Kevin, Thanks. This and Paul's comments helps to weight the pros and cons.
Well unfortunately I don't control the client application - but it will be the same client application that is always used and since a lot of the editing is done via javascript (custom open layers and GeoServer mix I think - I'll have to find out more of the details of that to better weigh the ramifications). I'm not quite sure how easy it will be to swing a WKB editor in there or if that would be overkill. It is not high precision stuff and for the most part I think all the transformations have already been done that will be. Thanks, Regina -----Original Message----- From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of Kevin Neufeld Sent: Monday, May 19, 2008 10:57 PM To: PostGIS Users Discussion Subject: Re: [postgis-users] Does ST_AsText truncate or round Hi Regina, Sorry I didn't respond sooner ... I was out camping for the weekend. Fun, fun :) Right, this is what I was thinking with my previous post recommending snaptogrid - force your geometries through WKT : geomfromtext(astext(the_geom)). Essentially, this would make all your geometries only as precise as their WKT representation. In a way, this enforces a very fine precision model on all your geometries. This should elicit the data consistency you were after ... assuming of course that all your table edits are performed using geomfromtext(). Bear in mind, though, that if you have a client program that modifies the table using WKB, you may be left with data in an inconsistent state. If this is the case, put a simple trigger on your tables forcing all UPDATES and INSERTS through WKT. I agree with Paul R, however, that a client application should not be transiting data using WKT, but rather WKB. It may take some work, but I would recommend that you upgrade your apps to perform all data edits using WKB. He was right when he mentioned that I fought with this for a while. I had a large database that utilized the full precision of PostGIS. I did some data editing using a simple WKT editor and suddenly found that geometries that used to touch no longer touch or they intersect incorrectly. I guess it boils down to: * A database that does not enforce any precision model, by default uses full precision - thus, all data edits should be done using full precision. * A database that enforces a precision model (including the one you suggested where X = GeomFromText(AsText(X)) must be true), could be edited using a editor with an equivalent precision model. In your case, a WKT enforced database could be edited using a WKT editor. However, if you can't guarantee that all editing is performed using WKT (including updating a geometry using transform() ), then triggers should probably be constructed enforcing the model on your spatial tables. Cheers, Kevin Paragon Corporation wrote: > Kevin, > > Actually this would help me with the second phase, but for the first > phase I just need consistency which I was hoping AsText would do for > me. It doesn't even need to be that good since I'm sure the other > side is using AsText in some fashion. So I just need consistency more than correctness. > > Where I start off with > > Orginal geometry X > > I think I need to guarantee that > > X = GeomFromText(AsText(X)) > > Seemed like the easiest way to assure that was to just force all my Xs to X > = ST_GeomFromText(AsText(X), somesrid) which I presume should be the same > as > > ST_SetSRID(AsText(X), somesrid) > > The back end editor I'm assuming is just taking the AsText(geometry) > representation and just returning that back when no edits on a > particular point, and so its being poisoned by its own database > query. I'm not working on that part so that's just my general observation. > > Thanks, > Regina > > -----Original Message----- > From: [EMAIL PROTECTED] > [mailto:[EMAIL PROTECTED] On Behalf Of > Kevin Neufeld > Sent: Friday, May 16, 2008 11:15 AM > To: PostGIS Users Discussion > Subject: Re: [postgis-users] Does ST_AsText truncate or round > > Have you considered using ST_SnapToGrid() and make all geometries in > you database respect a specified tolerance? > > Warning, snaptogrid may not behave as you might expect - this is not a > simple precision_reducer method (which I think would be valuable). > > Consider: > select > 'POINT(0.877 0.778)'::geometry ~= > snaptogrid('POINT(0.87689017271623 0.777984935324639)'::geometry, > 0.001); > > ?column? > ---------- > t > (1 row) > > No problem there, but then there's this: > select > 'POINT(0.956 0.66)'::geometry ~= > snaptogrid('POINT(0.956079571507871 0.659502930939198)'::geometry, > 0.001); > > ?column? > ---------- > f > (1 row) > > > To deal with this (I know it looks like a hack, but it works), I > convert to text and then back to a geometry. Wrapping this in a > trigger on your geometry tables will guarantee any geometry edits are > automatically reduced to a certain precision. > > select > 'POINT(0.956 0.66)'::geometry ~= > geomfromtext(astext(snaptogrid('POINT(0.956079571507871 > 0.659502930939198)'::geometry, 0.001)), -1); > > ?column? > ---------- > t > (1 row) > > > Cheers, > Kevin > > Paragon Corporation wrote: > >> Slight clarification about what I am trying to solve. >> >> After edits I am coming back with lines that used to be intersecting >> no longer intersecting and that still have the same AsText representation. >> >> My hope is to reduce the precision of my geometry in the database so >> the edits don't break the topology of my lines by doing something >> like this >> >> >> Update sometable >> SET the_geom = ST_SetSRID(ST_AsText(the_geom), mysrid) >> >> >> But I fear that if AsText is doing a rounding rather than a >> truncation, that I run the risk of breaking things that used to >> intersect. I am more concerned about under intersecting than over >> intersecting. Although I probably haven't thought this out enough so >> > maybe it's a non-issue. > >> Thanks, >> Regina >> >> >> >> -----Original Message----- >> From: [EMAIL PROTECTED] >> [mailto:[EMAIL PROTECTED] On Behalf Of >> Paragon Corporation >> Sent: Friday, May 16, 2008 12:01 AM >> To: 'PostGIS Users Discussion' >> Subject: [postgis-users] Does ST_AsText truncate or round >> >> Hopefully this is a simple question. I know that ST_AsText returns a >> geometry that is not necessarily as prescise as what is actually >> stored in the geometry field. I have an editor that relies on WKT >> > representation. > >> Anyrate just wanted to know if the ST_AsText does a rounding of the >> points or it does a truncation of the points. Also would be nice to >> know if I could control the precision of this since I will need to >> truncate my actual geometry accordingly so that both are in agreement. >> >> Thanks, >> Regina >> >> >> _______________________________________________ >> postgis-users mailing list >> postgis-users@postgis.refractions.net >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> >> >> _______________________________________________ >> postgis-users mailing list >> postgis-users@postgis.refractions.net >> http://postgis.refractions.net/mailman/listinfo/postgis-users >> > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > > > > _______________________________________________ > postgis-users mailing list > postgis-users@postgis.refractions.net > http://postgis.refractions.net/mailman/listinfo/postgis-users > _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users _______________________________________________ postgis-users mailing list postgis-users@postgis.refractions.net http://postgis.refractions.net/mailman/listinfo/postgis-users
