v.to.rast: ignore null entries of attribute values for use=attr

[metzm]

ignore null entries of attribute values for use=attr

if an attribute value is null, the corresponding vector features will not be rasterized

[veroandreo]

I applied the patch and it works as expected according to the change proposed:

g.copy vector=zipcodes,my_zipcodes
g.region vector=my_zipcodes res=100
v.db.update map=my_zipcodes column=ZIPNUM value=NULL where="ZIPNUM == 27596"
v.to.rast input=my_zipcodes output=my_zipcodes_rast use=attr attribute_column=ZIPNUM

I agree with this change, but please see the comment by Micha in grass-user for a different opinion: http://osgeo-org.1560.x6.nabble.com/Null-values-in-attribute-table-get-converted-to-0-zero-during-v-to-rast-td5419967.html

[HuidaeCho]

Personally, I agree with Micha. Polygons with null attributes deserve to be converted to raster cells in some cases. How would we know locations that exist, but do not have any data vs. non-existent locations? For example, county census data may not be complete and some counties may have no population data yet. When you convert this population data to raster with this change, you would need a secondary layer to identify those counties with no data vs. unincorporated areas (or just outside the data boundary)? I believe we should have an option to choose which one we want. Maybe, null_value= option with default NULL so you have to specify a target null value knowingly (e.g., -9999).

[metzm]

I agree with Micha. Besides, all possibilities can already be done without any changes.
If you don't want features with missing attribute values to be rasterized, use v.to.rast where="column is not null".
If you want another value than 0 (zero) as replacement, use v.db.update where="column is null" + v.to.rast use=attr.
This PR is now only printing a message if missing attribute values are replaced with 0.

[neteler]

Backported to relbranch78 in 9460807