[Talk-us] NHD import: what data quality is acceptable?

Apollinaris Schoell aschoell at gmail.com
Mon Jul 23 17:50:04 BST 2012


On Jul 22, 2012, at 5:25 PM, Greg Troxel wrote:
>> (2) One historic complaint I've seen about the NHD import is that it
>> clutters the map,
> 
> That seems like a bogus criticism; renderers can choose to render or
> ignore.
> 
> If the criticism is cluttering the database, that's a harder call, but
> water features are broadly important and it seems unreasonable to
> exclude them on clutter grounds.  Sooner or later we're going to need a
> layer concept in editors and probably the API.
> 

If the imported features are tagged correct it's all good. But I have seen imports of "streams" where you will never find water except while a huge storm. This wasn't NHD and I am not sure if it  applies here.  As long as the data is verified against nature in some places it's perfectly fine to import.

> 
>> (5) In the area I have in mind, there are very few ways that actually
>> would need to be conflated (a few major rivers). Is it likely to be
>> called vandalism if I confine myself to copying the OSM-specific tags
>> from the OSM ways onto the NHD ones? I trust a land survey rather
>> more than I do someone tracing a shoreline from Bing imagery: in
>> well-graded streams, the shorelines can be variable and ambiguous,
>> and NHD has pretty sound information about high water marks.
> 
> In my view, if you are acting in good faith and believe the post-edit
> map to be better than the pre-edit map, it is not even close to
> vandalism.

+1 

> 
>> (6) When I try a limited import, I get a lot of JOSM warnings about
>> waterways crossing highways. Do people think that all of these have
>> to be fixed before importing? In that case, I'll have to confine myself
>> to the very small area that has highway-crossing-stream that I can
>> visit myself (to try to settle what the boundaries of the bridge,
>> dam or culvert are, and what type of waterwork it is). Is it considered
>> acceptable simply to leave the crossings unmarked? (They still
>> render correctly in Mapnik, for what it's worth.)
> 
> waterways crossing highways is how it is.  They shouldn't be connected,
> because you can't navigate from one to the other.   Having the waterway
> improves the map, and the fact that it doesn't have detail about bridges
> or aqueducts etc. should not be a bar to the improvement.  No where
> else in OSM do we require quality standards to be met - just that the
> map after the edit is better.

agreed, to me the validator check in JOSM is too strict. some mappers workaround by adding a layer -1 to all waterways. But that's mapping for the validator and doesn't add any value but clutter to the DB.

> 
>> I'm trying to start with areas where I have firm local knowledge,
>> although clearly I cannot survey streams on private lands, so I
>> have to depend on NHD there. But the result is not going to be of
>> much use to me unless I can generalize what I learn to do better
>> NHD imports in areas that I know less well.
> 
> I would say that if NHD says there is a stream, and there's no data
> about hydrography in OSM near it, then the map is better off with the
> NHD data.  My impression is that NHD data that I see on the map while
> driving appears quite in sync with reality.

Any data source is more or less accurate in different regions. If you verify it in local areas with your knowledge it's ok to extrapolate it to county or even state level. But if you verify in New York it's not a good idea to import NHD in Arizona and Alaska with the same rules. 

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