[GRASS5] how to NULL in site lists?

Markus Neteler neteler at geog.uni-hannover.de
Fri Jan 26 17:02:32 EST 2001

Hi Andreas,

> i agree with Eric and Roger that the quick fix is not advisable.

.. I am convinced :-)

>But there is another more general consideration to the NULLs in site
>IMHO the site list format is for the storage of point data derived from
>measurements.  Usually the points are irregularily spaced. The site list
>contains the spatial data (coordinates) and the attribute(s) in a sort of
>1:1 database. So if you have no value (measurement) at a given point, there
>should be no site list entry/line.
This is my current workaround (wrote a small s.cut to extract columns
from a multi-att sites list).

>The problem comes from changing the sites format from one attribute to
>several attributes. This can not be modelled adequate with a 1:1 database
>Another approach would be to create a site list for each attribute/variable
>and to ommit the sites where no measurement/variable value is available
>(NULL attributes). I don't know if NULL values with site files are
>conceptually correct.

As you see the problems arises from multi-att site lists. An Example:
Say, you have numerous soil profiles with some parameters for each
horizont. So I save this in a 3D sites list:

x1 y1 z1 att1a att2a att3a                -> soil profile 1
x1 y1 z2 att1b att2b att3b
x1 y1 z3 att1c att2c att3c      
x2 y2 z1 att1d NULL(not measured) att3d   -> soil profile 2
x2 y2 z2 att1e att2e att3e

It would be great to allow such a NULL (or * or whatever),
because the sites format is easy to handle.
I don't know another way how to get in such data. And in
real life it is possible, that some values might be unknown.
In my case I have 280 horizonts in 24 profiles with each 43
parameters from the geo-ecological laboratory. It wouldn't be
fun to treat every parameter as an own sites list...

If the sites would be stored in a DBMS (maybe the forthcoming
internal DMBS for the vector atts), it should be possible to
define empty fields. In 5.0 it will be to complicated.

Just my 0.02Euro


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