[OSM-newbies] Systematic shift between streets and aerial photo

[Andrew Errington]

On Sat, 31 May 2014 18:14:49 Kjell Inge Tomren wrote:
> Hello!
> 
> I noticed there was a street name missing near my home and as my first edit
> i decided to add that name to OSM. The update was easy. I even did it from
> my tablet in bed this morning so i was inspired to do more. There are no
> numbers on the houses in my area, so i decided to register my own and I
> did it by pointing at my house in the background aerial photo and added
> the info for a house. But I noticed the position of the house was not
> correct vs the nearby streets.
> 
> After a closer look I noticed that there is a systematic shift between the
> streets in my area and the background aerial photo. Then I´m a bit stuck.
> Which is correct positioned? The street positions or the background aerial
> photo? How is it possible to correct a systematic error like this?
> 
> Here is a link to the area:
> https://www.openstreetmap.org/edit#map=18/62.45124/6.36599
> 
> Best regards,
> Kjlell Inge Tomren

Generally it's the aerial photo that is in the wrong place.  They are not 
always properly aligned, and even if they are, the alignment sometimes changes 
when a new set of aerial photos is released.

If there are any GPS traces visible in your area then you can use them to add 
roads and also to realign the aerial photos with 'real life'.  Note that GPS 
accuracy is also not perfect, but if you have enough traces you can see 
clearly where the roads actually are.

I use JOSM and there is an option for sliding the aerial photos around under 
the map layer, and for saving the offset position.  Other editors have similar 
options.

Glad you got started with OSM.  Please keep mapping, and have fun!

Best wishes,

Andrew