RU:Yahoo! Aerial Imagery

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Yahoo! Aerial Imagery appearing in JOSM using the WMS plugin with a downloader installed

Yahoo! provides Vertical Aerial Photographs as part of their map interface (one of several such services out there).

Yahoo! have agreed to let OSM use their aerial imagery (blog announcement about it).

You can use any of the three main OSM editors (Potlatch, Merkaartor, or JOSM), to edit OSM map data over the Yahoo! imagery. For JOSM, you need to install the WMS plugin to access the Yahoo! WMS.

Sadly metacarta no longer provide their opaque layer view [2]


The agreement allows us to derive our vector based map data from the Aerial photos owned by Yahoo! and release these derived works with our open content license (no longer copyright Yahoo!).

We are permitted to display the aerial photos alongside our data in Potlatch and the Java Applet (and any other online mapping tools) we are also now permitted to use plugins within JOSM (as of 13th July 2007 — See mailing list post)

Similar uses within other off-line mapping tools are also 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 your 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 capturing individual images for commercial purposes. Again, our assessment is that you're 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.

User: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."

Know any more? Feel free to add. See also discussion.

Using Yahoo! Aerial Imagery

The agreement with Yahoo could considerably simplify 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's 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 couldn'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 which is far from finished (doesn't have street names!) but is also far from a blank slate.

Next somebody (not necessarily the same person) can take a print-out of this road-layout with them, as they visit the area. As well as collecting street names, and noting other features. They can also spot corrections which 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 in JOSM is an alternative which 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 alway use the highway=road at least, which is used for road 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 totally 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 screenshot Potlatch with the background imagery switched off (to use less ink) or screenshot 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, since your work will show up on the map, however it is not a recommended approach since it will result in areas which 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, since some ways will never have 'name' tag even in their finished state (e.g. nameless footpaths)


Landuse 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.

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, so the highway=road tag should be used, until either on the ground classification or someone who knows the area comes and updates the data.
  • Roads from the sky look like a t-junction, but in reality are a turn to stay on the road.
  • Complex junctions are rarely drawn in a detailed form, which is required for routing or turn restrictions
  • Minimal one way restrictions
  • Divergence of Yahoo Images from up to 10m (can be solved by adjusting the Image to a GPS trace)


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 sketched in from the Yahoo imagery

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.

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

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.

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/км² rate quoted for Australian 2m resolution imagery, that would have cost AU$300,000.

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.)


See the list documenting Yahoo! Aerial Imagery/Coverage.