[postgis-users] questions/issues when creating a topology

Mon Jan 25 08:10:08 PST 2016

2) update. Apparently my "manual" brute force insertion does not work.

UPDATE temp_geom_bahia2 SET tg_geom_dump_utm = '(28,1,13314,3)' WHERE
cod_setor = '290390405000036'

The desired cell in column tg_geom_dump_utm  of table  temp_geom_bahia2 is
filled. But the topogeom still displays a hole for that area when I see it
in Qgis (even after updating).

Any ideas on how to insert that?

regards
Lucas

2016-01-25 13:26 GMT-02:00 Lucas Ferreira Mation <lucasmation at gmail.com>:

> Sandro,
>
> tks very much for the answers. After week to parse and try to implement
> your sugestions I think we made good progress. Of of the 316k polygons, we
> get 308 polygons that were not included.
> Bellow are some follow-up questions (in line bellow).
>
>
>> Message: 1
>> Date: Sun, 17 Jan 2016 14:23:25 +0100
>> From: Sandro Santilli <strk at keybit.net>
>> To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
>> Subject: Re: [postgis-users] questions/issues when creating a topology
>> Message-ID: <20160117132325.GA8245 at localhost>
>> Content-Type: text/plain; charset=us-ascii
>>
>> On Thu, Jan 14, 2016 at 09:57:48AM -0200, Lucas Ferreira Mation wrote:
>>
>> > 1) What is the role of ST_Force2D() function in
>> >  toTopoGeom(ST_Force2D(geom),...) ? The code fails if I omit this
>> function.
>> > However my data is already 2D.
>>
>> ST_Force2D should make no difference when the input is already 2D.
>> If it does there's a bug hiding somewhere and you're welcome to
>> file a ticket about it (with a way to reproduce).
>>
>>
> I actually managed to run most test cases without  the ST_Force2D()
> function. The only case it did make a difference was when creating a
> topology with the original SRID (4674 , SIRGAS 2000) and no tolerance
> parameter. With the  ST_Force2D() wrapipng the input polygons, the loop
> runs. Without it, there is a crash in the database (connection lost with
> database).
>
>
>> > 2) In most test I did, I got an error when uploading one or two polygons
>> > (out of a 900 polygons in Acre state). How can I add these afterward?
>> > Assume all the edges that define those error-polygons have already been
>> > added. How do I define a new face  by those edges and attribute that to
>> the
>> > topogeom for these error-polygons?
>> >
>> > Let's say polygon A can be defined by edges 1, 2, 3 and 4, which are
>> > already in the topology when other polygons were inserted. How do I
>> define
>> > face A and attribute that to the topogeom?
>>
>> Faces are automatically defined. You associate attributes with
>> TopoGeometry objects, which you can define in your case by specifying
>> the faces making up the polygon.
>> See http://postgis.net/docs/manual-dev/CreateTopoGeom.html
>>
>>
>  Ok, got it to work. To document for others. Fist I used QGIS to know the
> original polygon - face pairs. This is done manually, by overlaying
> serveral layers (orginal polygons, topgeom, face bounding boxes). For
> example, from the atached image, I learnt that  I should associate the
> polygon cod_setor=290390405000036 with face_id=13314 of the corresponding
> topology. This, this fixes the "hole" in the topology for that polygon
>
> UPDATE temp_geom_bahia2 SET tg_geom_dump_utm = '(28,1,13314,3)' WHERE
> cod_setor = '290390405000036'
>
> You may also try to re-run toTopoGeom on the previously failed
>> polygons. Sometimes the function works when finding a different
>> starting state.
>>
>
> This worked for 50 aditional cases of of the 358 errors (on the test with
> UTM reprojected polygons and with tolerance parameter of 1m).
>
> I'll now try to increase the tolerance parameter (to say 2m, 5m, 10m) to
> see if the remaining polygons get added to the topology.
>
>
>>
>> > 3) Is there a way to manually edit topologies, in QGIS for instance? I'm
>> > thinking of a workflow for repairing a map. After doing the automatic
>> fixes
>> > in Postgis and creating a topology, someone wants to manually correct
>> some
>> > feature. For instance, move an edge or a point, while maintaining the
>> > topological properties. Is there a QGIS plugin  for that?
>>
>> https://plugins.qgis.org/plugins/pgtopoeditor/
>>
>>
> I see cool options to remove find dangling edges, remove edges and points
> and find Orpaned topogeoms. But supose I wanto to change the shape of an
> edge, by mooving a few vertices among it's path in QGIS, while preserving
> the nodes where it intersects with other edges. Is it possible to do that
> (and preserve the topological properties)? What about moving nodes? Can
> pgtopoeditor manage that?
>
>
>
>> > 4) Is not including a tolerance parameter  in to... equivalent to a zero
>> > tolerance?
>>
>> Yes. This is something that might be good to change in the future
>> but for now it is the same.
>>
>> > 5) Does topology behaves differently depending on whether the SRID is in
>> > decimal degrees or meters? Ideally, I would prefer to have the tolerance
>> > parameter in meters, as it is a more intuitive unit and have a constant
>> > amount of "snapping" across the areas. However that means choosing a
>> > specific UTM zone for reprojecting each state. However states in Brasil
>> are
>> > quite large, spanning more then one UTM zone. Even Acre, the small
>> state I
>> > used for testing spans two UTM zones
>>
>> Topology makes no intepretation of projections.
>> Coordinates are always considered to be cartesian.
>>
>> > 6) I tested creating topologies with the original Sirgas 2000 decimal
>> > degree spatial reference (srid 4674, a decimal degree system, quite
>> similar
>> > to wgs84 but slightly more optimized for Brasil) and also with  a
>> >  reprojection to a UTM zone (srid 32719), which is where most of the
>> > polygons are located. The errors are not the same.
>>
>> When you pass no (or 0) tolerance, one is computed automatically based
>> on the minimum drift detectable by a "double precision" number.
>> Using lat/lon units usually means numbers closer to the 0 thus having
>> a smaller minimum tolerance. This changes overall results.
>>
>> > 7) Has there been any progress on some sort of index for
>> topogeometries, so
>> > spatial queries can run faster? (I found this 2013 tread
>> > <
>> http://osgeo-org.1560.x6.nabble.com/spatial-index-for-topology-td5085935.html
>> >on
>> > the topic, but nothing more recent). Of course one can always cast to
>> > geometry (topogeom::geometry) and index that, but that is duplicating
>> the
>> > information somewhat.
>>
>> No progress. A TopoGeometry object is externally defined so cannot really
>> be indexed on itself. The geometry cast is volatile for this reason,
>> and cannot be used in an index.
>>
>> What you could do is change the select queries to filter on bounding
>> box of components. It would be a pretty complex query as it'd have to
>> step by the "relation" table and the "layer" table to finally get
>> to the real data, which si already indexed.
>>
>> Or you could build a cache of bounding boxes, automatically updating
>> it whenever the primitives in the topology are updated, and use that
>> column as a filter.
>>
>
> this is interesting idea. Any example on this?
>
>
>>
>> --strk;
>>
>>   ()   Free GIS & Flash consultant/developer
>>   /\   http://strk.keybit.net/services.html
>>
>>
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