Диванное картографирование -- это рисование карты, не слезая со стула. Это спорная практика, и приветствуется не всеми. Диванные картографы не собирают данные на местности, а пытаются интерпретировать доступные источники данных:
- Sketching in features seen in aerial imagery such as bing. The most common data source for armchair mapping
- Raster map sources (where these are available for us to use e.g. scans of Out-of-copyright maps)
- Manual interpretation of OpenStreetBugs reports
- Automated Quality Assurance bugs
- Other people's GPS traces
- Merging vector datasets into OpenStreetMap (also known as "community import")
- A "fixup" phase after a conventional import
Обычный маппинг включает в себя выход на улицу, обозревание объектов вживую попутно зарисовывая их, делая схемы расположения объектов, фотографируя их или записывая GPS-треки, с последующим внесением собранных данных в карту OSM через редактор.
Диванное картографирование не включает в себя осмотр местности вживую, а представляет собой нанесение на карту объектов, видимых на спутниковых снимках, сущность которых можно однозначно интерпретировать (например, хорошо различимые дома, ограды, дороги), а также внесение данных из других уже имеющихся источников (стоит уделять больше внимания их достоверности).
Рекомендации по диванному картографированию
It is very important to follow these guidelines when contributing in an area where data already exists in OpenStreetMap, and where local people have been contributing. In general you can be more relaxed about armchair mapping in areas of the world which are more remote, or otherwise have less pre-existing data.
Learn to recognise valuable data. There are various tools and techniques, some more complex than others, for figuring out who's done what within the OpenStreetMap data. These are all valuable skills to avoid doing damage while armchair mapping.
Imagery can be wrong! This is the golden rule to remember.
- Check imagery alignment. Imagery is very frequently misaligned from reality with an offset in one particular direction. If all of the data is off to one side, try to shift the imagery to match. In Potlatch you do this by holding down the space bar while dragging. Although armchair mapping often involves moving roads to match with imagery, you should not go shifting large area areas of data where that shift is all in one particular direction, at least not unless you have reason to believe that existing data is from a less accurate source than your imagery.
- 'Things change. Imagery gets old. Road layouts change. Buildings get knocked down and rebuilt in different places. Imagery can be many years old. Armchair mapping involves blindly copying this information but in an area where locals have contributed, you should be very careful about this. Look out of tell-tale signs. If the whole area has building outlines drawn in, and you see just one missing, or one drawn in completely the wrong shape, don't just "fix" it. Alarm bells should be ringing. One approach would be to just leave a 'fixme' tag. (Conversely mappers could leave a note tag on elements or on a standalone node to warn of recent real world changes and out-of-date imagery. We have no firmly established routine for this)
If in doubt, assume that existing data has been mapped by local people on the ground. Sometimes you will be able to tell this by seeing source tags or just by contacting users to ask!
You can gain a better understanding of the reasoning behind these guidelines, with the 'Disadvantages and controversy' section below.
Рекомендации организаторам встреч
Coordinated armchair mapping events are not something we have experimented with often. When organising an intensive en-masse armchair mapping event, organisers have the opportunity to teach users about these guidelines and ensure that they are followed. They also have the opportunity to screw this up. Organisers should be acutely aware of all of the guidelines and their reasoning seen in the following section. In addition there are several steps which should be taken in advance of an event.
- Organisers must take steps to inform all participants about these guidelines and the issues around armchair mapping. This should include linking to this page from any publicity about the event (e.g. from a wiki page about the event) and perhaps copying or summarising sections of this page.
- Organisers should think carefully about target regions for armchair mapping, selecting areas where there is less pre-existing valuable data, and communicating these selected regions to the participants. Providing rigid indicators for regions to map, using tools such as MapCraft or the HOT tasking server, may be a good way to direct participants to map in sensible areas, however the set-up of these regions is then all the more important. Where participants are to be given more choice of where to do their armchair mapping, this increases the need for them to be educated about the guidelines and issues on this page.
- Organisers should take steps to contact the local mapping community and consult with them on target regions for armchair mapping. Where there is pre-existing contributed data, this means identifying key contributors and seek their buy-in to the idea. In the absence of pre-existing data, community contact channels at the regional or country level should still be used to inform locals however possible. This includes local wiki pages under Mapping Projects, country-level Mailing Lists and whatever Contact channel is most heavily used by locals.
Минусы и проблемы
OpenStreetmap treasures the contributions of local people who go out and map their neighbourhood. We love people to do that, and we would love more people to do that. In this way we can create a truly uniquely valuable map of the world. There is a sense in which this represents the heart and soul of OpenStreetMap. Nothing should be allowed to discourage this, and unfortunately armchair mapping is widely believed to do so. Three reasons:
- Although going out surveying isn't always convenient, there's some truth in saying that time we spend on armchair mapping is time which could have been spent on survey-based mapping. What's more, every mailing list post or blog post or wiki page describing armchair mapping could steer new visitors towards this approach and away from the more valuable process of mapping their own neighbourhood.
- It can be argued that even if armchair mapped data is not interfering with existing surveyed data, its sudden rapid arrival discourages people from doing the better survey-based mapping in a particular area (Note that similar arguments are made about imports. Perhaps armchair mapping can be regarded as a step on a sliding scale towards the damaging effects of mass data imports)
- Armchair mappers have often been found to be damaging or undoing the hard work of surveying mapping contributors.
The first two points are rather philosophical, difficult to argue against, but also difficult to accept as major problems depending on your outlook and how you weigh against the considerations of the value of armchair mapped data. The third point is a problem we can try to mitigate by laying down some armchair mapping guidelines, and trying to educate new users about what is accepted practice for armchair mapping.