NULLs in a CSV file are treated as 0 (zero).

[cgsbob]

ogr2ogr on a VRT produces 0 (zero) instead of NULL.

Here is some background info:

psha2002.vrt:

<OGRVRTDataSource>
  <OGRVRTLayer name="psha2002_bayarea">
    <SrcDataSource>/tmp/psha2002_bayarea.csv</SrcDataSource>
    <GeometryType>wkbPoint</GeometryType>
    <LayerSRS>NAD27</LayerSRS>
    <GeometryField encoding="PointFromColumns" x="longitude" y="latitude"/>
  </OGRVRTLayer>
</OGRVRTDataSource>

psha2002_bayarea.csvt

"Real","Real","Real","Real","Real","Real","Real","Real"

A few lines from psha2002_bayarea.csv:

longitude,latitude,pga_bc,pgamw_bc,modal_distance,modal_mag,pga_d,pgamw_d
-123,38.25,0.666,0.732,,,0.666,0.732
-122.99,38.25,0.694,0.77,,,0.694,0.77
-122.98,38.25,0.701,0.778,,,0.701,0.778

modal_distance and modal_mag have zero values where there should be nulls.

[Even Rouault]

I've fixed that in trunk in r15999 for all data types except for strings, as we have no way of distinguish empty string ( ,"", ) from NULL string ( _{) in the return values of CSVReadParseLine(). I'm not sure that this distinction makes sense in a CSV file anyway.}

[warmerdam]

I agree that NULL is not a meaningful concept for string fields in a CSV file.