
Geometry : 
Subject: Re: [geometry] distance between geometries
From: Adam Wulkiewicz (adam.wulkiewicz_at_[hidden])
Date: 20130902 15:29:52
Hi,
Barend Gehrels wrote:
> On 292013 0:04, Adam Wulkiewicz wrote:
>>
>> Initially I thought about providing ray queries. Those queries return
>> some number of objects hit by a ray.
>>
>>
>>
>>
>> Because we don't have a Ray concept I decided to use Segment and by
>> extension Linestring. I thought about a real intersection because
>> this algorithm will probably be the fastest one but more general
>> variations could also be useful. I'd like to allow users using any
>> algorithm they want in a strategy of the nearest() predicate. More
>> algorithms means more possibilities.
>>
>> Btw, Is this what you've thought?
>>
>>
>
>
> Yes, based on your proposal, I think that intersecting and objects
> close by are both interesting. But if you want the first 5, you might
> get ambiguities (closer to starting point, or closer to the line...)
>
Yes, this is why I'd like to allow users define their strategies and
provide some readytouse common ones. The default one would use
bg::comparable_distance so wouldn't take into account the position on
the line.
>> Yes, the knowledge about the position of a measurepoint in some
>> cases is useful, e.g. in the Linestring example above where the
>> position of a closest point must be calculated and then it's distance
>> from the beginning of the Linestring.
>>
>> I thought about something related to this and nearest predicate. If
>> we have two points inside a Polygon or Two Polygons intersecting one
>> Point, which one is the closest?
>>
>>
>>
>> If I remember correctly, currently I'm using distance = 0 if the
>> Point is inside (intersecting) the Box but one could also say that
>> the closest one is the one nearest to the centroid or further of the
>> border or even something else.
>
> I see, so this is another sort of ambiguity indeed. I don't know what
> users expect if they ask for the 5 nearest points and there are 10
> inside. But using the centroid is an interesting solution. However, if
> two points are located on the same distance (w.r.t. centroid), you can
> still get an arbitrary choice... I don't think that is really
> problematic, though I'm quite sure some users want to have a
> consistent choice in these cases.
>
Exactly my way of thinking. Thanks.
Best regards,
Adam
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