[postgis-users] Off topic : Relative distance measurement

Øyvind Vestavik oyvindve at idi.ntnu.no
Thu Jan 25 06:03:11 PST 2007

```Thanks Ken.

I suspected this this was not a good idea :-) What I ended up doing was
setting the measure based on absolute distances, so if two points are
less than 200 m apart they get score 1.0, 200 - 400 meter apart -> 0.9
and so on.

ons, 24,.01.2007 kl. 11.11 -0800, skrev Ken Southerland:
> You seem to be making this harder than necessary.  First off is
> EquatorialLength the circumference of the earth?  If so, you are obviously
> using a number that is way too big for comparison and its not surprising that
> everything is near one.
>
> If EquatorialLength is a variable depending on the data, then use a smaller
> number and/or stretch the axis through some exponent. y = 1 - (x/C)^n
>
> However, if your EquatorialLength is constant for all calculations and if all
> you want to do is see if something is close then why not just decide what
> close enough is (which you would do anyway after transforming the number to
> something between one and zero only much more abstractly) and just check to
> see if distance() is less than that number or not.
>
> i.e.  If you decide that close enough to be the same place (given that it has
> the same names) is 100 meters than just say that anything less than 100
> meters is the same place and anything greater than 100 meters is not the same
> place.
>
> Ken
>
>
> On Wednesday 24 January 2007 9:53 am, Øyvind Vestavik wrote:
> > Sorry for posting off topic but I was wondering if anyone have any
> > thoughts on comparing two points using distance() or something similar
> > and then converting the retrieved distance into a value between 0 and 1
> > where 1 means that the points are the same and 0 means the points are
> > as far apart as possible.
> >
> > i tried something like the formula below, but unless using a lot of
> > desimals the metric will return 1 for all but very long distances.
> >
> > simGeo = 1 - (distance / (EquatorialLength / 2))
> >
> > The reason for this strange request is that I'm comparing gazetteer
> > entries from two different sources based on their names (implemented as
> > term vectors) and their locations (Points). The hypothesis is that if
> > the entries share many names and have Identical or near identical points
> > representing the place, they are the same place.
> > To have the metrics comparable they should both be a value between 0 and
> > 1, something like this.
> >
> > ((simVSP x weightVSP) + (simGeo x weightGeo))/2
> >
> > If anyone has any tips on how to transform for instance a number of
> > meters to a value between 0 and 1 ?
>
--
Kind Regards
Øyvind Vestavik

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