[postgis-devel] Dropped DM (Time) dimension with intersections
Obe, Regina
robe.dnd at cityofboston.gov
Thu Oct 9 05:17:10 PDT 2008
I've been giving this some thought in my sleep and my assumptions are
probably wrong. Just thinking out loud.
It just occurred to me, correct me if I am wrong, there is no real
mathematical difference between x y, and x z and
y z. We have as far as I am concerned arbitrarily chosen to call our
flattened world the x y plane.
That would mean any shape that is a parallel to a plane we can deal with
is valid. Getting back to your polygon with all Z's the same, it can be
in fact all x's the same, all y's the same and I think numerous others
I'm too stupid to visualize, which to me covers an awful lot of use
cases. Hmm I really need to get another chemistry set.
Now as a stepping stone to having a true 4D index, is it simpler to
have a 3 Rtree indexes (x y, x z, y z) - so an intersection in all would
imply intersection. Of course I guess this doesn't deal with polygons
that are not parallel to an index plane or maybe it does.
Thanks,
Regina
-----Original Message-----
From: postgis-devel-bounces at postgis.refractions.net
[mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of Paul
Ramsey
Sent: Tuesday, October 07, 2008 1:00 PM
To: PostGIS Development Discussion; PostGIS Users Discussion
Subject: Re: [postgis-devel] Dropped DM (Time) dimension with
intersections
Unfortunately, there's no such thing as a 3D polygon, except for
trivial cases (the triangle, the shape with all Z's the same).
Everything else is unclear on how to interpret the enclosed "plane"
(if that is what it is) formed by an irregularly elevated boundary. So
we can store the things, but there's really no decent way to interpret
them in generality. For that we need the real stuff, Surfaces,
volumes, etc.
I think the "low hanging fruit" is probably more the "infrastructural
requirement". We need a 4D index. That will allow us to handle things
like 4D time tracks and point clouds efficiently, and form the
indexing basis for future volumetric objects.
P.
On Tue, Oct 7, 2008 at 9:44 AM, Obe, Regina <robe.dnd at cityofboston.gov>
wrote:
> I'm not sure how low hanging the fruit :), but first off would be
being able
> to do intersections and indexable ST_DWithin with 3D polygons and
> linestrings and so forth. For example when I place a cable up on a
roof I
> need to know if I'm hitting another piece of equipment.
>
> Higher fruit - being able to support volumetric geometries. Right now
we
> support 2d-3D polygons and lines and you can form wireframes with
those, but
> no true volumetric stuff. But then what do I know, I'm just parroting
> things I've heard in whispers and those whispers are getting louder is
all
> :)
>
> There is still the issue of being able to display 3-D geometries
without
> spending a fortune on proprietary stuff which is not a PostGIS issue,
but
> has to gain in momentum to make PostGIS 3D more powerful (e.g. uDig
for 3D
> or OpenJump for 3D or OpenLayers for 3D?)
>
> Thanks,
> Regina
> ________________________________
> From: postgis-devel-bounces at postgis.refractions.net on behalf of Paul
Ramsey
> Sent: Tue 10/7/2008 12:30 PM
> To: PostGIS Development Discussion
> Subject: Re: [postgis-devel] Dropped DM (Time) dimension with
intersections
>
> Perhaps elabourate on what better 3D support would be? There's the
> surface object hanging around. There's the issue of maintaining higher
> dimensional coordinates through lower dimensional transforms (which
> you saw the result of a few days ago). There's elabourating the
> complete set of 3D objects and relationships (gulp).
>
> It's not clear to me what is the "low hanging fruit" that will make 3D
> users happiest in the shortest time.
>
> P.
>
> On Tue, Oct 7, 2008 at 8:54 AM, Obe, Regina
<robe.dnd at cityofboston.gov>
> wrote:
>> Margie,
>>
>> Unfortunately I think the answer is no. Most of the work going on in
1.4
>> is
>> to improve speed of existing functionality and reorganize the source
to
>> make
>> it more maintainable.
>>
>> Someone please correct me if I am wrong.
>>
>> I for one would be very elated if we had better 3D support since
CityGML
>> and
>> similar initiatives are becoming more of a hot topic around here.
>>
>> Thanks,
>> Regina
>> ________________________________
>> From: postgis-devel-bounces at postgis.refractions.net
>> [mailto:postgis-devel-bounces at postgis.refractions.net] On Behalf Of
>> Huntington, Margaret (US SSA)
>> Sent: Tuesday, October 07, 2008 11:47 AM
>> To: postgis-devel at postgis.refractions.net
>> Subject: [postgis-devel] Dropped DM (Time) dimension with
intersections
>>
>> Hello,
>>
>> Currently I'm using PostGIS 1.3.3. From past discussions and
from
>> testing, both the st_intersection st_extend methods return 2D results
with
>> 4D input geometries. As a temporary work-around, I had hoped
>> st_intersection might work with 3DM geometries. (Plan was to
interpolate
>> the altitude value within the function call if PostGIS could
calculate the
>> time dimension). I found time components are also dropped by
>> st_intersection with 3DM geometries. I abandoned usage of 4D
bounding
>> boxes
>> since these too effectively degrade 4D geometries down to 2D
geometries
>> (altitude and time are zeroed out).
>>
>> polyGeometry geometry;
>>
>> bbGeometry geometry;
>>
>> intersectionGeometry geometry;
>>
>> coorddims smallint;
>>
>> -- both polygon and linestring have an expected zmflag
value of
>> 1
>>
>> polyGeometry := 'SRID=4326;POLYGONM((0 0 0, 0 10 4, 10 10
4, 10
>> 0
>> 0, 0 0 0))'::geometry;
>>
>> bbGeometry := 'SRID=4326;LINESTRINGM(0 0 1.5, 10 10
>> 2)'::geometry;
>>
>> intersectionGeometry := st_intersection(polyGeometry,
>> bbGeometry);
>>
>> -- st_intersection method drops the time dimension; zmflag
>> value
>> of 0
>>
>> coorddims := st_zmflag(intersectionGeometry);
>>
>> If I were to download the subversion snapshot, the current 1.4
version,
>> might st_intersection work with either 3DM or 4D geometries? Are
bounding
>> boxed or st_extend improved for either 3DM or 4D geometries? I had
>> incorporated polygons only as a possible work-around to the bounding
box
>> and
>> st_extend 2D limitations.
>>
>> Margie
>>
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