Re: [Tagging] length=

[Mike Thompson]

Here I diverge. If the hedgerow is an improtant part of the landscape
then I'd map it .. even if it is not at your required level of
'accuracy'. Reason: it is the relationship between the objects on the
map that is important rather than the absolute accuracy of any one
object.
+1

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Re: [Tagging] length=

[Warin]

On 31/01/2015 1:47 AM, SomeoneElse wrote:

On 30/01/2015 14:12, St Niklaas wrote:

> From: François Lacombe 
>
> Since OSM editing tools aren't AutoCAD you can't be 100% precise on the
> geometry.


Exactly so.



Francois if you’re using JOSM you’re be able to work up till 0,06 - 
0,04 =0,02 m accuracy




No.  Unless you can measure accuracy on the ground to that level of 
precision, you simply can't*.


And you should not use anything, AutoCAD included, to enter data at a 
greater precision (accuracy) than what is available. Unfortunately the 
data is stored as being absolutely accurate and precise so if the end 
user is so inclined they could try to use it as such. They would quickly 
be disappointed with OSM! OSM is not a source of absolute accuracy.



On 31/01/2015 1:47 AM, SomeoneElse wrote:
A good rule of thumb for OSM is "don't try and map more accurately 
than your sources".  If you only have aerial imagery, or only have a 
few GPS traces, don't try and map every last hedgerow, since you 
simply don't know how accurate the sources that you're working from 
are.  Instead, go out and collect more data.  For example, once you 
know how aerial imagery compares to lots of GPS traces (and vice-versa 
- GPS traces can have a systematic offset due to terrain and even 
"what side of a road people are allowed to walk down") you're in a 
much better position to contribute.


Here I diverge. If the hedgerow is an improtant part of the landscape 
then I'd map it .. even if it is not at your required level of 
'accuracy'. Reason: it is the relationship between the objects on the 
map that is important rather than the absolute accuracy of any one 
object. If the relationship between the objects is representative of 
what is on the ground then it conveys usefull information to the end 
user and should be mapped.


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Re: [Tagging] length=

[SomeoneElse]

On 30/01/2015 14:12, St Niklaas wrote:

> From: François Lacombe 
>
> Since OSM editing tools aren't AutoCAD you can't be 100% precise on the
> geometry.


Exactly so.



Francois if you’re using JOSM you’re be able to work up till 0,06 - 
0,04 =0,02 m accuracy




No.  Unless you can measure accuracy on the ground to that level of 
precision, you simply can't*.  The imagery sources availble to OSM 
aren't that accurate, and even the aggregated traces of many, many 
consumer GPS units won't hit that accuracy.  0.06m is a tiny amount 
compared to the amount that natural processes can cause a particular 
"location" to move**.


A good rule of thumb for OSM is "don't try and map more accurately than 
your sources".  If you only have aerial imagery, or only have a few GPS 
traces, don't try and map every last hedgerow, since you simply don't 
know how accurate the sources that you're working from are.  Instead, go 
out and collect more data.  For example, once you know how aerial 
imagery compares to lots of GPS traces (and vice-versa - GPS traces can 
have a systematic offset due to terrain and even "what side of a road 
people are allowed to walk down") you're in a much better position to 
contribute.


Cheers,

Andy

PS:  Please don't reply to digest posts largely unedited.  The very 
first line in your reply mail was, quoted, "When replying, please edit 
your Subject line so it is more specific".  Even if you do edit a digest 
post down it'll still destroy the threading of the mailing list.  Many, 
many people rely on this to make the process of reading lists such as 
"tagging" less of a chore (it's much easier to "mark a thread as read" 
than it is to go through every post about semicolons, patron saints or 
pipelines one by one).  If you don't have a mail reader than can handle 
non-digest mail, you probably need to change the way that you read mail.


* The exception is where you're triangulating from points measured with 
an accuracy beyond what consumer kit can provide.  I'm looking forward 
to seeing "triangulation via theodolite from X" in an OSM source tag :)


** http://www.bbc.co.uk/news/science-environment-12732335
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Re: [Tagging] length=

[François Lacombe]

I understand your point.

Nevertheless, using a compass + indicating direction=* will be more precise
and exact than drawing the box according to aerial pictures (often
according where you think the box is)
Same for length=* or any other physical properties.

I've no problem with hardcore micromapping. OSM is a meaningful project for
that.
But sometimes, the precision and scale issues force us tu use simpler
primitives.

*François Lacombe*

fl dot infosreseaux At gmail dot com
www.infos-reseaux.com
@InfosReseaux 

2015-01-28 10:22 GMT+01:00 Martin Koppenhoefer :

>
> 2015-01-28 9:13 GMT+01:00 François Lacombe :
>
>> Some small features can actually be summarized as nodes when drawing
>> their shape sounds irrelevant regarding the cluttering it introduces.
>>
>> I'm sorry that was trivial for me.
>> I won't draw a circle for a 5cm diameter pole and so on...
>>
>>
>>
>
>
> while for a pole (especially a quite small one) geometry does indeed not
> give you a great gain, it does provide additional information for other
> small shapes even if they are on the limit of what we can represent, e.g. a
> telephone booth, a bench, a street cabinet, etc., because the geometry
> gives you orientation information and also other positioning information
> relative to close by objects (e.g. adjacent or with a gap, in line or
> shifted) and allows for further details like the position of the
> door/entrance, etc.
>
> I admit this is hardcore micromapping and mostly I don't do it like this
> myself but use a node, just wanted to point out that there are situations
> where more detailed geometry does make sense also in the case of tiny
> objects.
>
> cheers,
> Martin
>
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Re: [Tagging] length=

[Martin Koppenhoefer]

2015-01-28 9:13 GMT+01:00 François Lacombe :

> Some small features can actually be summarized as nodes when drawing their
> shape sounds irrelevant regarding the cluttering it introduces.
>
> I'm sorry that was trivial for me.
> I won't draw a circle for a 5cm diameter pole and so on...
>
>
>


while for a pole (especially a quite small one) geometry does indeed not
give you a great gain, it does provide additional information for other
small shapes even if they are on the limit of what we can represent, e.g. a
telephone booth, a bench, a street cabinet, etc., because the geometry
gives you orientation information and also other positioning information
relative to close by objects (e.g. adjacent or with a gap, in line or
shifted) and allows for further details like the position of the
door/entrance, etc.

I admit this is hardcore micromapping and mostly I don't do it like this
myself but use a node, just wanted to point out that there are situations
where more detailed geometry does make sense also in the case of tiny
objects.

cheers,
Martin
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Re: [Tagging] length=

[Martin Koppenhoefer]

2015-01-27 17:13 GMT+01:00 Mike Thompson :

> > The way in OSM is only a (sometimes not precise) drawing of an existing
> feature and can be different from the reality.
>
> How precise is the value of the "length" tag? From what is the value
> derived?
>



to make sense on ways, I'd expect it to represent either a measurement in
the real world (e.g. with tape) or the indication you can get from a sign
(e.g. length of a bridge). Measuring e.g. in JOSM or GIS from the
cartography or from imagery or calculating it from the coords does not make
sense (but I doubt people do it like this).

cheers,
Martin
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Re: [Tagging] length=

[François Lacombe]

2015-01-28 9:14 GMT+01:00 Martin Vonwald :

>  I don't see why anyone would want to do it that way instead of simply
> drawing a box, but I accept the fact, that some users do, so it's fine for
> me.
>

It's more precise to use tools like compass or meters to get the cabinet's
(or any other small feature) dimensions and put in tags value instead of
drawing a box.
Drawing a box isn't really precise and there will be a kind of data loss.

It's only a scale matter and such thought only apply to prismatic shapes.
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Re: [Tagging] length=

[Martin Vonwald]

2015-01-28 8:58 GMT+01:00 Florian Lohoff :

> On Tue, Jan 27, 2015 at 04:18:57PM +0100, Martin Vonwald wrote:
> > 2015-01-27 16:13 GMT+01:00 François Lacombe :
> >
> > > I personally recommend to use the length key while mapping street
> cabinets
> > > as nodes.
> > > 
> >
> > On a node it makes perfect sense. At least as long as it is not
> > possible/wanted/allowed to provide the geometry.
>
> Does it ? I cant think of any application where this makes sense.
> A node does not have an orientation so why can it have a length?
>
> If it has a length it does not make sense to use a node.
>

Read my second sentence again. Some mappers do not want to draw geometry
for some small feature. See e.g. man_made=street_cabinet. There you have a
length and width. Together with the key direction one can determine the
geometry. I don't see why anyone would want to do it that way instead of
simply drawing a box, but I accept the fact, that some users do, so it's
fine for me.

Best regards,
Martin
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Re: [Tagging] length=

[François Lacombe]

2015-01-28 8:58 GMT+01:00 Florian Lohoff :

>
> Does it ? I cant think of any application where this makes sense.
> A node does not have an orientation so why can it have a length?
>
> If it has a length it does not make sense to use a node.
>
> Flo
>

Since OSM editing tools aren't AutoCAD you can't be 100% precise on the
geometry.
Some small features can actually be summarized as nodes when drawing their
shape sounds irrelevant regarding the cluttering it introduces.

I'm sorry that was trivial for me.
I won't draw a circle for a 5cm diameter pole and so on...

That's why tags are intended for several kind of primitives instead of only
one sometimes.


*François Lacombe*

fl dot infosreseaux At gmail dot com
www.infos-reseaux.com
@InfosReseaux 




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Re: [Tagging] length=

[Florian Lohoff]

On Tue, Jan 27, 2015 at 04:18:57PM +0100, Martin Vonwald wrote:
> 2015-01-27 16:13 GMT+01:00 François Lacombe :
> 
> > I personally recommend to use the length key while mapping street cabinets
> > as nodes.
> > 
> 
> On a node it makes perfect sense. At least as long as it is not
> possible/wanted/allowed to provide the geometry.

Does it ? I cant think of any application where this makes sense.
A node does not have an orientation so why can it have a length?

If it has a length it does not make sense to use a node.

Flo
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Re: [Tagging] length=

[Warin]

On 28/01/2015 3:26 AM, Mike Thompson wrote:

E.g.: a 17-degree mapped mountain road or San Francisco street's will get
its actual length a 5% bonus compared to its real-life counterpart.

True, but such roads are not very common (17 degree ~= 30%). There are
a few short streets in places like San Francisco that are this steep.
In these cases if someone actually went out and measured them with
suitably precise equipment it might make sense to have some sort of
length tag, but I would suggest something to differentiate it from a
purely horizontal measurement. Of course, you could also drape the OSM
data over a suitably detailed DEM (digital elevation model) to get
your answer as well.




Or you could tag the slope/gradient of the road .. I've tagged a short 
section of my street at 17% .. that is averaged .. it peaks at 23%. You 
can then use that to work out the travelled distance, height gained/lost 
etc.


As for tracks being out by uptp 20% .. that is faily normal expection on 
outback roads .. they are dirt and if the grader driver (or his boss, 
the local farmer etc) think it would be better off over there .,. then 
that is where it goes .. can add some distance to what is on the map or 
road sign. I've come across a road that was realigned in such a manner 
for over 100 miles.


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Re: [Tagging] length=

[Eric SIBERT]

Le 27/01/2015 16:34, Martin Vonwald a écrit :

Ok - understood. Although I doubt, that there is real usage for that
example. But I had a quick look in overpass: besides aeroways it is
quite often used on bridges and tunnels, where the actual (official)
length can be observed. Makes sense.


Indeed, for tunnels, I just put the length indicated at the entrance in 
note=*...

... and some other contributors transfered it to length=*.

Eric


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Re: [Tagging] length=

[moltonel 3x Combo]

On 27/01/2015, Mike Thompson  wrote:
>> The way in OSM is only a (sometimes not precise) drawing of an existing
>> feature and can be different from the reality.
>
> How precise is the value of the "length" tag? From what is the value
> derived?

In my experience of hiking trails in Ireland, the length advertised
on-site and in leaflets is often 10-20% off (as measured by my treking
GPS that takes relief into account). I expect OSM + SRTM to be more
precise. YMMV of course.

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Re: [Tagging] length=

[Brad Neuhauser]

Quick note: for milestones, the tag used is distance, not length. There's a
wiki page with all unit tags if you're into that sort of thing:
http://wiki.openstreetmap.org/wiki/Map_Features/Units

On Tue, Jan 27, 2015 at 9:40 AM, Michał Brzozowski 
wrote:

> Just FYI, at OSM-PL people map highway milestones - as nodes near
> highways - which is less prone to error due to people editing
> geometry..
> The map: http://osmapa.pl/konkursy/pikietaz/
>
> Michał
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Re: [Tagging] length=

[Mike Thompson]

> E.g.: a 17-degree mapped mountain road or San Francisco street's will get
> its actual length a 5% bonus compared to its real-life counterpart.

True, but such roads are not very common (17 degree ~= 30%). There are
a few short streets in places like San Francisco that are this steep.
In these cases if someone actually went out and measured them with
suitably precise equipment it might make sense to have some sort of
length tag, but I would suggest something to differentiate it from a
purely horizontal measurement. Of course, you could also drape the OSM
data over a suitably detailed DEM (digital elevation model) to get
your answer as well.



> Depends on the accuracy
> you need, mostly for analysis purposes.

If you need that level of accuracy you may want to consider data
sources in addition to OSM.

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Re: [Tagging] length=

[Mike Thompson]

Thanks to everyone for sharing their thoughts.

> I personally recommend to use the length key while mapping street cabinets as 
> nodes.

Agree, length makes sense on nodes


> The way in OSM is only a (sometimes not precise) drawing of an existing 
> feature and can be different from the reality.

How precise is the value of the "length" tag? From what is the value derived?


> To be more precise : OSM is based upon a 2D geospatial database.
In my mind, a road climbing a mountain won't have the same length in
reality than in the DB : the Z dimension may have influence too.

For most highways this will have little difference.  10% (10 vertical
meters per 100 horizontal meters) is considered a steep highway grade.
In this example, the actual road length will be 100.5 meters
(sqrt(10^2 + 100^2)). For ski slopes and aerial trams, yes, it will
have a larger influence.


> it is quite often used on bridges and tunnels, where the actual (official) 
> length can be observed.

Perhaps "official_length" or "signed_length" might be more indicative
of what actually is being recorded.

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Re: [Tagging] length=

[fly]

Am 27.01.2015 um 16:34 schrieb Martin Vonwald:
> 2015-01-27 16:26 GMT+01:00 François Lacombe  >:
> 
> In my mind, a road climbing a mountain won't have the same length in
> reality than in the DB : the Z dimension may have influence too.
> 
> 
> Ok - understood. Although I doubt, that there is real usage for that
> example. But I had a quick look in overpass: besides aeroways it is
> quite often used on bridges and tunnels, where the actual (official)
> length can be observed. Makes sense.

But how to handle it one ways considering splitting and combining ways ?

For bridges and tunnels this info belongs to the area or the relation
but not to an unclosed linear way.

About aeroways we already have an own thread as we have more problems
with them.

cu fly




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Re: [Tagging] length=

[Yann Kacenelen]

Hi Mike, hi François, hi all,
 
I don't find the "length=*" tag redundant either for linear objects
like streetlines, especially in the case of pretty steep ones: as
they're projected for mapping purposes from 3D to 2D, their actual
length is not the one you measure on the map but you should consider
dividing it by cosinus(A) where A is the angle of inclination between
the local horizontal plan and the linear feature. Pythagore said it, man
;-)
E.g.: a 17-degree mapped mountain road or San Francisco street's will
get its actual length a 5% bonus compared to its real-life counterpart.
Pick a 100-meter-or-more road, it's not just a bunch of cm. Depends on
the accuracy you need, mostly for analysis purposes.
 
http://en.wikipedia.org/wiki/Grade_%28slope%29
http://www.datapointed.net/visualizations/maps/san-francisco/streets-slope/
 
My 2c,
 
- Yann


>>> tagging-requ...@openstreetmap.org> 27/01/2015 16:15 >>
( mailto:tagging-requ...@openstreetmap.org) 
Date: Tue, 27 Jan 2015 16:13:17 +0100
From: François Lacombe 
To: "Tag discussion, strategy and related tools"

Subject: Re: [Tagging] length=
Message-ID:

Content-Type: text/plain; charset="utf-8"

Hi Mike,

I don't find it redundant since the length=* tag can give the real
length
of a feature.
The way in OSM is only a (sometimes not precise) drawing of an
existing
feature and can be different from the reality.

The wiki can be completed with such a nuance.

I personally recommend to use the length key while mapping street
cabinets
as nodes.
http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dstreet_cabinet

All the best

*François Lacombe*

fl dot infosreseaux At gmail dot com
www.infos-reseaux.com
@InfosReseaux <http://www.twitter.com/InfosReseaux>

2015-01-27 16:04 GMT+01:00 Mike Thompson :

> I have noticed a "length" tag used on some linear ways [1]. It seems
> that this is redundant, as the length could be calculated from the
> geometry itself.  It could also be the source of future errors
should
> someone split the way, for example to add more detailed tagging,
such
> as varying smoothness.
>
> The length key does have a wiki page [2], but the page doesn't have
> much content.  Perhaps it is only intended for nodes?
>
> What do you think?  Should the length tag be used with ways and
> relations where the length can be calculated from the geometry
itself?
> Am I missing something?
>
> MIke
>
> [1] https://www.openstreetmap.org/way/324078614
> [2] http://wiki.openstreetmap.org/wiki/Key:length
>
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Re: [Tagging] length=

[Michał Brzozowski]

Just FYI, at OSM-PL people map highway milestones - as nodes near
highways - which is less prone to error due to people editing
geometry..
The map: http://osmapa.pl/konkursy/pikietaz/

Michał

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Re: [Tagging] length=

[Martin Vonwald]

2015-01-27 16:26 GMT+01:00 François Lacombe :

> In my mind, a road climbing a mountain won't have the same length in
> reality than in the DB : the Z dimension may have influence too.
>

Ok - understood. Although I doubt, that there is real usage for that
example. But I had a quick look in overpass: besides aeroways it is quite
often used on bridges and tunnels, where the actual (official) length can
be observed. Makes sense.

Thanks,
Martin
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Re: [Tagging] length=

[François Lacombe]

2015-01-27 16:18 GMT+01:00 Martin Vonwald 
>
>
> But on a way? Hm... Any real-world examples for me?
>

To be more precise : OSM is based upon a 2D geospatial database.
In my mind, a road climbing a mountain won't have the same length in
reality than in the DB : the Z dimension may have influence too.

Dealing with this problem with a length key instead of introducing a 3rd
dimension in OSM is dirtier but far more simpler ;)


*François Lacombe*

fl dot infosreseaux At gmail dot com
www.infos-reseaux.com
@InfosReseaux 
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Re: [Tagging] length=

[Bryan Housel]

I just did a quick search in iD.  We display the “Length” field only on the 
Runway preset. 
(I think it does make sense there)

Thanks, Bryan


> On Jan 27, 2015, at 10:04 AM, Mike Thompson  wrote:
> 
> I have noticed a "length" tag used on some linear ways [1]. It seems
> that this is redundant, as the length could be calculated from the
> geometry itself.  It could also be the source of future errors should
> someone split the way, for example to add more detailed tagging, such
> as varying smoothness.
> 
> The length key does have a wiki page [2], but the page doesn't have
> much content.  Perhaps it is only intended for nodes?
> 
> What do you think?  Should the length tag be used with ways and
> relations where the length can be calculated from the geometry itself?
> Am I missing something?
> 
> MIke
> 
> [1] https://www.openstreetmap.org/way/324078614
> [2] http://wiki.openstreetmap.org/wiki/Key:length
> 
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Re: [Tagging] length=

[François Lacombe]

Hi Mike,

I don't find it redundant since the length=* tag can give the real length
of a feature.
The way in OSM is only a (sometimes not precise) drawing of an existing
feature and can be different from the reality.

The wiki can be completed with such a nuance.

I personally recommend to use the length key while mapping street cabinets
as nodes.
http://wiki.openstreetmap.org/wiki/Tag:man_made%3Dstreet_cabinet


All the best

*François Lacombe*

fl dot infosreseaux At gmail dot com
www.infos-reseaux.com
@InfosReseaux 

2015-01-27 16:04 GMT+01:00 Mike Thompson :

> I have noticed a "length" tag used on some linear ways [1]. It seems
> that this is redundant, as the length could be calculated from the
> geometry itself.  It could also be the source of future errors should
> someone split the way, for example to add more detailed tagging, such
> as varying smoothness.
>
> The length key does have a wiki page [2], but the page doesn't have
> much content.  Perhaps it is only intended for nodes?
>
> What do you think?  Should the length tag be used with ways and
> relations where the length can be calculated from the geometry itself?
> Am I missing something?
>
> MIke
>
> [1] https://www.openstreetmap.org/way/324078614
> [2] http://wiki.openstreetmap.org/wiki/Key:length
>
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Re: [Tagging] length=

[Martin Vonwald]

2015-01-27 16:13 GMT+01:00 François Lacombe :

> I personally recommend to use the length key while mapping street cabinets
> as nodes.
> 
>

On a node it makes perfect sense. At least as long as it is not
possible/wanted/allowed to provide the geometry.

But on a way? Hm... Any real-world examples for me?

Best regards,
Martin
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[Tagging] length=

[Mike Thompson]

I have noticed a "length" tag used on some linear ways [1]. It seems
that this is redundant, as the length could be calculated from the
geometry itself.  It could also be the source of future errors should
someone split the way, for example to add more detailed tagging, such
as varying smoothness.

The length key does have a wiki page [2], but the page doesn't have
much content.  Perhaps it is only intended for nodes?

What do you think?  Should the length tag be used with ways and
relations where the length can be calculated from the geometry itself?
Am I missing something?

MIke

[1] https://www.openstreetmap.org/way/324078614
[2] http://wiki.openstreetmap.org/wiki/Key:length

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