[GRASS-user] displaying volumes from xyz vector in nviz

Benjamin Ducke benjamin.ducke at oxfordarch.co.uk
Thu Aug 21 04:55:07 EDT 2008

To generate a volumetric model of your point data, you need to use
an algorithm that can interpolate an attribute in 3D space.

To start with, you need to make sure that your input points map is
actually a 3D geometry, not just 2D points with X,Y and Z attributes
attached (us v.info to find out).

v.to.rast3 simply takes the 3D point positions and creates a single
voxel value at every point location in the input map. Most likely, this
is not what you want and you should look into v.vol.rst instead.

The question then is WHAT to interpolate. In your attribute table list
below, I cannot see an attribute that could actually be used for
interpolation. The only thing you have are integer categories.

If you want to show point density in 3D: there is currently no GRASS
module to calculate that. So the only thing you can do is calculate
density for 2D slices at different Z ranges separately (use v.select
to query out points for each Z range, then run v.kernel on them) and
stack the resulting 2D raster maps on top of each other to form a voxel
cube using r.to.rast3.


ian summers wrote:
> Dear all,
> I have a vector point file and have tried to run the program v.to.rast3 
> though in the 'column name' section I am unsure what to enter? the 
> column attributes for the data are as follows.
>  > INTEGER/cat
> I have tried entering cat, id and z in this box but in the output I 
> receive the message 'position not in queue'.
> I have also tried to create the volume using r.to.rast3 and though it 
> appears as though a file is created when it comes to display in nviz 
> only a plain 2D white tile appears. The values in my Z column are 
> negative as I am modelling a lake.  I am unsure whether this holds any 
> bearing on outcome.
> Hope someone can help.
> Ian Summers
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Benjamin Ducke
Senior Applications Support and Development Officer

Oxford Archaeology Ltd
Janus House
Osney Mead
Oxford, U.K.

Tel: +44 (0)1865 263 800 (switchboard)
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benjamin.ducke at oxfordarch.co.uk

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