Bug report #20753

WKT output is wrong for EMPTY geometries

Added by Johannes Kroeger over 1 year ago. Updated about 1 year ago.

Status:Open
Priority:Normal
Assignee:-
Category:Geometry
Affected QGIS version:3.5(master) Regression?:No
Operating System: Easy fix?:No
Pull Request or Patch supplied:No Resolution:
Crashes QGIS or corrupts data:No Copied to github as #:28573

Description

QGIS geometries' asWkt() method returns "GeometryType ()" or simple an empty string '' instead of "GeometryType EMPTY" for empty geometries.

http://www.opengeospatial.org/standards/sfa defines for example for a POINT:

<empty set> ::= EMPTY

<point text> ::= <empty set> | <left paren> <point> <right
paren>

From my understanding this means that an empty Point would be represented as "POINT EMPTY".
QGIS however represents it as empty string ''.

>>> p = QgsGeometry.fromWkt('POINT EMPTY')
>>> p.isEmpty()
True
>>> p.asWkt()
''

History

#1 Updated by Johannes Kroeger over 1 year ago

Actually, it looks like something weird is being created for empty geometries. They all have geometry type 3?

>>> for type in "POINT", "LINESTRING", "POLYGON", "GEOMETRYCOLLECTION", "MULTIPOINT":
>>>   type = type + " EMPTY" 
>>>   g = QgsGeometry.fromWkt(type)
>>>   print(type)
>>>   print(g.type())
>>>   print(g)
>>>   print(g.asWkt())
POINT EMPTY
3
<QgsGeometry: >

LINESTRING EMPTY
3
<QgsGeometry: >

POLYGON EMPTY
3
<QgsGeometry: >

GEOMETRYCOLLECTION EMPTY
3
<QgsGeometry: >

MULTIPOINT EMPTY
3
<QgsGeometry: >

#2 Updated by Loïc BARTOLETTI about 1 year ago

I think there are several issues raised in the ticket.

First of all I think your reasoning is right, there is missing information on empty geometries. For example, postgis returns "POINT EMPTY" when creating such a geometry.

There is indeed an isEmpty method that could be used for geometries except when there are points since the method always returns false. By creating a point by QgsPoint(), it should be able to return " POINT EMPTY " and not QgsPoint(0, 0) as it is today.

For me it's interesting to have an empty point, but I wonder if it's a bug to fix or if it's bigger (aka for QGIS 4)?

#3 Updated by Loïc BARTOLETTI about 1 year ago

related issues #19190 and #20754

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