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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18 thoughts on “Bing Aerial Photos for OpenStreetMap – Great, But Are They Recent?

  1. Thanks Martijn, excellent post! I decided to use your date overlay to check my current areas of interest: Aruba and Colombia.

    Besides the fact that it is almost impossible in Aruba and Colombia to zoom in far enough to map, the dat stamp go back to the year 2000.

    As far as I am concerned, bing is no improvement to current sat. imagery. We have to keep trying to get access to hi-res imagery in other ways, especially for [HOT] mapping.

  2. I don’t think the dates presented are always correct:
    In Budapest, Hungary there were “major” changes to one of the squares, which happened around 2006/2007. The Bing images don’t include these changes, but claims the images are from Apr/2009.
    (Case in point: Móricz Zsigmond körtér, Budapest, Hungary)

  3. Seems to be no date output for Horsens, Denmark if you zoom in to where you get high quality images. I guess it could be MS gives no date, or an error your scripts?

  4. First of this Bing imagery is great even if outdated!
    Second, but more importantly, thanks to all the OSM experts for making it so easy to use.
    However, can anyone tell me why many tiles are missing and replaced by an icon; Camera in circle with a diagonal line!
    Example: My small town of Rheinbach in Germany. North Rheinbach has Bing imagery, south Rheinbach doesn’t.
    N 50.6247 E 6.95

    NOTE: email down at the moment so please reply on this page

    Merry Christmas

    dcp

  5. Hi, this utility looks like it would do exactly what I need but it doesn’t appear to work anymore? Has it been shut down? (can’t tell since my work server blocks a lot of web traffic – the problem might be on my end).
    Thanks

  6. What do you make of a range of dates that span almost a year? Example: June 2010-May 2011. Do think I can make the assumption the latest date is probably the highest zoom level (largest scale)? Thanks for the analyzer – it’s sweet!

    • Thanks!
      I don’t know about the date span – it’s the data that Microsoft / Bing attaches to the tile. Maybe they purposefully obfuscate the exact image date, maybe it’s lazyness on their part – we’d have to have them weigh in on it themselves.

  7. This is terrific! However, I wonder how accurate the dates are. I was browsing different satellite images through terraserver.com and it appears that the dates on the satellite images from the license holders could be inaccurate.

    I know that some of the dates terraserver is listing for images from Globexplorer are wrong. (e.g., image1 showing partial home construction with a date of 1/1/00, image2 showing empty lot with a date of 1/1/01, and image3 showing completed home with a date of 1/1/02.)

    Thank you again for this page. Either you\’ve managed to fly below the radar, or the licensing issues you were worried about weren\’t really issues at all. /knockonwood that your work continues to stay up.

    • I would like to include that the Bing Imagery appears to be higher quality than any other freely available imagery in the United States. The Bing imagery at one of our study sites in Chile is the not as crisp, but still the best resolution I have been able to find to date!

  8. Dear Martijn van Exel
    Congratulations for your excellent and very usefull work. Can you tell me if there is the possibility to hide the layer with the transparent tile showing the photo date?

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