[GRASS-stats] readVECT6 problem fo importing null data

[Roger Bivand]

On Thu, 29 Nov 2007, Jarosaw Jasiewicz wrote:

>
>>
>>  I can reproduce the problem, which seems related to DBFIsAttributeNULL not
>>  being handled adequately on either side (both are using the OGR library).
>>  Could you please try to write the temporary shapefile directly using
>>  v.out.ogr, and then try to read it with readOGR() and with readShape*()
>>  with * as Points, Lines, or Poly depending on what getinfo.shape() says?
>>  I'll try to debug using the DBF code in the foreign package to start with
>>  - numeric fields should have value /* NULL numeric fields have value
>>  "****************" */ from dbfopen.c, but do not seem to.
>> 
> I tried what you sugested. I can't probably help more, but I found that:
>
> - v.out.ogr produce dbf with 0 instead null;
> - readOGR() reads dbf with 0 instead null;
> - if grass dbf table null are treated as 0, but displayed as null;
>
> I use RdbiPgSQL to import data with null values and nad readVECT6 to import 
> vector and use  substitute () to merge them (if needed). it is a temporary 
> solution

Jarek,

Thanks - I am adding comments to the readOGR() and readVECT6() help pages. 
OGR does not promise to handle missing values at all, because there are no 
standards as such - so this affects GRASS vector handling generally.

Two questions if you have time:

1) can you try to read from PostgreSQL directly using readOGR() - I guess 
that your GDAL/OGR includes the PostgreSQL/PostGIS driver?

2) can you see whether the missing values are visible correctly within 
GRASS, maybe with db.select (the v.db.univar script assumes that there 
are no missing values)? If they are seen, then the problem is in 
v.out.ogr not checking.

I can see isNull fields in dbmi.h; they seem to be used in the fetch.c 
files for mysql, odbc, ogr, postgres, and sqlite drivers in db/drivers/. I 
can only see this retrieved in vector/v.univar/main - but Martin Landa 
probably added this recently, and he is active. There are traces in 
lib/db/dbmi_base/value.c, lib/db/dbmi_base/xdrvalue.c, and 
lib/db/dbmi_client/select.c.

I think that Martin will be interested in this, so maybe it's worth 
checking. I do not run PostgreSQL, so I'm afraid I can't check myself. If 
you do, maybe post to grass-dev?

Roger


>
> Jarek
>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no