Yahoo! Aerial Imagery
Yahoo! provides Vertical Aerial Photographs as part of their map services, and allows this to be used within Potlatch and formerly standalone OpenStreetMap editors. We were able to trace from yahoo to more quickly and accurately create maps in some parts of the world, and we are very grateful to Yahoo! for this support during the crucial early years of our project's development between 2007 and 2011.
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Yahoo imagery is no longer available outside Potlatch as of 13 September 2011: [1] |
In September 2011 Yahoo partially shut down their aerial imagery service. Luckily we have, since 2010, had a similar agreement with Bing (Microsoft). On the whole bing's imagery is superior, although not everywhere. Cities in Pakistan for example were covered by the no-longer-available Yahoo imagery, but are not available (at hi res) in bing.
Yahoo! never exactly gave us explicit permission, but stated that their terms and conditions allow using their aerial imagery (blog announcement about it) for the purposes of tracing. We could use any of the three main OSM editors (Potlatch, Merkaartor, or JOSM), to edit OSM map data over the Yahoo! imagery.
TODO: past tense/rework or blow away the rest of the details on this page
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Using Yahoo! Aerial Imagery
The use of Yahoo imagery could simplify considerably the mapping work flow. Previously we were starting with a blank canvas. We had no copyright-free information (apart from very out-of-date Out-of-copyright maps) and were forced start from scratch, hitting the streets with our GPS Units. Now, using the aerial photos of urban areas, we could make a start at mapping areas without having to go to there, which makes a pretty big difference!
Unfortunately, however, we do still need to visit the streets we are mapping, at least at some stage. Aerial images give us the street layout, but there is other information we need to gather: most crucially, the street names. We will be faced with a terrible temptation to look at existing maps, just to get the names of streets, but we can't do that! It's copying! We can't do it for all the same reasons that we can't trace over street layouts from other maps or pinpoint individual latitude/longitude based on other maps. Technically it's breach of copyright, and technically we could be rumbled if we did it. It's frustrating, but we will just have to go and look at the street signs to find the street names!
While we are there, there's other information we need to gather too. We need to record things like one-way restrictions. We need to make a note of any other Map Features such as pubs and post offices, none of which can be gathered from aerial photos.
Sketching over the imagery
Working with aerial imagery lends itself to a very different overall mapping approach. It is possible to sketch in the road layout for large expanses of urban areas, without going there. This yields a map that is far from finished (doesn't have street names!) but also is far from a blank slate.
Next somebody (not necessarily the same person) can take a print-out of this road layout with them (see Walking Papers) as they visit the area, collecting street names and noting other features. They also can spot corrections that need to be made. Often streets can be obscured from the aerial view. Sometimes it looks like there is a street, when in fact there isn't.
How to sketch?
As described above, mapping can take place in two phases: sketching and then more complete surveying. However the preferred approach for putting these sketched roads into the database needs to be decided and clarified (here).
The new Potlatch editor is the easiest way of tracing over the imagery. WMS plugin in JOSM is an alternative that is a little more fiddly to set up.
In Potlatch you can sketch in untagged ways (a way with no highway= or other such keys set). But if you are sketching roads you should always use the highway=road at least. It is used for roads that are not classified yet (don't mix up with the highway tag "unclassified", which is a certain type of road!).
One problem is that neither Mapnik nor Osmarender (the layer as configured by default and rendered by Tiles@Home) will show untagged ways. Maplint shows untagged ways, but it also shows all the other lint and doesn't show the complete areas of map. This makes it harder to get a quick feel for the level of phase-one completeness in an area. It's also harder to get a print-out suitable for use while surveying. Solutions to this might be to screen-shot Potlatch with the background imagery switched off (to use less ink) or to screen-shot JOSM and invert the image to get a white background. More fiddly solutions involve running custom settings of Osmarender or Kosmos (e.g.), or using an image editor to splice together Maplint with other renderings. (Image:Shoho Cake inc segments.png is an example of this.)
An alternative might be to set a highway tag but not a name tag. This is tempting, because your work will show up on the map. However it is not a recommended approach, because it will result in areas that appear to be fully mapped, but in fact nobody has done a proper survey. The two-phase approach definitely requires a second phase! Unnamed ways are not sufficiently distinct from named ways in any of the default map layers, as they are currently set up. It might not be feasible to make them more distinct, because some ways never will have a 'name,' tag even in their finished state (e.g. nameless footpaths).
Land Use
Land use is usually easy to determine from Yahoo imagery and, in many cases, infeasible to obtain using GPS surveying. So, it's a good thing to sketch.
Bodies of water, woodland and residential areas all show up really well and are worthwhile to trace. Tracing residential areas is possibly more useful than trying to trace residential roads. A marked residential area with no roads is clearly visible on rendered maps and can help a GPS user to find areas that need detailed surveying.
Yahoo imagery boundaries
It can be useful to others to add a way that defines the boundary of each area of high-resolution imagery. The convention is to tag it with something like note=Extent of Yahoo imagery for London.
There are lists and maps of areas of high resolution here.
Problems with tracing Yahoo
This section is to list problems with tracing from Yahoo Imagery, and may cause problems with later editing.
- Roads and paths under trees cannot be seen.
- Roads are unnamed and not easy to classify. In this case the highway=road tag should be used until either on-the-ground classification is done or someone who knows the area updates the data.
- Roads from the sky look like a T-junction, but in reality are a 90-degree turn to stay on the road.
- Complex junctions rarely are drawn in a detailed form, which is required for routing or turn restrictions
- Minimal one-way restrictions
- Divergence of Yahoo images up to 10 meters (can be solved by adjusting the image to a GPS trace)
Achievements
The availability of the Yahoo! imagery has made possible a number of tasks which would most be almost impossible without it. Some of these achievements are listed here:
The whole of the original London Congestion Charging Zone was mapped over a single weekend by a team of people using Yahoo! imagery. London is notoriously difficult to map using conventional GPS-based methods due to the urban corridor effect giving poor traces.
- Baghdad maps
The whole road and rail network of the Baghdad area has been mapped using the Yahoo! imagery. As have some parks, forested areas, etc. People on the street are showing real interest in filling in the road names to make this an actually useful map, it's not just a promotion piece.
- Mashhad, Iran
The whole road and rail network of the Mashhad, Iran area has been mapped using the Yahoo! imagery. As have some parks, forested areas, etc. Street and place names are slowly coming in as well. A GPS-track was used to verify that the Yahoo! imagery indeed was accurate and has very little (<2m) offset in that area
- St Albans and surrounding area
Not quite so grand as the above cases, but every last bit of woodland, every golf course, every lake and the main rivers/streams/lakes were mapped, which isn't possible with Landsat and would be very laborious by ground-based surveys.
- Sydney and surrounding area
Completion of the greater metropolitan railway network, completion of the inner city unclassified road network, many arterial major roads that were previously in as stretches here and there, several complex road interchanges not practical to map on the bike, several major rivers (also strangely difficult to map on a bike) and adjusted many problems due to inner city urban canyoning. Plus a good few golf courses, parks and sports pitches. At the AU$160/km2 rate quoted for Australian 2m resolution imagery, that would have cost AU$300,000.
- Darwin to Port Augusta (Australia)
Using the new Potlatch editor the highway between Darwin and Port Augusta (ie the whole of the central North - South Highway), together with the highway to Ayers Rock & the Olgas, has been completed. Most of the imagery is low resolution. (The road has now been surveyed along stretches and has been nudged and tagged accordingly.)
Coverage
The imagery covers the whole of the U.S., but elsewhere it generally covers only rectangular areas around some cities. See the list documenting Yahoo! Aerial Imagery/Coverage.
Accuracy
Yahoo!'s aerial imagery is pretty accurate. More so than consumer GPS units in general. In some parts of the world (e.g. Italy) there is noticable offset though, much more than simple consumer GPS units. For more detail see: Yahoo! Aerial Imagery/Accuracy.
Legalities
Yahoo takes the position that if we derive vector-based map data from the aerial photos owned by Yahoo! they are no longer copyright Yahoo!, so we can release them under any license we want, CC-BY-SA, ODbL, ... Anyone who has traced from Yahoo imagery may accept our new contributor terms and does not have to change their contributions.
We are permitted to display the aerial photos alongside our data in Potlatch and the Java Applet (and any other online mapping tools). Note that as of late 2011, Yahoo are shutting down their mapping unit and this imagery service will be switched off without notice.
We are also permitted to use plug-ins within JOSM (as of 13th July 2007 - See mailing list post). Similar uses within other off-line mapping tools also are likely to be permitted, although developers might like to discuss the details before going to a lot of development effort.
Yahoo's last statement on the legalities, from Scott Regan, is..
On the usage of our Aerials API, again I can reiterate that the OSM's usage of the free API for overlaying GPS and tracings appears to be well within the terms of use. To address some other questions that have been raised, we don't see any red flags with the GPX files overlays approach that is used by OSM community members vs. our restriction on plotting real time GPS coordinates or points that are less than 6 hours old. And the other question regarding the mechanics of how imagery is cached in a transient manner by the OSM tracing app also does not violate the constraint preventing the capture of individual images for commercial purposes. Again, our assessment is that this is well within the terms of use. I hope this helps address some of the concerns of the community, please continue to let us know how we can help as things progress.
Mikel has been working on liaising with Yahoo! people about this agreement. His comment is:
- "Even though there are open questions, I suggest that we can continue as we have confidently. OSM continues on even with the outstanding questions about attribution, etc. The use of Yahoo! imagery can continue as well. The spirit of what we are doing is sound, there are only particulars which need sorting out"
We don't have a written agreement explaining exactly what is permitted. It seems to be more a case of agreeing an interpretation of their Terms of Use.
- Also from Mikel: "There's multiple individuals, project managers and lawyers, involved in the decision. The first pass on the subject was a more cautious reading of the ToU, but at request, they (Yahoo) invested significant time in re-examining the particulars."
