[Qgis-user] to line or not to line

[bernhard.stroebl at jena.de]

Hi Gerhardus,

as I experience similar questions at my work, too, I would like to make 
some statements in order to (hopefully) clarify things for you:
GIS is about modelling spatial data. QGIS as any other GIS-desktop 
software enables you to create maps that show your data. If you need a 
_beautiful_ map in a certain scale it might be necessary to _draw_ the 
map! As modelling calls for abstraction, your data might not be 
sufficient for this beautiful map.
So if you want to calculate all rivers' length or model discharge you 
need to represent your rivers as lines (probably digitized in drainage 
direction). These lines will be ok for maps from say 1:200000 till 
1:500000. Smaller scales would need generalization (e.g. less windings), 
as you enter larger scales you would like your rivers as polygons.
In short: If you need you rivers modelled, model them (as lines), if you 
need a beautiful map, draw it, if you need both, do both :-)

regards

Bernhard

Am 29.10.2011 19:50, schrieb Gerhardus Geldenhuis:
> Hi
> This question is not as much QGIS specific but more about generic GIS
> concepts.
>
> In all of the tutorials I have watched so far rivers is represented as
> lines. This makes sense and if you were to do later analysis I would imagine
> a line would make it easier to get information like the length of all rivers
> in an area. However rivers do not play nice and have varying widths. I could
> adjust the line width but that would only be accurate for a certain part of
> the river. My question is thus how do you represent a river of any feature
> for that matter, of varying width and substantial length. If the river is a
> polygon then I would loose the ability to extract length data or is that
> assumption wrong? Do I represent a river/feature as both a line and a
> polygon then?
>
> Regards
>
>
>
>
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