[GRASS-user] Strange behavior with v.patch

[Markus Metz]

On Fri, Dec 8, 2017 at 11:46 PM, Peter Tittmann <ptittmann at gmail.com> wrote:
>
> Hi,
>
> In attempting to patch two polygon layers I am confused as to the the
results i am getting. My understanding of v.patch is that it would be
similar to a `union` operator in SQL (not a spatial union) wherein rows of
one or many tables are concatenated to a new table. However, the results
i’m getting are not in aligned with that expectation.
>
> Here is a link to an image of the two tables (one orange, one green) (
https://pasteboard.co/GXjWm6P.png )
>
> The second image ( https://pasteboard.co/GXjVSQE.png) is a result of
>
> # combine all clustered polygons into one table
> gsc.parse_command('v.patch',
>                   flags = 'e',
>                   overwrite = True,
>                   verbose= True,
>                   input = <green>, <orange>,
>                   output = ssPoly)
>
> The blue represents the result of the v.patch command. I don't understand
why all of the area covered by the two source vectors is not covered in the
patched vector.

You have patched two area vectors together and the output contains most
likely topological errors. Try to clean the output of v.patch with v.clean
-c tool=break type=boundary.

HTH,

Markus M

>
> My only thought it it has something to do with the categories, or keys?
>
> Thanks in advance for any guidance.
>
> Peter
>
>
>
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