On 21/04/10 00:21, Volker Wichmann wrote:
Hi Moritz,
I fully agree with you that GRASS should not change its concepts. But
your arguments are misleading: just cleanning up your vector data does
not imply that your results are more correct - the cleaning process
itself introduces new errors. so it depends on your application if it is
more appropriate to count a point twice or to let the cleaning algorithm
take the choice to which polygon it will be added. The "dirty" approach
somehow introduces some kind of fuzziness. With SAGA we also encourage
the user to know her/his data - but we leave it to the user to decide if
the she/he wants to do a certain calculation or not. It's in her/his
responsibility to interpret the results correctly. This is a thing which
software can't take the responsibility for.
I think this is exactly what I said:
>> I see as one of the trademarks and strengths of
>> GRASS its rigour in pushing the [user] as far as possible towards a
>> careful and thoughtful use of the data.
You can decide how to clean the data, but you have to make a choice. If
you just let the user do what he wants without thinking about the
implications of the data quality for his results, then I would wager
that many analyses contain mistakes without anybody knowing about them.
As you so rightly say, it's one of these questions about whether
software should impose certain limits or leave complete freedom to the
user to make his own mistakes. Not currently active as a developer, but
active in using GRASS for teaching and research, I see daily how GRASS
allows us to avoid a series of mistakes (cf also the debate about
on-the-fly reprojection). And when there are so many other tools out
there to do analyses on spaghetti data, and so little developers
available for GRASS, I'd rather see those spend time on the core
strenghts of GRASS, then on trying to implement this particular feature.
But as always, that's only my less than 2 cents worth...
Moritz
Moritz Lennert schrieb:
I don't want to make this discussion go on too long, but
On 20/04/10 13:38, G. Allegri wrote:
1 - we had to make a simple points in polygon count. The polygon layer
wasn't 'clean' (we hadn't perfect boundary adjacency for example), and
it was made of hundreds of thousands of polygons. The v.in.ogr
process, and the necessary clean, was taking too much time respect to
the simple operation we needed, so we imported it in saga, and with a
few clicks we had our result.
This is typically the example where quick and dirty "works", but where
it might contain imprecise results if you do not ensure clean
polygons: any point falling into sliver polygons will be counted
double. So this is exactly why I would plead for not allowing such
operation in GRASS, as I see as one of the trademarks and strengths of
GRASS its rigour in pushing the use as far as possible towards a
careful and thoughful use of the data.
Moritz
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