null values in GML

[Mateusz Loskot]

Steve Lime wrote:
> Mateusz is most likely correct. MapServer extracts attributes and
> converts when necessary (not very often). If the asterix 
> always represents NULL then we could represent as something else (empty
> string?) in the DBF code. What does GDAL/OGR
> do with NULLs in its output?

OGR reader for Shapefile skips reading null attributes, what means that
no value is assigned. Field without any assigned value is marked as
not set (OGRFeature::IsFieldSet() ==  FALSE).

Shapefile reader for OGR detects null field if:
- string field value is empty, strlen() == 0
- numeric value is full-of-asteriks or full-of-blanks
- date field value is 00000000
- boolean field value is ?

Cheers
-- 
Mateusz Loskot
http://mateusz.loskot.net