<HTML dir=ltr><HEAD><TITLE>Re: [postgis-devel] Dropped DM (Time) dimension with intersections</TITLE>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Which begs the question - what exactly does this tell me?</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>SELECT ST_AsEWKT(ST_ConvexHull(ST_Collect(ST_GeomFromEWKT('LINESTRING(1 2 3,3 4 5, 3 6 7)'), ST_MakePoint(0, 2, 0))));<BR></FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2>Result</FONT></DIV>
<DIV dir=ltr><FONT face=Arial size=2>"POLYGON((0 2 0,3 6 7,3 4 5,1 2 3,0 2 0))"</FONT></DIV>
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<FONT face=Tahoma size=2><B>From:</B> postgis-devel-bounces@postgis.refractions.net on behalf of Obe, Regina<BR><B>Sent:</B> Tue 10/7/2008 1:51 PM<BR><B>To:</B> PostGIS Development Discussion<BR><B>Subject:</B> RE: [postgis-devel] Dropped DM (Time) dimension with intersections<BR></FONT><BR></DIV>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Paul,</FONT></DIV>
<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Ah nevermind I think I'm beginning to understand what you are saying. A LINESTRING is well-defined I think if you have varying altitudes correct, but a Polygon with varying altitudes is not well defined, since its unclear what area is covered.</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>I wish I still had my chemistry set to help me visualize these concepts.</FONT></DIV>
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<DIV dir=ltr><FONT face=Tahoma size=2><B>From:</B> Obe, Regina<BR><B>Sent:</B> Tue 10/7/2008 1:31 PM<BR><B>To:</B> PostGIS Development Discussion<BR><B>Subject:</B> RE: [postgis-devel] Dropped DM (Time) dimension with intersections<BR></FONT><BR></DIV></DIV>
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<DIV dir=ltr><FONT face=Arial color=#000000 size=2>Paul,</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>I'm a little confused about your statement. I guess I haven't thought about 3D stuff all that much, but are you saying if you have varying Z of a polygon or line it has no meaning or ambiguous meaning?</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>To me its a 2D object in 3D space and if I glue each of those faces together, I could get what would appear to be a volumetric thing - a wireframe if you will. My visualization abilities are kind of weak though but to me something like</FONT></DIV>
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<DIV dir=ltr><FONT face=Arial size=2>LINESTRING(1 2 3,3 4 5)</FONT></DIV><FONT face=Arial size=2></FONT></DIV>
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<DIV dir=ltr>makes sense and woud represent a line not completely horizontal relative to Z?<BR></DIV></FONT></DIV>
<DIV dir=ltr>Thanks,</DIV>
<DIV dir=ltr>Regina<BR></DIV>
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<DIV dir=ltr><FONT face=Tahoma size=2><B>From:</B> postgis-devel-bounces@postgis.refractions.net on behalf of Paul Ramsey<BR><B>Sent:</B> Tue 10/7/2008 12:59 PM<BR><B>To:</B> PostGIS Development Discussion; PostGIS Users Discussion<BR><B>Subject:</B> Re: [postgis-devel] Dropped DM (Time) dimension with intersections<BR></FONT><BR></DIV>
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<P><FONT size=2>Unfortunately, there's no such thing as a 3D polygon, except for<BR>trivial cases (the triangle, the shape with all Z's the same).<BR>Everything else is unclear on how to interpret the enclosed "plane"<BR>(if that is what it is) formed by an irregularly elevated boundary. So<BR>we can store the things, but there's really no decent way to interpret<BR>them in generality. For that we need the real stuff, Surfaces,<BR>volumes, etc.<BR><BR>I think the "low hanging fruit" is probably more the "infrastructural<BR>requirement". We need a 4D index. That will allow us to handle things<BR>like 4D time tracks and point clouds efficiently, and form the<BR>indexing basis for future volumetric objects.<BR><BR>P.<BR><BR>On Tue, Oct 7, 2008 at 9:44 AM, Obe, Regina <robe.dnd@cityofboston.gov> wrote:<BR>> I'm not sure how low hanging the fruit :), but first off would be being able<BR>> to do intersections and indexable ST_DWithin with 3D polygons and<BR>> linestrings and so forth. For example when I place a cable up on a roof I<BR>> need to know if I'm hitting another piece of equipment.<BR>><BR>> Higher fruit - being able to support volumetric geometries. Right now we<BR>> support 2d-3D polygons and lines and you can form wireframes with those, but<BR>> no true volumetric stuff. But then what do I know, I'm just parroting<BR>> things I've heard in whispers and those whispers are getting louder is all<BR>> :)<BR>><BR>> There is still the issue of being able to display 3-D geometries without<BR>> spending a fortune on proprietary stuff which is not a PostGIS issue, but<BR>> has to gain in momentum to make PostGIS 3D more powerful (e.g. uDig for 3D<BR>> or OpenJump for 3D or OpenLayers for 3D?)<BR>><BR>> Thanks,<BR>> Regina<BR>> ________________________________<BR>> From: postgis-devel-bounces@postgis.refractions.net on behalf of Paul Ramsey<BR>> Sent: Tue 10/7/2008 12:30 PM<BR>> To: PostGIS Development Discussion<BR>> Subject: Re: [postgis-devel] Dropped DM (Time) dimension with intersections<BR>><BR>> Perhaps elabourate on what better 3D support would be? There's the<BR>> surface object hanging around. There's the issue of maintaining higher<BR>> dimensional coordinates through lower dimensional transforms (which<BR>> you saw the result of a few days ago). There's elabourating the<BR>> complete set of 3D objects and relationships (gulp).<BR>><BR>> It's not clear to me what is the "low hanging fruit" that will make 3D<BR>> users happiest in the shortest time.<BR>><BR>> P.<BR>><BR>> On Tue, Oct 7, 2008 at 8:54 AM, Obe, Regina <robe.dnd@cityofboston.gov><BR>> wrote:<BR>>> Margie,<BR>>><BR>>> Unfortunately I think the answer is no. Most of the work going on in 1.4<BR>>> is<BR>>> to improve speed of existing functionality and reorganize the source to<BR>>> make<BR>>> it more maintainable.<BR>>><BR>>> Someone please correct me if I am wrong.<BR>>><BR>>> I for one would be very elated if we had better 3D support since CityGML<BR>>> and<BR>>> similar initiatives are becoming more of a hot topic around here.<BR>>><BR>>> Thanks,<BR>>> Regina<BR>>> ________________________________<BR>>> From: postgis-devel-bounces@postgis.refractions.net<BR>>> [<A href="mailto:postgis-devel-bounces@postgis.refractions.net">mailto:postgis-devel-bounces@postgis.refractions.net</A>] On Behalf Of<BR>>> Huntington, Margaret (US SSA)<BR>>> Sent: Tuesday, October 07, 2008 11:47 AM<BR>>> To: postgis-devel@postgis.refractions.net<BR>>> Subject: [postgis-devel] Dropped DM (Time) dimension with intersections<BR>>><BR>>> Hello,<BR>>><BR>>> Currently I'm using PostGIS 1.3.3. From past discussions and from<BR>>> testing, both the st_intersection st_extend methods return 2D results with<BR>>> 4D input geometries. As a temporary work-around, I had hoped<BR>>> st_intersection might work with 3DM geometries. (Plan was to interpolate<BR>>> the altitude value within the function call if PostGIS could calculate the<BR>>> time dimension). I found time components are also dropped by<BR>>> st_intersection with 3DM geometries. I abandoned usage of 4D bounding<BR>>> boxes<BR>>> since these too effectively degrade 4D geometries down to 2D geometries<BR>>> (altitude and time are zeroed out).<BR>>><BR>>> polyGeometry geometry;<BR>>><BR>>> bbGeometry geometry;<BR>>><BR>>> intersectionGeometry geometry;<BR>>><BR>>> coorddims smallint;<BR>>><BR>>> -- both polygon and linestring have an expected zmflag value of<BR>>> 1<BR>>><BR>>> polyGeometry := 'SRID=4326;POLYGONM((0 0 0, 0 10 4, 10 10 4, 10<BR>>> 0<BR>>> 0, 0 0 0))'::geometry;<BR>>><BR>>> bbGeometry := 'SRID=4326;LINESTRINGM(0 0 1.5, 10 10<BR>>> 2)'::geometry;<BR>>><BR>>> intersectionGeometry := st_intersection(polyGeometry,<BR>>> bbGeometry);<BR>>><BR>>> -- st_intersection method drops the time dimension; zmflag<BR>>> value<BR>>> of 0<BR>>><BR>>> coorddims := st_zmflag(intersectionGeometry);<BR>>><BR>>> If I were to download the subversion snapshot, the current 1.4 version,<BR>>> might st_intersection work with either 3DM or 4D geometries? Are bounding<BR>>> boxed or st_extend improved for either 3DM or 4D geometries? I had<BR>>> incorporated polygons only as a possible work-around to the bounding box<BR>>> and<BR>>> st_extend 2D limitations.<BR>>><BR>>> Margie<BR>>><BR>>> ________________________________<BR>>><BR>>> The substance of this message, including any attachments, may be<BR>>> confidential, legally privileged and/or exempt from disclosure pursuant to<BR>>> Massachusetts law. It is intended solely for the addressee. If you<BR>>> received<BR>>> this in error, please contact the sender and delete the material from any<BR>>> computer.<BR>>><BR>>> ________________________________<BR>>><BR>>> Help make the earth a greener place. If at all possible resist printing<BR>>> this<BR>>> email and join us in saving paper.<BR>>><BR>>> _______________________________________________<BR>>> postgis-devel mailing list<BR>>> postgis-devel@postgis.refractions.net<BR>>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-devel">http://postgis.refractions.net/mailman/listinfo/postgis-devel</A><BR>>><BR>>><BR>> _______________________________________________<BR>> postgis-devel mailing list<BR>> postgis-devel@postgis.refractions.net<BR>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-devel">http://postgis.refractions.net/mailman/listinfo/postgis-devel</A><BR>><BR>> ________________________________<BR>><BR>> The substance of this message, including any attachments, may be<BR>> confidential, legally privileged and/or exempt from disclosure pursuant to<BR>> Massachusetts law. It is intended solely for the addressee. If you received<BR>> this in error, please contact the sender and delete the material from any<BR>> computer.<BR>><BR>> ________________________________<BR>><BR>> Help make the earth a greener place. If at all possible resist printing this<BR>> email and join us in saving paper.<BR>><BR>> _______________________________________________<BR>> postgis-devel mailing list<BR>> postgis-devel@postgis.refractions.net<BR>> <A href="http://postgis.refractions.net/mailman/listinfo/postgis-devel">http://postgis.refractions.net/mailman/listinfo/postgis-devel</A><BR>><BR>><BR>_______________________________________________<BR>postgis-devel mailing list<BR>postgis-devel@postgis.refractions.net<BR><A href="http://postgis.refractions.net/mailman/listinfo/postgis-devel">http://postgis.refractions.net/mailman/listinfo/postgis-devel</A><BR></FONT></P></DIV></DIV>
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