access or motor_vehicle tag was set, then I'd expect a warning that the route may be on private or inaccessible tracks.

Irrespective of the MapRoulette challenges, just looking at OSM today in my view the most sensible defaults for highway=track without any access tags set would be motor_vehicle=private + bicycle=yes + foot=yes but with an added note that it is actually unknown.

Another anecdote, sometimes I map building footprints I can see from imagery alone, I don't know the type of the building, or building:levels so I just tag it as building=yes (ie. building of known type). Sometime later somebody walks past with StreetComplete and it asks them what type of building it is and how many stories, and sets the tags. Without me first adding the generic building way, the other mapper never would have completed it by choosing the building type.

I wasn't trying to give a definitive exhaustive list of what is a highway=track so yes tracks can be used by people for recreation too.

I see your point Ian, you don't want to see these roads/tracks mapped unless they have complete access restrictions set, and I'm trying to argue that it's better to have them partially mapped even incompletely so that other mappers and surveyors can complete it later. Either without the access tag or access=unknown https://wiki.openstreetmap.org/wiki/Tag:access=unknown, which is much preferable in my opinion than setting access=private just because you don't know access.

It means data consumers can treat it as truly unknown and mappers can be prompted to survey it (this happens with StreetComplete for amenity=parking).

access=private should be reserved when you really know it's private access, if you're not sure best to set access=unknown and then once you know explicitly set access=yes (public access) or access=private (no public access) etc.

Perhaps the problem is with how some routers assume a lack of access on tracks mean they are public, so the issue is with those data consumers not with OSM's data.

> IMO, the key problem with the Aus road network in OSM (except perhaps in Qld and N Aus) is not that it has too few roads, but that it has too many. We already have a big problem in Victoria with heaps of “paper roads” and private roads that are tagged as open and public. It’ll take years to remove those we already have. I dread to think that 1000s of new ones will continue to be drip fed onto the map from a Map Roulette challenge.

I made it clear through the MapRoulette instructions that these should be checked with another source, so "paper roads" which can't be seen on imagery shouldn't be added though this process.

> It may seem crude but, given the immense size of these datasets, I actually view this “soft launch” as you call it, as a “TIGER import by stealth”. 😉 yeah, I know it’s not a formal import but if an organised mapping team took up the challenge and added 200,000 roads without including suitable access tags, local mappers will be cursing this initiative for decades.

Yep which is why this feedback here is important, but critically any organised mapping team would need to flag there intentions before steam rolling through it.

> I can’t believe that you think you are erring on the side of caution by omitting suggested government access data. In what way can this be a prudent approach? Even if 10% are wrong, the rest are going to be helpful. Hence in contrast to your approach, I suggest “we err on the side of caution and include the government tag”. 
>
> So, please add as many tags as possible that indicate likely access and trust the gov data for now. That’s a better default position than no data. Perhaps add a note tag saying something like “access data is from Vic gov and may need to be updated after ground truthing” or the like.

Again the lack of the access tag doesn't imply access=yes, it just means it's not set yet. Same comments as above about no access tag being better than access=private + fixme=actually I don't know access but just set private. Borderline "tagging for the buggy data consumer"?

> [As an aside, your code says that “4: 'motor_vehicle=no', // permanently closed” but I assume you mean no access to vehicles. This does not forbid  pedestrian access].

>From memory the Vicmap documentation said a value of 4 meant permanently closed, I took the assumption that means permanently closed to vehicles, and hence selected the suggested tag as motor_vehicle=no and not the wider access=no.

> Any suggestion that 100-200,000 private roads could be added to OSM and that problems can be sorted out later is a joke. Who is going to do this? Every hour spent fixing up errors could be better spent on building the map. It would take decades to weed about these problems. There aren’t enough local mappers in rural regions as it is. We don’t want future mappers to be burdened with the task of fixing tags that could have been 99% avoided had a couple of lines of code been added initially.

I don't think these MapRoulette challenges are adding problems, the intent is to add tracks that exist in both data and imagery. It's not the goal to complete the access restrictions tagging of these tracks.

> Filter the Vic and SA datasets using land tenure (public vs private) in whatever way is practical, and separate both into two separate Challenges, one titled “Priority challenge: unmapped tracks on public land” and one titled “unmapped roads on private land” or similar. The filtering may not be perfect but it should separate most groups reasonably well.
> 
> The public land dataset will be a great resource and will potentially be used by many people. The private roads are of less immediate value to data users (and potentially of negative value if private access is not tagged), hence the need to keep them separate.
> 
> Tracks on public land (most of which will be in State Forests and parks) can be added as is, including any access tags indicated in the gov datasets plus a caveat note tag on the need to ground check access. The basic assumption is that access is public unless it is tagged otherwise through your data wrangling.
> 
> For all tracks that are not on public land, instead of highway=track, why not set the default tags as highway=service, access=private (and maybe even service=driveway) with a note that says something like “private access is assumed based on mapped land tenure and may prove inappropriate after ground truthing”  or similar.

The crown land tenure dataset I found from Vicmap didn't seem to fully cover public land so I need to work out the best way to do it. But overall yes, I think we could split the challenges into on public land and not.

Different data consumers of OSM have different priorities, and so do mappers so that's all fine, some data consumers like emergency services (and the people who called them) might find private roads just as important to have as public roads.

> It’s obvious from the Map Roulette map that the majority of the roads on private land will be short driveways, so highway=service will be a better default tag than track for most.

I though I mentioned in the challenge instructions that mappers might need to evaluate the highway classification and if unsure they can either be highway=road or skipped.

> Again, sorry for the long message. I’ve spent the best part of the last year working on rural roads, including days of work adding access tags on private roads. We need to move on from the standard OSM position of “more is better” to a recognition that, outside of forests and reserves, we have a great map of public roads in SE Aus. Geometry and tags can always be improved but we definitely don’t need heaps more roads added to the public network (which is what effectively happens if private roads don’t have an access tag). The suggestion that 100s of 1000s of private roads can be dumped on without adding private access tags will set us backwards, not forward, by years.

> So, if you ignore everything else, P-L-E-A-S-E use “access=private” as the default tag for every driveway/service road/track that is not clearly on public land. If in doubt, tag it private and add a caveat note to inform future mappers how the access tag was derived. Thanks again, Ian

I would argue that private should be the assumed default already for a highway=service+service=driveway and for highway=track without an access tag. And the lack of the access tag is already a note for future mappers to request that it be set explicitly. So I think Joseph makes a good point in his last paragraph.

If we can come to some agreement about this we can update https://wiki.openstreetmap.org/wiki/OSM_tags_for_routing/Access_restrictions#Australia. There is also https://wiki.openstreetmap.org/wiki/Proposed_features/Defaults for encoding these assumed defaults into the OSM data itself. Though I'd still argue that explicitly tagging access is always preferred over assuming a default.



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