[Tagging] Housenumber interpolation with regularly skipped numbers

[Richard Bullock]

> On Sun, Oct 11, 2009 at 9:31 AM, Richard Bullock <rb357 at cantab.net> wrote:
>> I was replying to the assertion by Anthony that interpolation doesn't
>> specify which house numbers are present between two numbered nodes - just
>> that some unspecified number is present.
>
> I was wrong.  I assumed the word "interpolation" was being used
> properly.  It isn't.
>
When I say "interpolation" I of course mean, when I use the tag 
addr:interpolation=foo

But we are using interpolation in this case to determine approximate 
positions of houses without needing to put each house in as a separate node. 
We (I've seen at least one website actually doing this) are using linear 
interpolation between house number a and house number a+b to determine that 
house number a+c (where 0<=c<=b) lies approximately c/b of the distance down 
the interpolation way from house number a.

>> If you type in a house number on the same street that does exist, it
>> takes you to the interpolated position of that house.
>Why do you think that's an interpolated position?

On Google Maps;

When you type in, say, 2 Lindow Fold Drive, the marker points you to the end 
of the road, not at the house itself - look at the Satellite photography.

When you type in 4 Lindow Fold Drive, the marker moves down the road, still 
not pointing at the house

...

When you type in 30 Lindow Fold Drive, the marker is close to the other end 
of the road - still not pointing directly at the house.

Try typing in 16 or 18, and the marker becomes a circle and does not point 
to anywhere on the ground.

I'm not saying they are definitely using something similar to us - but the 
fact that the lowest number points to the end of the road, and the higher 
numbers you input get you further down the road - and yet don't actually 
point to the correct house directly - suggests that they may have done some 
linear interpolation as well.

The fact that 16 or 18 do not give you positions on the ground are strongly 
suggestive that Google know that these house numbers are missing.