Yahoo! Aerial Imagery/Accuracy

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This page describes Yahoo! Aerial Imagery but all of the points will also apply to bing (which these days we are mostly using instead of Yahoo). We have a page Using Imagery which covers the points more generally. TODO:merge content to there

The Yahoo! Aerial Imagery is offset from reality in places. Every imagery source has a limit to its accuracy, and Yahoo imagery is no exception, however consumer grade GPS units certainly have accuracy limits too. If you think you've discovered an area where yahoo imagery is offset from reality, please bear in mind first of all that it's actually far more likely that one GPS trace is wrong! Consumer GPS units will often record tracks with an offset from reality. Recording several tracks on different occasions might allow you to diagnose a yahoo accuracy problem however. You shouldn't ignore this, but try to move to yahoo imagery in the editor to better suit the recorded tracks and existing data.

This generally isn't a high priority problem to try and tackle. Initially when looking at mapping an area, it's fine to assume that the Yahoo imagery is placed accurately (as long as there is not already mapping data and GPS-traces in OSM to verify against), and there's lots of work to do, in the U.S. for example, lining up TIGER data with Yahoo.

Mapping with an offset

We could aim to use multiple GPS readings to determine what offset Yahoo imagery has, and it's worth noting that local people may have already attempted this. If you are looking at an area which has already had some work done on it, you should probably think twice before moving all the roads onto the yahoo imagery.

The idea is to use your GPS traces, and any other GPS traces uploaded in the area. If there are several of them along the same road, and the traces seem trustworthy (for example: no significant wobbling on straight roads) then supposedly you have found that Yahoo imagery is offset. At this point you can then re-align the imagery layer onto those traces as follows:

  • In Potlatch you can move the background by holding space and dragging the background
  • In JOSM you can move the image layer by selecting it and choosing the Adjust tool Adjust WMS-Layer.png: then drag the imagery to fit the traces.

Examples of inaccurately placed yahoo imagery

Please link examples here. Link to the potlatch edit view, where we can see the imagery and then show your GPS tracks (Upload them) You should have several GPS tracks recorded on different occasions.