I've just had some thoughts on this and wanted to see if my assumptions
are correct. I'm going to deal with one line segment in relation to the
neatline at a time.
The neatline will be created as a LineString in a clockwise direction
(right hand rule).
The line segment to create the polygon must be trimmed so that it does
not extend beyond the neatline and must already follow the RHR.
The neatline and the line segment will be noded by using the union
operator to create a MultiLineString.
The diagram below shows a + for the start/end of each line segment and
arrows for the direction and numbers for each segment
------+---->------
| \ |
| \ 4 |
1 ^ \/ v 2
| \ |
| \ |
+------<----+-----
3
Each noded line it the MultiLineString will be processed.
Create a list to store line segments that are part of the polygon and
put the original line segment in it.
If the start of a noded line string shares the same coordinate as the
end of original line string then given that the line and the neatline
follow the RHR the noded segment must be appended to the end of the list
of polygon line segments.
In the diagram segment 3 will be matched
If the end of a noded line string shares the same coordinate as the end
of the original line string the that the line and the neatline follow
the RHR the noded segment must be inserted at the beginning of the list
of polygon line segments. This check is required as you will get two
line segments around the start of the neatline geometry
In the diagram segment 1 will be matched.
After processing all the segments we will have the polygon.
Now if you have the classic double sided river case where both lines
cross the neatline then what you can do is create a new polygon from the
intersection of the two geometries.
Can anyone see any problems with this.
Paul
P.S. again ignore the hole case
The neatline geometry is created
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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