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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