Bing Aerial Photos for OpenStreetMap – Great, But Are They Recent?

Bing, Microsoft’s decision engine that also includes Bing Maps, the former Virtual Earth platform, announced a week ago that OpenStreetMap can use all their worldwide aerial imagery to improve their free and open wiki world map. As these things tend to go with OpenStreetMap, the ink of the blog post announcing this news was not even dry when new versions of the OpenStreetMap editors were ready that included the Bing Maps Aerial layer as a backdrop. As soon as the formal go-ahead was given by the Bing legal team, the new editors went live. Since then, active OpenStreetMap volunteers have been tracing, checking and completing data like there’s no tomorrow, kicking OpenStreetMap into a higher gear once again.

Data Quality

As the initial excitement is wearing off and mappers get more acquainted with the new Bing layer, a need arises for more profound insight into this emerging primary mapping resource. One question that almost immediately pops up when using the Bing Aerial layer for OpenStreetMap editing is: what is the quality of this imagery? This question pans out into a few different quality dimensions. Firstly, the resolution of the images – which seems to be generally better than the Yahoo aerial imagery that OpenStreetMap has been relying on as a background layer. Secondly the spatial accuracy, which can be an issue for large scale mapping. Spatial accuracy can vary in aerial imagery, due to the process of orthorectification. The error is usually not very big, but an error of even a few metres can be prohibitive for large scale mapping. Lastly, the the age of the photo is a crucial quality component. This temporal dimension of the Bing imagery data quality is particularly interesting when OpenStreetMap contributors work on an area without knowing the current ground reality. Another case where imagery age is of particular consequence is when an area has data from a previous import of known age and origin. It only makes sense to refine the map based on Bing imagery when that imagery is actually of a more recent date than the import. With the recent 3DShapes import in the Netherlands, we are dealing with that exact situation. It would be good to be able to easily ascertain if the Bing imagery for a particular tile is of more recent date than the 3DShapes. So I built a tool to do just that.

The Bing Date Map shows a full screen Bing Aerial map using the Bing Maps SDK. Every 256×256 tile of imagery is overlaid with a transparent tile showing the photo date for that tile. This information is extracted from the HTTP headers:

mvexel$ curl -I 'http://ecn.t3.tiles.virtualearth.net/tiles/a12020033.jpeg?g=587&n=z'
HTTP/1.1 200 OK
Cache-Control: public, max-age=31536000
Content-Length: 7255
Content-Type: image/jpeg
Expires: Sat, 03 Dec 2011 17:02:31 GMT
Last-Modified: Mon, 23 Aug 2010 17:40:15 GMT
ETag: "7"
Server: Microsoft-IIS/7.5
X-VE-TFE: DB30022438
X-VE-TILEMETA-CaptureDatesRange: 7/1/2004-7/31/2004
X-VE-TILEMETA-Product-IDs: 3
X-VE-TBE: 0023726
X-AspNet-Version: 2.0.50727
X-Powered-By: ASP.NET
Date: Fri, 03 Dec 2010 17:02:31 GMT
Connection: keep-alive

The X-VE-TILEMETA-CaptureDatesRange key gives a date range for the capture date that we are going to use. Usually, the range seems to be one month, so that’s the resolution we’re going to work with. I wrote a PHP script that takes a tile quadkey, which is a unique geographical identifier, as input. It retrieves the HTTP HEAD as displayed above, parses the dates into a legible format and writes the date string onto a transparent, 256×256 pixel PNG image which is the output of the script. Using the Bing Maps SDK, it is then very straightforward to overlay this pseudo-tileserver as a TileLayer onto the Aerial base map:

var tileSource = new Microsoft.Maps.TileSource({uriConstructor: 'http://server/tile.php?t={quadkey}'});    
var tilelayer= new Microsoft.Maps.TileLayer({ mercator: tileSource, opacity: 1 });
map.entities.push(tilelayer);

The result looks like this:

You can give it a try here. I might (be forced to) close it down as firing HTTP HEAD requests directly at Microsoft’s tile servers probably violates their terms of use.

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