[Qgis-user] Problem using draw tool's with snap enabled

[Luigi Castro Cardeles]

Hi list,

i am doing some tests with the drawing tool using *versão do qgis do
rodrigo* and snap active.

I am using postgis layers (one for line and another for polygon).

I have some polygon and want to draw lines on the boundary of that polygon
(image
http://picasaweb.google.com.br/lh/photo/S82AQ6dp00-YdDZktcIf7A?feat=directlink
)
At image 1 the yellow one has id=1 and the other has id=3.


i have draw both using snap to segment to draw a line on top of polygon's
boundary.

If i use the postgis function to calculate the distance between each line
and the polygon, i obtain strange results.

I think that both distance's should be zero since the lines are on top of
polygon boundary's but instead i get 3.53443747694303e-016 for line with
id=1 (the yellow one) and get zero for line with id=3. The zero answer is
right but the other one is not.

I do this same operations using OpenJUMP and obtain the expected result
(zero distance for both lines).
If i do this same operations using QGIS unstable 1.2.0, the distances are
zero (right result).

Is this a bug on 1.1.0 version?


If i was not clear enough, please let me know what information i need to
clarify.

----*walkthrough*----

1- create postgis tables
CREATE TABLE line ( id integer NOT NULL,  CONSTRAINT line_id_pk PRIMARY KEY
(id));
CREATE TABLE polygon ( id integer NOT NULL,  CONSTRAINT polygon_id_pk
PRIMARY KEY (id));
SELECT addgeometrycolumn('public','line','the_geom',4326,'LINESTRING',2);
SELECT addgeometrycolumn('public','polygon','the_geom',4326,'POLYGON',2);

2-load the layer at qgis

3-create a polygon and draw lines using the snap tool.

4-run the distance function
SELECT id,st_distance(a.the_geom, b.the_geom) from line a, polygon b


Thanks,
Luigi Castro Cardeles
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