Proposal talk:Clearance (construction)

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Use maxheight

maxheight=value exists and is well used. Why have another? --Lulu-Ann 12:32, 28 July 2009 (UTC)

Because there is a difference between the legal height restriction and the physically possible height, and there are uses for both types of information. --Tordanik 14:49, 28 July 2009 (UTC)

Placa17.gifBrasil A-37.svg

Because I want to mark the difference of these two signs. They can appear together or by themselves, with different values depending on lane and so on. --Skippern 23:39, 28 July 2009 (UTC)
Ah, I got it. I have never seen signs of the 2nd type here in Germany. Then it makes sense. You got my support. --Lulu-Ann 08:48, 29 July 2009 (UTC)
maxheight:physical=value is a useful proposal. But why do you want to change all already tagged maxheight=value items to maxheight:legal ? This seems not the OSM-way to me (change a widely used tag). Additionally the proposal of "clearance" in a way, that doesn't seem intuitive (usually used to express the space between the top of a vehicle and the border of an obstacle) doesn't make sense to me. Also, this tag clearance is exactly what maxheight:physical=value expresses more literally. --Dieterdreist 14:20, 1 August 2009 (UTC)
Adding a new maxheight:legal=* doesn't magically make all existing maxheight=* wrong. It only make the existing tagged roads a little less accurate (as maxheight doesn't say whether te limit is legal or physical), just as the original definition of the tag was. By allowing a maxheight:physical=* value to be tagged we can specify that a certain restriction is physical, and than it will be only logical to allow the same with maxheight:legal=*. By denying the legal version of the specification you will make many already tagged points wrong, as the old description suddenly will no longer fit. If you plan to visit all the places tagged with maxheight=* to verify whether they all are legal or to correct the physical ones to use maxheight:physical=*, than go ahead. As no height restriction (physical or legal) are put on the map for Brazil yet, than you do not need to come here to check, but as Alv have mentioned other places, than you might need to take a trip to Finland, and probably many other places. --Skippern 06:39, 6 August 2009 (UTC)

I think that the proposal should recommend always tagging the lowest applicable(*) maxheight in maxheight=*, even when the other is tagged as maxheight:legal or maxheight:physical; "conventional" users of the data are not interested in whether it's a physical limit or a legal limit, just that they can and may use the road section to get through. *) the minimum value of the pair: legal limit, highest physical limit on any single lane. Alv 11:08, 31 July 2009 (UTC)

why mix different proposals?

The section "for marine usage" seems a different proposal to me. Why is it mixed with maxheight:physical? Furthermore I don't see the point of changing best-tagging-practise to put the limit on another way as the one it applies to (the element under the bridge). --Dieterdreist 14:20, 1 August 2009 (UTC)


The proposal currently lacks a definition of the tag. --Tordanik 14:51, 28 July 2009 (UTC)

I just put in a definition. Could probably be better language, but it defines how I intend the tag to be used. --Skippern 23:48, 28 July 2009 (UTC)
It's a good definition overall, but it also needs to make clear whether - in case of a bridge crossing another road - the tag is supposed to be placed on the way under the bridge or on the bridge itself. (I'm hoping its the way under the bridge, btw.) --Tordanik 11:05, 30 July 2009 (UTC)
Actually it doesn't harm (and states more explicitly correct usage), but logically there is no question about this: a tag is always valid for the way it is applied to, and not to other ways around. --Dieterdreist 14:24, 1 August 2009 (UTC)
I also added a note about marine usage of the tag, since I would do these different. For the road I would place it on the road, but for marine usage I would locate it on the bridge. --Skippern 00:31, 31 July 2009 (UTC)
you shouldn't have done this. Make a new proposal for a new tag. --Dieterdreist 14:24, 1 August 2009 (UTC)
I can easilly put it up for separate votes if that makes it better for you. As I see it any clarification of the maxheight tag is part of this proposal, wether on land or at sea. --Skippern 01:59, 2 August 2009 (UTC)

Marine Usage

I guess this tag can be used to mark "free sailing clearance" under bridges. In that case I would tag it as clearance:sailing=* and put it on a section of bridge where the recomended sailing channel is, or the node marking the highest point. A bridge might have several sailing channels under it, and thus requiring this value on various points. --Skippern 01:48, 30 July 2009 (UTC)

I would like "marine" better than "sailing", as it applys for motor boats aswell. --Lulu-Ann 07:05, 1 September 2009 (UTC)
OK for me. --Skippern 23:16, 1 September 2009 (UTC)

Request for comments?

I did not find the email to talk mailing list with the rfc at the date given... --Lulu-Ann 10:20, 30 July 2009 (UTC)

[OSM-talk] [tagging] Feature Proposal - RFC - Clearance dated Thu Jul 30 02:45:13 BST 2009 (I follow Brazilian time, so it was about 22:45 on the 29th when I sent the mail) --Skippern 00:23, 31 July 2009 (UTC)

Tag name

(from the mailinglist)

There are lots of mappers that don't read the wiki pages at all and lots of mappers that only give them a cursory glance. So when introducing new tags it should be important that the tag itself is as descriptive as possible.

When comparing the words maxheight and clearance, it isn't obvious at a glance that they will be used for related but decidedly different things. The end effect of that will be that people are going to use one when they "should" have used the other.

When using maxheight / maxheight:physical / maxheight:legal the words themself already tell most of the definition.

maxheight -> for places where the difference is academic / for people who don't care about the difference

maxheight:physical -> the name says it all: whatever fits under it

maxheight:legal -> a legal restriction of some kind

--Cartinus 02:25, 31 July 2009 (UTC)

I can accept these names. This will not affect already tagged areas though it might be an idea to verify if already tagged maxheight are physical or legal in some areas. This can only be done by re-surveying such areas. --Skippern 09:18, 31 July 2009 (UTC)
As mappers do not usually run around with 4m folding rules, you can probably assume that most maxheight tags in the database over 2m are legal maxheights. --Lulu-Ann 10:13, 31 July 2009 (UTC)
I also expect that maxheight has been used almost exclusively for legal maxheights. It's the only thing that is signed in many countries, and people don't tend to measure the heights (afaik). In addition to this, the illustration on the wiki (traffic sign) strongly hints at legal meaning. --Tordanik 13:27, 31 July 2009 (UTC)
Unless a maxheight:physical sign is put up at that point... --Skippern 05:38, 1 August 2009 (UTC)
yes, but as far as this discussion went, those are only present in Brazil, and according to tagwatch so far there is no maxheight-tag at all in all of south-america. So there is no possible confusion about this. Just let maxheight be what it was and introduce maxheight:physical according to your local needs. --Dieterdreist 14:30, 1 August 2009 (UTC)
It seems that in the Netherlands there are also different signs for legal[1] and physical[2] maxheight. --Cartinus 19:32, 19 October 2009 (UTC)
Just a note: some Finnish maxheights now present, especially those because of tram power cables, are, as far as the legal text is concerned, only physical limits. There's plenty of sections with a legal limit at the start of a section leading only to a bridge and lane specific physical limits at the bridge conserned... You can't (normally) use a way regardless of whether it's a legal or physical limit; a maxheight=* anyway. It's still good to add the physical limit if it's known. Alv 23:27, 1 August 2009 (UTC)
In case of Finland, are these physical limits signed? If not than maxheight=* is probably the right tag, since it is not specified, while a signed one is maxheight:physical=*, this would be an argument for a maxheight:legal=* tag. In the saying Dieterdreist have tried to hijack this discussion by entering the word legal in the description of maxheight=* as well as a link to the unwritten page maxheight:physical=*. Shouldn't we complete this proposal process before starting to edit tag definitions? --Skippern 01:45, 2 August 2009 (UTC)
Yes, they're signposted with rectangular (horizontal) signs and the text and symbols are just like the yellow one at the start of this discussion page. The legal restriction signs are of the round type at the previous intersection. If a 4.3 meter high vehicle can't use a road, it should be in the tag maxheight, regardless of whether it's a legal or physical limit; If the physical limit is higher than the legal and someone wants to disregard the legal restriction, the physical limit can be informative and given as maxheight:physical. When there's either of :physical or :legal and plain maxheight, you can deduce which value the plain tag refers to. Alv 11:37, 2 August 2009 (UTC)
I today looked more intensively for these signs and noticed a place with both legal and physical "4.0 m" signs on the same pole (tram power cables being the most likely reason, although there's a pedestrian bridge, too.) I've also seen "4.2 legal restriction, applies to rightmost lane only" and then lane specific physical signs reading (from right to left) "4.2", "4.4", "4.4". There's generally a nation specific height limit that's guaranteed to pass unless signposted, likely 4.6 meters for most European countries. Alv 18:18, 2 August 2009 (UTC)
I totally agree with you Alv. For general route planning, you would not care if the restriction is physical or legal, and any maxheight* tag of lower value than your vehicles height should be avoided, the only concern is when your destination is located so that you have to enter a portion road with such lower value, and than knowing wether the restriction is physical or legal than you can decide wether you can ignore the restriction on the portion in question (I would never ignore a physical limit, though could ignore a legal limit with the right arguments in place). --Skippern 13:19, 2 August 2009 (UTC)

Special transports

Even in countries where they're not signposted, these values are usable for planning routes for special transports - hauling complete houses and such as they do sometimes. Measuring the free height under every cable or tree branch high above won't likely happen anytime soon (except by those executing such transports), but such information can be used to find the alternatives for them to check for correct heights. But to really describe unambiguously if any particular wider-than-one-lane load fits is probably not even feasible with common tagging. I'm thus in favor of maxheight:physical as describing the maximum physical height on any one of the lanes in their direction.

Idea: maybe some of those companies might be interested in providing their height measurements or gps logs, even when they now rely on commercial data for their planning. Alv 08:40, 31 July 2009 (UTC)

Width, length, weight...

Why is this discussion only about height? There are many similar cases possible where a road is signed as not supporting anything wider, longer or heavier than a certain value, but special transport would allow it. Only question would be of course how to measure it, and I see the same problem with the height. From the moment there's some curvature in the road surface, or when there are some non-straight shapes of the overhead obstruction. And even then one could possibly work around it, as I've seen special transports sometimes where they temporarily removed traffic signs or light poles. Therefore I'd say that the yellow diamond shaped signs as shown on the page aren't really the ultimate physical height restriction in many cases, and as such maxheight:physical is a bad name for this tag, as the actual ultimate maximum height may be even slightly higher depending on vehicle on obstruction. maxheight_clearance would be more appropriate, and would be possible to extend to maxwidth_clearance, maxlength_clearance etc. --Eimai 12:54, 2 August 2009 (UTC)

I see your point, but width=* can be used to tag a physical maxwidth whereas maxwidth=* is the legal restriction, for weight I do not know if there are any physical limits, if there are go ahead and suggest them. A physical length restriction, I cannot think of any other usages than for example ferries crossing rivers or fjords, most of which are long enough to handle all vehicles of normal legal maxlength (in Norway vehicles longer than 22m need special aproval and therefor lengths of ferry decks longer than 22 meter can safely be assumed as no physical length restriction). Also, if your transport is so high that you start planning a route where you can dismount blocking structures, than you would probably survey the route independently of signed restrictions, and make sure that any barriers can be altered for your transport to safely pass. If that is really what you are going to tag, than why not specify even more? maxheight:physical:removable=* and maxheight:physical:nonremovable=* comes in mind. --Skippern 13:11, 2 August 2009 (UTC)
Length restrictions can usually be found here in city centers where long vehicles wouldn't be able to turn around corners in narrow streets.
Width clearance isn't necessarily the road width either. Just put a curve in a narrow road and the maximum width would vary according to the length of the vehicle.
Weight restrictions are put in place for two reasons: getting trucks away from residential streets (where the restriction isn't physical: the road could well support much more). Other reasons are that the road just isn't built to handle more: a delicate road surface that would break if several heavy vehicles would pass it daily, or a bridge that can't support more. In the bridge case, the restriction is normally a very safe limit, and the actual limit would be higher. And if the bridge would be temporarily closed for other traffic, that vehicle could safely pass as well. In that case there would be a second limit, but don't ask me how to measure that :-)
Point is though, that from the moment you have to disobey a certain legal restriction, you need special permits for that, and then we're entering the special transport section, in which case you probably wouldn't make use of a simple route planner but rather go out measuring things on the ground together with some engineers.
And that's what I'm not sure about what that clearance sign really means here: does it say that vehicles up to that limit are always safe to pass (i.e. would it still have a few extra centimeters spare room?), so if the vehicle just fits according to the sign, should those engineers bother to measure it up? (because if a bridge is exactly four meters high, and your vehicle is exactly four meters high as well, you wouldn't fit, so would the clearance sign say four meters there or, say, 3.95m?).
Anyway, the point I was just making is that (a) ":physical" to denote that sign is a bad description of what is really tagged here, namely a sign, not the actual maximum possible height, and (b) denoting this as the physical limit opens the door to the other restrictions, which probably aren't signed, wouldn't be possible to measure or depend even more heavily on the shape of the vehicle. --Eimai 13:44, 2 August 2009 (UTC)
First of all I would like a clear definition for tags to be used on signs that are available in the area I map. When a sign saying the physical height says 4 meter, I would tag it as 4 meter. If I run up to it with a measuring tape, will I than see 4.00m or 4.15m? I really don't know. I do not plan to measure actual heights as that would steal too much time from actually getting roads on the map, but maybe that can become a hobby for when the map is getting completed?
As you say yourself, actual width restriction on a courved road might also be limited by the length, so tagging that will be a nightmare, but the physical width of the road should be easy to tag. If you think you might get in trouble with that, than you can take the grade of the courve from the map (implying that the node density gives good enough resolution on the courve) and calculate that yourself, than you would know if your 15m by 2.60m truck will fitt through that 2.80m wide courve. --Skippern 13:57, 2 August 2009 (UTC)

Round tunnels

crude sketch of a tunnel cross-section - not exactly to the measures refered on the left
Torbogen small.jpg

What is the physical maxheigt, if the lower border of a tunnel is rounded, so you can not pass with a width= 3m, height=3.5m, but you can pass with a width=2m, height=4m vehicle? --Lulu-Ann 15:00, 4 August 2009 (UTC)

This is an extreme case, though maybe not that uncommon as you would think... I came with this proposal to tag signed maxheight:physicals as they differ from maxheight:legal. If there is a sign indicating a physical height, than I suggest the value on the sign are put in. If not than I really have no idea how to do it. Maybe somebody else might be able to come up with a tagging scheme for this? --Skippern 01:39, 5 August 2009 (UTC)
For now I'd tag the height any as-wide-as-legally-possible road legal vehicle (2.55 meter wide in most countries) could be to fit under it. That still leaves undecided whether the value should be "for the lanes in drivers direction" or "anywhere in the tunnel" - I know a short tunnel where buses must drive on the center line, effectively blocking the tunnel for as long as they are in there, but there are normal lanes marked on the ground for both directions. A more descriptive scheme would make sense but defining it could start with either of
  • maxheight:physical:width:2=4; maxheight:physical:width:3=3.5 (and so on) or
  • maxheight:physical:lateral:division=1 (how far apart the next values are) + maxheight:physical:lateral=1.5;3.4;4;4.2;4;3.4;1.5
Alv 09:57, 5 August 2009 (UTC)