[postgis-users] Same, similar et al

[Stephen Woodbridge]

On 5/9/2012 12:06 PM, Bob Pawley wrote:
> Hi Steve
>
> FYI - I'm not involved in the geography side of Postgis, I'm developing
> an application to analyze engineering drawings.

I spend 20+ yrs of my career working with Computervision CAD/CAM 
systems, most of that at Computervision in development, so I know a lot 
about mechanical drawing, modeling, drafting, and machining.

> So, same in shape even
> if the shapes are spatially diverse is what I was looking for.

OK, but most of my questions are still valid.

* same shape but different location?
* same shape but scaled, different location?
* same shape but rotated about Z at different location scaled or not?

* don't care about topology

* "same shape" - implies same construction in terms of entities make up 
the shape. So:

+-----+-----+ is different from +-----------+ ?

because of the extra node or not?

You might find that is you normalize all shapes into 1.0 x 1.0 unit box 
and then do the comparison that this might simplify differences due to 
scale.

-Steve W

> However, since this function doesn't exist I will need to develop
> something.
>
> Thanks for the info.
>
> Bob
>
>
> -----Original Message----- From: Stephen Woodbridge
> Sent: Wednesday, May 09, 2012 8:54 AM
> To: postgis-users at postgis.refractions.net
> Subject: Re: [postgis-users] Same, similar et al
>
> On 5/9/2012 11:40 AM, Bob Pawley wrote:
>> Hi
>> I have two geoms that are similar in shape but don’t share the same
>> space nor the same size.
>> Is there a function or method to compare the two shapes in this
>> situation??
>> The functions and operators I have found, seem to only look at spatial
>> sameness.
>> Or is my only option to spatially transfer and size one geometry to the
>> other?
>> Bob
>
> Bob,
>
> I don't think there is anything in PostGIS that does this. A bigger
> question in my mind is how do you want to define "similar"? For example:
>
> 1. identical but scaled differently?
> 2. identical but positioned or rotated differently?
> 3. topologically the same, ie: a coffee mug and donut have the same
> topology but not the same shape
> 4. the same visual presentation but made up of different number of segments
> 5. and the list goes on and on
>
> -Steve W
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