[Qgis-user] vector editing in QGIS - questions and issues (PostGIS biased)

[Christopher Barker]

Maciej Sieczka wrote:

> Another thing - IMO users more often need snapping than they don't.

I don't know that that's true -- I don't think I ever need snapping. 
Though maybe I'm mis-understanding what snapping means -- I think it 
means that when you move a point (for instance), it's final coordinates 
will be adjusted to fit a pre-defined (though changeable) grid -- i.e. 
rounded to a given resolution. I can see how this is useful, but why do 
you think it's the most common need?

 > Help
> users not to digitize overlapping features by default, eg. promote using 
> snapping rather than not using it. I'd be also in favor of enabling the 
> QGIS "topological editing" feature by default.

I'm confused about what snapping has to do with overlapping features? Or 
is the idea that if a user intends to put two points at the same 
coordinates, snapping makes that easy, where without snapping, it's easy 
to have two polygons overlap (or have a gap), when you intend them to 
abut each-other? In that case, maybe it would be best to have a "snap to 
existing nearby point" mode -- unless that's what snapping already is, 
in which case, I'll shut up now...

> What I hate most in GIS are all those overlapping polygons and lines 
> which I have to tediously correct topology for, re-calculate their area 
> and length, explain the vendor what is wrong about his data. Please 
> promote topological practices.

I do agree with this. When editing, it should be clear to the user what 
the topology is, and easy to make it what they want. i.e. highlight 
overlapping polygons, having a "snap to nearby point", etc.

By the way, I don't think this fits into common GIS data standards, but 
it seems to me that the "right" way to deal with these issues is to 
define polygons in two steps: a set of points, and a set of polygons 
defined on those points. That way, two adjoining polygons would have 
their shared vertexes be the SAME point -- not two different ones that 
happen to have the same coordinates. Could this data structure be easily 
supported?

-CHB


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Christopher Barker, Ph.D.
Oceanographer

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