It's not a one-liner, if that's what you mean by "easy"... 8^(
There's two main parts to this problem:
1. Ensuring that the linework is correctly noded and snapped (so that
both the river line and the mapsheet boundary contain the same node
points).
2. Building the polygonal topology for the line-polygon overlay and
extracting the relevant resultant polygons
For the sake of simplicity I'm going to assume you already have #1 8^)
The formal, rigorous way to do #2 is to build a topology with the edges
labelled with the river side information (e.g. which side of the line is
water). You can then label the topology faces from that information and
extract river polygons.
The cheesy, fast way to do #2 is to toss your noded linework into the
Polygonizer and get a bunch of polygons back. Then determine a point
which lies on the river inside the mapsheet (for instance, offset a
point a very small amount from the midpoint of the river line on the
water side). Use a point-in-polygon test to find the polygon which
corresponds to the river.
Paul Austin wrote:
I have a data set which uses line strings to represent polygons. In
some cases these line strings do not form a closed ring directly,
instead it uses the neatline of a mapsheet to virtually close the
polygon within that sheet (the full polygon would be across multiple
sheets).
In the following example there is a line for river that goes from the
bottom to the top of the sheet with the river on the right.
+---------+----------+
| \ |
| \ |
| \ |
| \ |
| \ |
+------------------+-+
Is there an easy way given a line string geometry and a polygon
neatline to create a new polygon for the river portion on the right?
Cheers,
Paul
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--
Martin Davis
Senior Technical Architect
Refractions Research, Inc.
(250) 383-3022
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