[Tagging] RFC proposed water property key 'ephemeral '

Nick Bolten nbolten at gmail.com
Fri May 25 10:09:16 UTC 2018


> The tag provides mapper a way of tagging rivers and streams that is
presently not available.

> That will vary .. year to year and decade to decade ... to much change?

> This too will vary.

I think something might be getting lost in translation, since those
questions were rhetorical. I'm describing my understanding of the proposal
as it exists on the wiki, hoping it matches the intent.

> Intermittent does not equal ephemeral.

I now understand that after reading a few reviews, but the wiki proposal
does not make that very clear. From the wiki articles, an ephemeral body of
water "is only present for short duration", and intermittent "is used to
indicate that a body of water does not permanently contain water". From
those definitions alone, I'd assume that a river could have both
`intermittent=yes` and `ephemeral=yes` and be perfectly valid. However, my
new and improved understanding of hydrology definitions would say these are
separate and incompatible categories of rivers/streams. Page 6 of this EPA
document [1] has definitions that make these things clearer, maybe a
similar approach could be taken with the wiki.

> At present in OSM the tag to indicate an  intermittent flow that only
occurs in winter is intermittent=winter

> The proposal for ephemeral flow in autumn would be
ephemeral=autumn

> Combining these would give
> intermittent=winter
> ephemeral=autumn

> I would think that is easy enough to understand and matches the present
tagging scheme in use.

This is actually undocumented, and taginfo has no examples of seasons being
used as values for the `intermittent` key. With that said, I was also
suggesting that seasons might more appropriately be values for keys
describing "frequency/type of flow" like `intermittent` or `ephemeral`,
rather than having a separate `seasonal` tag, due to the ambiguity of
having more than one flow mode over the duration of a year. Something like
`intermittent:seasonal=<season>` might also work. Maybe I'm missing a
proposal for seasonal values for `intermittent`?

The idea I'm proposing is that there may be a 'catch-all' key that
describes the 'type' of thing that `ephemeral` or `intermittent` are, and
to set `intermittent`, `ephemeral`, or otherwise (`perennial`?) values when
appropriate. What about `flow=ephemeral` or `flow=intermittent`? Then, if
you need to set more specifics, you could use `flow:ephemeral=winter`. This
also provides a convenient hint for what is being described: the flow type
of the stream.

> The intermittent tag already exists .. do you want to change it?

I think we're both considering changes to the `intermittent` tag as
potential options.

> And what if the stream has a seasonal flow in say winter and an ephemeral
flow possible in spring?
> For this proposal this would be tagged
> seasonal=winter
> ephemeral=spring

This is super confusing. Namely, the `seasonal` tag's documentation
indicates whether the feature is present, but in this case it seems to be
indicating that there is a 'full' flow. In other words, I would've assumed
you should tag it as `seasonal=winter;spring` and `ephemeral=spring`. This
becomes extra confusing when combined with the `intermittent` tag, for
which the wiki entry acknowledges there exist different interpretations for
the same tag combinations.

> The same can be said for roads. how stale is that data .. etc.

I'm just trying to get a better handle on the use case. How rapidly does
this tag become stale? Would data consumers use the tag? Would they instead
want a time series of observations, and if so, how frequently? It feels
like a tag that is science-ie, but I don't know whether, for example,
scientists would use it.

1.
https://www.epa.gov/sites/production/files/2015-03/documents/ephemeral_streams_report_final_508-kepner.pdf

Best,

Nick


On Fri, May 25, 2018 at 2:16 AM Warin <61sundowner at gmail.com> wrote:

> On 25/05/18 15:00, Nick Bolten wrote:
>
> If I understand this proposal right, the goal of this tag is to provide
> more specific information for the general likelihood of whether a water
> feature (only rivers?) exists: when it comes and goes every days.
>
> The tag provides mapper a way of tagging rivers and streams that is
> presently not available.
>
>
> This sounds like there are two pieces of information that are desirable to
> map:
> - What is the frequency of flow/presence?
>
> That will vary .. year to year and decade to decade ... to much change?
>
> - Does this frequency vary over seasons?
>
> This too will vary.
>
>
> These seem like questions that deserve separate tags, for flexibility, and
> could benefit from a 'subtag' approach. Let's say you want to describe a
> river that always has current in the spring and summer, is intermittent in
> the fall, and has no flow in the winter. This could look like:
>
> intermittent=yes;ephemeral;no
> intermittent:ephemeral=autumn
> intermittent:yes=winter
> intermittent:no=spring;summer
>
>
> Intermittent does not equal ephemeral.
>
> At present in OSM the tag to indicate an  intermittent flow that only
> occurs in winter is
> intermittent=winter
>
> The proposal for ephemeral flow in autumn would be
> ephemeral=autumn
>
> Combining these would give
> intermittent=winter
> ephemeral=autumn
>
> I would think that is easy enough to understand and matches the present
> tagging scheme in use.
>
>
>
>
> The first tag declares the frequency, but doesn't specify when each tag
> applies. The other 3 describe how this frequency varies over seasons. The
> presence of more than one value for `intermittent` would be a hint for
> incremental mapping/QA: there should be one `intermittent:<value>=<season>`
> for every value in `intermittent`. Tagging a single water feature as always
> `ephemeral` would also be simple and get combined with the seasonal tag:
> `intermittent=ephemeral` `seasonal=<season>`.
>
> The intermittent tag already exists .. do you want to change it?
>
> And what if the stream has a seasonal flow in say winter and an ephemeral
> flow possible in spring?
> For this proposal this would be tagged
> seasonal=winter
> ephemeral=spring
>
>
> I also have a follow-up about the use cases for this data. For example,
> for the cases I can imagine off the top of my head (scientific,
> humanitarian), it's valuable to know how stale the data is, how the person
> collecting it determined the value (memory, general impression as a
> local?), and ideally have a yearly report of flow (in some form or
> another), and so a wikidata key or external db may serve your use case
> better. Unfortunately, this raises a lot of hard questions about linking
> OSM and time series data that I don't think are solved, but it the
> discussion could be valuable in itself.
>
>
> The same can be said for roads. how stale is that data .. etc.
>
> The present tagging has no entry for ephemeral. None.
> If you want more data .. such as flow rates .. then put it forward. It
> would need to include perennial flows, as well as seasonal, etc.
> I don't think anyone wants that data in OSM ... yet.
>
>
> Nick
>
> On Mon, May 21, 2018 at 3:35 PM Warin <61sundowner at gmail.com> wrote:
>
>> Hi,
>>
>> Following from the discussion on "Seasonal, intermittent, and ephemeral
>> water tags" I have created this proposal.
>>
>>
>> Hopefully the definition is tight enough that it excludes intermittent,
>> seasonal and any other thing that is not ephemeral.
>>
>> I have also included a guide on determination so that mappers can see
>> how to determine if something is ephemeral.
>>
>>
>> So here it is ...
>>
>> https://wiki.openstreetmap.org/wiki/Proposed_features/ephemeral
>>
>>
>> Is it clear? Do you understand it?
>>
>>
>> Would you use it? If so where?
>>
>>
>> Any thing else?
>>
>>
>>
>>
>>
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