Hi Christoph, Me, too, I’m appreciating your remarks. May I ask you to give us some hints:
- how can we distinguish a 'drainage shaped landscape' from a stream? Actually, what do you mean with this term? - same question for 'fossil river'. Is that something like a dry valley? Thanks in advance. Viele Grüße Arne > Am 08.04.2020 um 20:05 schrieb Rafael Avila Coya <[email protected]>: > > Hi, Christoph: > > Thank you very much for this informative and thorough assessment! > > O 08/04/20 ás 17:38, Christoph Hormann escribiu: >> Thanks for the additional info, that makes things a bit clearer. >> >> Based on this it seems your suggested tagging is not right. You base >> the classification into waterway types on the HYP attribute - which is >> not what this indicates apparently. > > > As explained already, I based that on checking a sample. We could leave the > waterway tag empty, but that would slow the process and more tiresome, and if > you have that most of HYP=1 and 2 are mostly rivers, and HYP = 4 are most of > the time streams, we can put that as default, and users will correct when > needed. I can't see any problem in this. > >> More generally speaking i have doubts about the wisdom in importing >> large parts of the data. Looking over it a large fraction of the >> waterways in the data set are not correct. This is already hinted in >> the information you provided. Most of the waterways (more than 40k, >> i.e. more than 90 percent) are indicated to be dry, that is without any >> evidence of present day water flow. Looking over the data i could find >> a lot of cases where a drainage shaped landform was interpreted to be a >> stream and that stream was then continued downhill without any physical >> indication of even historic water flow - sometimes along tracks misread >> to be streambeds, sometimes also right across villages and other human >> built structures. > > > There are actually some examples of those waterways in the workflow wiki. > Examples of ways that should not be imported (deleted). I can add more > examples if you let me know, maybe in an appendix to avoid making the wiki > difficult to follow because too many captions. > >> In subtropical Africa it is very common that due to climate change (both >> recent human made and natural changes over the last few thousand years) >> as well as immensely intensified groundwater use valleys created by >> water flow - with often indications of that visible in imagery - do not >> carry even sporadic water flow any more at present time. This is >> called a fossile waterway. According to OSMs verifiability principle >> mapping such structures as waterway however is clearly wrong. >> >> The problem i see is that importing such data where a large portion of >> the features are factually incorrect will either result in >> >> * a lot of incorrect data in serious need of cleanup imposing a serious >> debt on the local community. >> * a lot of work to evaluate every single one of the >40k features to >> assess if it really represents a verifiable waterway. My estimate >> would be that this work might be more efficiently invested in mapping >> those from scratch. > > > We commit errors all the time when mapping remotely, not only adding wadis > that might not carry water all the year round. We, as mappers, put all our > interest in mapping the best way possible, anyway. It's the responsability of > the mappers to decide, with a validation too. Nothing will be perfect, as > usual. Take any two experienced users, put them to map waterways in a certain > area, and you won't get the same osm file. > > I put you one example: We recently asked the community on helping the UN > mission in Somalia to map some features in some areas of the country. One > project ( https://tasks.hotosm.org/project/7918 ) asked for waterway mapping > in an area of the south, an area where there aren't any of these > candidates-to-be-imported waterways. The project was divided in 100 tasks > (squares), and one user, Arne Kimmig, who is participating in this thread, > did 92% of them, I think with a very good quality. > > As nobody jumped in, I've been validating all tasks of that project, and made > some modifications, corrections, and additions to most of the tasks. I've > validated 83% already, so almost finished. But I am sure if a 3rd comes in > there would be modifications, etc, etc. With this import, that is basically a > waterways mapping where we integrate the data in a controled way, we will > follow the same process. > >> Now this of course varies a bit across the coverage area - in the >> western part a significant fraction of the HYP=4 waterways show >> indications they could be legitimate intermittent streams based on >> available images (though you often have to spend a lot of time looking >> for hints for that). In the east this is much less so and i would >> probably consider the majority of features bogus. > > > We will actually start from the East. For each new project (this import is so > huge that it will need 15, if no more, TM projects) we can check the data and > decide. It's not a matter of putting data that is 80% bad, but my > understanding after sampling in general is that the majority of ways are ok > for importing, some with modifications. If some areas look not to be worth > the effort, we will comment them with the interested users and the local > community, and in case we think they aren't interesting we drop those areas > and continue with others. We want to improve the map, that's the aim. > >> At the same time the positional accuracy of many of the features is poor >> with positional errors in the order of often 50-100m. This is another >> indication that importing this could be quite wasteful in terms of time >> spent. > > Not many in my opinion. Participants are being asked to correct geometries > when needed. But I've seen thousands of waterways everywhere that have less > accuracy than most of these waterways. So I don't see any issue here, really. >> >> Overall i would probably say that for mappers invested in improving the >> area this data could be useful to help identify where there are >> possibly waterways to map. But as an actual import where the mapper >> just does a quick verification and tag adjustment before adding the >> feature more or less as is it seems less suitable. And actual import >> would bear a high risk of well meaning mappers adding a lot of >> incorrect data because of the principal difficulty of proving a >> negative (it is hard to reliably prove a waterway from the data does >> not exist in reality and many likely will be inclined to trust the data >> being correct in such cases). > > > I get your point. And I will add more emphasis on that in both wikis about > the fact that waterways don't have to be uploaded when they aren't ok. The > aim is clear: help mapping waterways taking this dataset as a base, but not > trusting it blindly. They aren't asked to prove the incorrectness, because > users are free to decide what they think, and so too the validators. > >> Existing waterway data in the area is - while being incomplete - fairly >> decent quality it seems and it would be unfortunate if that was diluted >> by low reliability data. > > > So let's make sure this doesn't happen. With a well documented workflow wiki, > remembering well the goal of this data integration, a good and constant > communication among participants and a proper validation, I am sure the > results will be fairly ok and the map improved and enriched. > > Thank you again for placing your valued time in reviewing. I really > appreciate it. > > Cheers, > > Rafael. > > > _______________________________________________ > Imports mailing list > [email protected] > https://lists.openstreetmap.org/listinfo/imports _______________________________________________ Imports mailing list [email protected] https://lists.openstreetmap.org/listinfo/imports
