> What you think is an error probably not error to me/others and your corrections will just make more troubles to me/others.
Let's get this very clear. What I'm after is a tool to guide me into overlapping buildings only. Nothing else. Once I have them I load them into the todo list and visually inspect each one before deciding which if any to delete. I assume we are in agreement that the buildings should not be mapped twice? The area of interest to me is rural Africa. I assume if I spot a building that has been mapped twice manually there is no objection to me deleting it? I do realise I should do a changeset comment for each and wait a month for a reply but realistically there are too many. This is no different to a manual inspection of the buildings and deleting one if I see it mapped twice or would you prefer me to retag one somehow so the academics can find a new subject for a PhD? I make no comment as to why it has been mapped twice. It doesn't really interest me other than to prevent it happening again. I have no interest in anything other than simple buildings. Accurate building information is very important in parts of Africa. It is used for population estimates for a start. You take the floor area and multiply by the average number of people per square metre. This doesn't work if you map buildings as a node. If you have very limited resources where do you put a new school? I've seen one instance where the official estimate was a thousand buildings in one section. An OSM supporter managed to arrange a very special mapping event. It was restricted to GIS professionals from local governments apart from one or two experienced mappers. They very accurately mapped six thousand buildings. The quality of the mapping was some of the highest I've seen. This had a huge impact on the resources allocated for schools water supplies etc. in the area. OSM is a very cost effective tool for local government planning when you have very little money. The NGOs are also very interested in this type of information. How many doses of vaccine do we send? Pierre can probably give you more information on why accurate building outline information is important. We need a decent building_tool plugin for iD but that's another story. Cheerio John On 25 November 2017 at 03:47, SandorS <sandor...@gmail.com> wrote: > John, > > by a step of abstraction, you are touching one of the most complex and > complicated issues related to the OSM source data. Namely, replications and > overlaps as its consequences. Let us focus on area overlaps and especially > on the kind you have mentioned – when the overlapping geometries, with high > probability, represent the same object or parts of it. There are 100s of > thousands of these in the source data. The consequences are large/huge > number of errors/anomalies present in all today publicly available OSM > based maps. I am not sure what JOSM (and other tools) can do with these > errors but obviously very little because they are there. The issue has been > up for discussion several times on this and other OSM forums. > > The majority of the replications are cased by bot/programmatic/mass edits. > We have really large number of edits-over-edits-over... and, as you know, > checking whether two almost overlapping geometries represent the same > real-world object is far beyond a simple exercise and the power of a > “script” solution. This many replications are the reason why we don’t like > (even don’t want) extensive use of mass edits, quick fixes and similar > options (exception, with DWG’s approval). There are several typical area > overlapping classes/types. To be short, just in few words and links to > illustrative examples (arguments) from the lake area objects. > > The outer border polygons exactly overlap but the areas have different > hole structures (covers case of identical geometries). For instance here > https://osm.org/go/y3Q5zNY- (missing islands, how many?). > > Outer polygon section of a smaller area exactly overlaps an outer border > section of a larger area like here https://goo.gl/aAvEkM (exact fragment > overlaps, how many?). > > Corridor replica (overlaps) when some border polygons of two geometries > are so close to each other that with a very high probability the geometries > represent the same real world object, like here https://goo.gl/DtF2nA. > > Just to mention some and of course, many combinations of the former cases. > While a solid/robust data preparation should detect and correct/repair (and > it does) the mentioned overlap cases, still there is an overlap class > raising a dilemma what to do. These are when a smaller area is strictly > inside of a larger in the same class, e.g. lakes (the smaller outer is > strictly inside a larger outer and outside any holes of the larger) like > here https://goo.gl/7HUX43 or here https://goo.gl/TjfP9o or here > https://goo.gl/H8E3L5 or here https://goo.gl/ZY64u3 ... not to mention > the confusion in the Venezia lagoon. Trusting the tags only, these areas > are redundant and should be ignored but according the Wiki rules and > trusting the geometry these smaller areas should be holes. > > Having in mind only raster map-making, the mentioned anomalies/errors are > not so important (blue is blue for lakes, green is green for forests...). > At the same time for the OSM based GIS and vector map-making the overlaps > present serious issues/problems. Just take the procedures like > defragmentation, data generalization, tiling, rectangular clipping and so > on. > > Finally, anomalies/errors in the OSM source data in certain cases > represent a valuable attribute. There is no richer and larger source of > serious and complex issues for research and academia related to topology, > polygon algebra, algorithms, programing... than the OSM source data. So, > insisting on corrections, especially by mass editing, should not be a OSM > strategy. These errors are there, some will go and many new will come. > Instead, strengthen you data preparation tool and offer it to users with > arguments. But please, do not touch the source data. What you think is an > error probably not error to me/others and your corrections will just make > more troubles to me/others. > > Regards, Sandor. > > > > > > > > Sent from Mail <https://go.microsoft.com/fwlink/?LinkId=550986> for > Windows 10 > > > > *From: *john whelan <jwhelan0...@gmail.com> > *Sent: *22 November 2017 00:19 > *To: *OpenStreetMap talk mailing list <talk@openstreetmap.org> > *Subject: *[OSM-talk] finding overlapping buildings > > > > Can someone describe a method I can locate these in JOSM. I'm not after > crossing buildings but just those that are mapped twice so two buildings > with 50% or more overlap. > > > > Straight duplicates aren't a problem but ones that are drawn twice by two > different mappers are. Yes I know it shouldn't happen but it does. > > > > Thanks John > > >
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