[postgis-users] Check identical point

[Nicklas Avén]

Hallo

The ST_DWithin fuction will only check the 2d distance and not care it the points are far from each other in z direction.

If you need to also check they are close in z-direction I guess the easiest way is to calculate the 3d distance between the points with Pythagorean theorem. The cleanest maybe is to make a function of it like this:


create or replace function pointdistance3d(p1 geometry, p2 geometry)
returns double precision as
$$
select sqrt((st_x($1)-st_x($2))^2 + (st_y($1)-st_y($2))^2 + (st_z($1)-st_z($2))^2);
$$
language 'sql';


then to get some speed of it you can use st_dwithin to do a first index-based search. That is as said only a 2d calculation, but if the distance is more than your tolerance in 2d it will be also in 3d.

I think the most effective way of doing this is to only do the bounding box comparison that is built into st_dwithin and then go straightly to the 3d calculation. That would look something like this if we use the tolerance 0.1 meters (requires based coordinate sytem):

Select * from table1, table2 where table1.the_geom && ST_Expand(table2.the_geom, 0.1) and
table2.the_geom && ST_Expand(table1.the_geom, 0.1) and pointdistance3d<=0.1;

Hope that helps

Nicklas


Try:
>   ST_DWithin(point1, point2, 0.00001) 
>
>-F
>
>>
2010/5/22 eehab hamzeh <eehab40 at hotmail.com>
>>
Hello 
>
>I want to check if two points are identical, the points has x,y,z coordinates and they are not exactly the same, i need to check the intersection between them with tolerance value. any direction of how to do that.
>
>Thank 
>
>kind regards
>>

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