[OSM-talk] Aerial photo offsets

[Andrew Errington]

On Mon, November 7, 2011 11:09, mick wrote:
> On Sun, 6 Nov 2011 19:38:06 +0900
> Andrew Errington <a.errington at lancaster.ac.uk> wrote:
>
>
>> Hi everyone,
>>
>>
>> Forgive me if this has been covered elsewhere, but I am wondering what
>> can be done about the need for aerial photo offsets.  There's very
>> little in the wiki[1]
>>
>> In Korea (as elsewhere, I assume) the aerial photos are not always
>> accurately aligned with reality.  Across the country different offsets
>> are needed (sometimes no offset).  What I do is look for a set of GPS
>> traces from a very visible landmark and then use them to align the
>> aerials.  If there are none I will often find a visible landmark, such
>> as a park, or a running track in a stadium, and make several GPS traces
>> of my own on different days, so that I can use them to line up the
>> aerials.  I know that one trace is insufficient.
>>
> When I first got my GPS (Garmin GPS 60) I traced the fence lines at home.
> I found that the accuracy was not very good - between +- 12 to 30 metres
> so I traced the GPS unit on the top of each post so I could accurately
> repeat the logging then took readings each day, at different times, and
> plotted them in a CAD program with a circle centred on the reading with a
> radius of the error range. At the end of the month almost all the circles
> were reduced to under 1/2 a metre.
>
> I guess everyone knows this trick.

No, I don't think so, and that's the problem.  In fact, when I first
started tracing aerials I didn't realise that they might be offset.  I
thought that a huge company such as Yahoo! would have access to the
highest quality imagery (maybe they do, but we don't).  I made the same
assumption with Bing more recently, and I was disappointed to find I still
have to move the image layer before I can use it.

This is the reason that I suggested in my original message that mappers
should receive a warning when they turn on aerial imagery and try to trace
from it.  I realise that turning on the imagery is a required action in
JOSM, but it's automatic in Potlatch, so maybe Potlatch should always warn
when it is launched.

The problem is that when mappers start mapping solely from aerials without
adjusting them first they will do one of two things:
1) They will map clusters of buildings and roads in the wrong place, which
must later be moved, or
2) They start 'correcting' roads and buildings that are already mapped
accurately, but they move them to match the location on the aerial, which
of course is in the wrong place (see 1 above).

Either of these actions is counterproductive.  We should be harnessing
mappers' efforts efficiently.  No-one wants to re-map an area where
everything is 20m offset, and new mappers might feel they are wasting
their time when they see all of their contributions have been moved,
despite them taking the best care.

As we get more mappers who start using the easiest methods to get started
(tracing aerial imagery) we are going to see more problems, but I don't
know the best way to deal with it.  (Yes, I have just encountered exactly
this problem, and now I have to fix it).  (Yes, I know I don't *have* to
fix it, but I feel compelled to, especially as I have GPS data for a group
of streets nearby that I can't 'join up' because the roads they are
joining to are in the wrong place).

Best wishes,

Andrew