[OSM-talk] Map search accuracy

[David Earl]

On 20/12/2007 15:06, Tom Hughes wrote:
> In message <yek8x3pv2is.fsf at dellow.uk.cyberscience.com>
>         Tom Hughes <tom at compton.nu> wrote:
> 
>> In message <1198161611.5262.96.camel at tomchancepc.BIOREG.LOCAL>
>>         Tom Chance <tom at acrewoods.net> wrote:
>>
>>> I've noticed that searching for small features exposes a slight
>>> inaccuracy with the placement of the red arrow. For example, searching
>>> for "Edgecombe House" and then zooming in twice shows this:
>>>
>>> http://www.openstreetmap.org/?lat=51.470354&lon=-0.085346&zoom=18&layers=B0T
>>>
>>> Which is not exactly far off, but then it is pointing at the wrong
>>> building so if you followed the arrow at the default zoom level from the
>>> search link (which doesn't show building names to help you spot the
>>> mistake) you'd end up in the wrong place!
>> Well the first thing you need to do is to work out what part of
>> the tool chain is at fault - are those the correct coordinates
>> or not is the first question?
> 
> Based on this:
> 
>   http://maps.google.co.uk/maps?f=q&hl=en&geocode=&time=&date=&ttype=&q=51.470354,-0.085346&ie=UTF8&ll=51.470357,-0.086893&spn=0.00403,0.013036&t=h&z=17&om=1
> 
> it looks like the we are pointing at the coords right, and the problem
> is the location returned by the namefinder.

Namefinder chooses the location by taking the lat/lon of the middle node 
of the way. No processing is done on it. The node it has chose for this 
way is the northernmost corner. If you get the building in JOSM and look 
at the points you can see that the latitude and longitude are identical 
to those of this point (to 6 dp anyway).

The arrow is then offset a small amount upwards from the node in order 
that it doesn't get in the way of most captions (eg. pub name, street name).

So I think it is doing what I intended in this case.

Possibly for areas I should take the centre of gravity (though this 
means doing a lot more processing of tags to derive what ways are areas, 
and centre of gravity is not ideal for less regular shaped areas - same 
issue as people have been talking about re the parking symbol recently).

David