OpenStreetMap
Yahoo imagery less precise than multiple GPS tracks
Posted by Jean-Marc Liotier on 18 November 2009 in English.I have been busy mapping along the Petite Cote in Senegal. I have found numerous occurrences of Yahoo sourced roads with multiple concurring GPS tracks lying some distance away - clearly showing that the road is not following the correct path. I corrected them with the more precise path. That shows that when multiple GPS tracks are available and concur, they should be relied upon more than third party sources.
Discussion
Comment from ctheile on 18 November 2009 at 15:05
Salu Jean
J'ai "mappé" quelques routes dans la petite cote et quand je suis pas trops paresseux je continue de la faire. J'ai corrigé déjà quelques routes (p.ex. saly- ngaparou- somone. Lesquelles routes to sont faux selon ton GPS ?
Cordialement
Christian
Comment from andrewpmk on 18 November 2009 at 15:38
Generally this is a result of misaligned Yahoo imagery. To align it to GPS press space+arrow keys (in Potlatch) or use the little move WMS button (in JOSM). Usually once the Yahoo imagery is aligned properly, you can rely on it for tracing. In some cases Yahoo is spot on, in other cases it needs a bit of manual adjusting.
Comment from Jean-Marc Liotier on 18 November 2009 at 17:16
@ctheile - La N1 et quelques un de ses affluents à l'ouest de Sinndia étaient légèrement à côté. Rien de grave, et j'ai faites les corrections...
Comment from Jean-Marc Liotier on 18 November 2009 at 17:18
@andrewpmk - Thanks for the tip : indeed some of the cases encountered were slight misalignment of a whole stretch and would have benefited from the method you suggest. Other cases were simply were the definition of the GPS tracks was greater than the definition of roads traced from Yahoo imagery which was otherwise correct.
Comment from Anna_AG on 18 November 2009 at 20:52
I can only partially agree with your statement that multiple GPS traces are more
accurate that some satellite imagery.
Mapping even medium density suburban areas with high resolution imagery is
generally more accurate than GPS, and allows up to add significantly more
detail than any GPS survey could complete. If you think about it, there are
only two errors the picture can have, one of position and of scale. Assuming
that the GPS trace confirms that the Image is correct on the 4 corners of a
chosen square area, the rest in the middle must be pretty much right, or at
least to acceptable tolerances.
My recent experience of mapping Kaduna served as a good example of this.
Kaduna was a town I used to visit frequently some years ago, then I noticed
that it was in high res Yahoo imagery. A number of OSMers started contributing
and generated the map you see below
http://www.openstreetmap.org/?lat=10.5221&lon=7.4339&zoom=13&layers=B000FTF
Recently I spent three weeks there and devoted my spare time to adding the
details such as street names and amenities. My GPS traces confirmed that
there was no significant error in the positioning of the Sat imagery over
the scale of the town.
However what it did teach me was that there would have been absolutely
no way to have mapped this medium sized town extensively on GPS alone.
There is still much work to be done on Kaduna and only limited resources.
GPS traces give us excellent information for areas not imaged and for
assistance in adding the extra information such as amenties, on the ground
surveying and streetnames. But for high detail work the image is essential.
This essentailly explains why Abuja and Lagos, the capitals of Nigeria
are not on the map yet.
Cheers b
Comment from Jean-Marc Liotier on 18 November 2009 at 21:27
@brig Thanks - I just discovered the use of the Yahoo imagery underlay, and I agree that it is very practical for mapping my city where high resolution imagery is available. Indeed I would not have been able to achieve this level of detail with my 1/15 Hz GPS logger. The case I reported was a rural area where no high resolution imaging was available - in that case the GPS traces seem to provide better resolution, even though the roads traced from Yahoo imagery are most often correct if slightly offset.
Comment from HannesHH on 19 November 2009 at 20:24
Yes, be careful. Yahoo can be terribly inaccurate even in big european cities.