[GRASS-dev] 3D polygons (faces) and attributes

Benjamin Ducke benjamin.ducke at oxfordarch.co.uk
Tue Sep 1 06:40:21 EDT 2009

Hi all,

I have some 3D polygonal data in 3D shapefiles that I need to process
in GRASS and then export to VTK for interactive exploration.
The data represents what archaeologists call "profiles", i.e. vertical 
cross-sections through the soil that document soil layers. 
I think the term "catena" exists in some geosciences for roughly the 
same thing. The data is synthetic, i.e. digitized from idealized 
hand-drawings done in the field. As such, it  has no actual W-E or N-S 
extent, only top to bottom.

I realized that v.in.ogr cannot import such data. Even though it reads
the 3D coordinates just fine, it treats the topology as 2D, trying to
calculate X-Y areas and finding labeling points to attach the attributes.
Which fails of course, as no 2D centroid can be calculated for a 3D
area that has no extent in the X-Y plane!

So I went on to patch v.in.ogr to import 3D lines w/o area checking and
that at least gave me an option to import the geometries. I then added
support for 3D faces which allowed me to import a better representation.

But now I am stuck on the attribute side of things:
How do I attach attributes to GV_FACE type geometries? 
I suppose it should be equivalent to how it is done for GV_BOUNDARIES? 
But using V_KERNEL instead of V_POINT for attaching the atts?
Does anyone here have any experience with this? 
Any other GRASS modules that need to do the same thing so I could learn
from them?



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