Re: [Tagging] units and notations for depth

[Paul Johnson]

On Tue, Feb 19, 2019 at 5:19 AM Tony Shield 
wrote:

> Quite right. For OSM purposes I suggest depth in metres, if water is
> tidal=yes also tidal_range in metres.
>
> I can't think of any reason to try to replicate nautical charts and tide
> tables. And when planning navigation I do not consider not using formal
> navigation charts and tables specific to that purpose.
>

More casual use cases for small craft might benefit from this, as small
craft *very rarely* carry the official charts (they're expensive and they'd
get ruined rather easily).  Depth is less of a concern for these since
often this kind of craft is often designed to be casually (and gently) run
aground on a river or lake beach anyway (and is a common thing to do at
Sauvie Island since it's the only way to take a boat to the beach instead
of fighting for a parking space and still get out of the (usable part of
the) shipping channel on that part of the Columbia River.  But even chart
that superficially maps the major features (where the water is, where the
markers are, where the rocks that protrude out or visibly close to the
surface are, etc).
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Re: [Tagging] units and notations for depth

[Tony Shield]

Quite right. For OSM purposes I suggest depth in metres, if water is 
tidal=yes also tidal_range in metres.


I can't think of any reason to try to replicate nautical charts and tide 
tables. And when planning navigation I do not consider not using formal 
navigation charts and tables specific to that purpose.


TonyS


On 19/02/2019 10:56, Colin Smale wrote:


On 2019-02-19 11:11, Tony Shield wrote:

Depth of water in tidal areas can vary enormously. People using depth 
for navigation and general use would expect to use depth at MLWS 
(Mean Low Water Springs) and add increments based on tide tables, I 
suggest that OSM does the same. Use the tidal tag and perhaps a 
tidal-range tag, in metres, tidal_range is the difference between 
MHWS (Mean High Water Springs) and MLWS.


note - Spring tides are the highest and lowest forecast tides and are 
based on the relative position of the earth, moon and sun.


Indeed, they are astronomical predictions, but not forecasts. They are 
not in themselves good enough for planning a (route to a) berth in a 
harbour for example. There are many other factors influencing the 
actual water level, like global/local weather conditions and seiche 
waves. Depth is of course also affected by dredging operations...


Should OSM aim to replicate the Admiralty Charts and tide tables?



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Re: [Tagging] units and notations for depth

[Colin Smale]

On 2019-02-19 11:11, Tony Shield wrote:

> Depth of water in tidal areas can vary enormously. People using depth for 
> navigation and general use would expect to use depth at MLWS (Mean Low Water 
> Springs) and add increments based on tide tables, I suggest that OSM does the 
> same. Use the tidal tag and perhaps a tidal-range tag, in metres, tidal_range 
> is the difference between MHWS (Mean High Water Springs) and MLWS. 
> 
> note - Spring tides are the highest and lowest forecast tides and are based 
> on the relative position of the earth, moon and sun.

Indeed, they are astronomical predictions, but not forecasts. They are
not in themselves good enough for planning a (route to a) berth in a
harbour for example. There are many other factors influencing the actual
water level, like global/local weather conditions and seiche waves.
Depth is of course also affected by dredging operations... 

Should OSM aim to replicate the Admiralty Charts and tide tables?___
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Re: [Tagging] units and notations for depth

[Tony Shield]

Depth of water in tidal areas can vary enormously. People using depth 
for navigation and general use would expect to use depth at MLWS (Mean 
Low Water Springs) and add increments based on tide tables, I suggest 
that OSM does the same. Use the tidal tag and perhaps a tidal-range tag, 
in metres, tidal_range is the difference between MHWS (Mean High Water 
Springs) and MLWS.


note - Spring tides are the highest and lowest forecast tides and are 
based on the relative position of the earth, moon and sun.


TonyS

On 19/02/2019 08:01, Colin Smale wrote:


On 2019-02-19 04:03, Graeme Fitzpatrick wrote:

Not opposition, but for tidal areas, isn't this going to have the 
same problems of whether you mark the "coastline" at the high- or 
low-tide line?

Coastline is MHW, that is settled isn't it?
Water heights are more problematic though, because there are different 
datums across the world. AFAIK there is no global default datum apart 
from WGS84 geoid height which no mariners ever use directly. All water 
levels/heights therefore need to be related to a datum.
Depth however is local and is just the difference in height between 
the water level and the bottom. Can you even have a negative depth? 
Floating above the water?


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Re: [Tagging] units and notations for depth

[Colin Smale]

On 2019-02-19 04:03, Graeme Fitzpatrick wrote:

> Not opposition, but for tidal areas, isn't this going to have the same 
> problems of whether you mark the "coastline" at the high- or low-tide line?

Coastline is MHW, that is settled isn't it? 

Water heights are more problematic though, because there are different
datums across the world. AFAIK there is no global default datum apart
from WGS84 geoid height which no mariners ever use directly. All water
levels/heights therefore need to be related to a datum. 

Depth however is local and is just the difference in height between the
water level and the bottom. Can you even have a negative depth? Floating
above the water?___
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Re: [Tagging] units and notations for depth

[Warin]

On 19/02/19 14:03, Graeme Fitzpatrick wrote:


On Tue, 19 Feb 2019 at 12:45, Warin <61sundow...@gmail.com 
> wrote:


The wiki has no units for depth, I would suggest these be the same as
height.


Makes sense - default as m's, but can be marked as ft depending on 
local standards


There are also problems with estimation and variability.

Some are using the tilde mark '~' to indicate 'approximately'.

Some are using '-' for between eg depth=0.5-0.7 for between 0.5
and 0.7.

Any opposition or better ideas???


Not opposition, but for tidal areas, isn't this going to have the same 
problems of whether you mark the "coastline" at the high- or low-tide 
line?


You'd probably need something along the lines of
Average_low-tide=0.8
Average_high-tide=1.9


The depth at the high tide mark would usually go from 0 to some negative 
number.  and average would be a negative number.


The depth at the low tide mark would usually go from 0 to some positive 
number. So it could be mapped depth=0-1


I would not map those. But it does raise the problem of using'-' for 
'between' if there is any negative number to be used.


Is using 'to' an acceptable method to represent 'between'?

This comes out of the river navigation problem when I looked at what 
people were doing for variable depths.

--
For tidal or seasonal etc I would only tag the expected range .. thus

Average_low-tide=0.8
Average_high-tide=1.9

would become

depth=0.8-1.9 (or 0.8to1.9)


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Re: [Tagging] units and notations for depth

[Graeme Fitzpatrick]

On Tue, 19 Feb 2019 at 12:45, Warin <61sundow...@gmail.com> wrote:

> The wiki has no units for depth, I would suggest these be the same as
> height.
>

Makes sense - default as m's, but can be marked as ft depending on local
standards


> There are also problems with estimation and variability.
>
> Some are using the tilde mark '~' to indicate 'approximately'.
>
> Some are using '-' for between eg depth=0.5-0.7 for between 0.5 and 0.7.
>
> Any opposition or better ideas???
>

Not opposition, but for tidal areas, isn't this going to have the same
problems of whether you mark the "coastline" at the high- or low-tide line?

You'd probably need something along the lines of
Average_low-tide=0.8
Average_high-tide=1.9

Thanks

Graeme
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[Tagging] units and notations for depth

[Warin]

The wiki has no units for depth, I would suggest these be the same as 
height.


There are also problems with estimation and variability.

Some are using the tilde mark '~' to indicate 'approximately'.

Some are using '-' for between eg depth=0.5-0.7 for between 0.5 and 0.7.

Any opposition or better ideas???




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