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I am preparing a map which is concerned with showing two line features and there relative location to each other. The line features are 20 m apart and at a scale of 1:50 000 or bigger on A3 I want to show which the lines a little separated from each other. The features follow a common path, but change sides irregularly, i.e. one feature is on the south for while, then the north, then th common path changes north south and the line features then alternate between sitting on the east or west of the path.

I would like to do this in QGIS. Is there some sort of automagical dynamic offset control for lines? I can imagine how to rotate symbols based on an adjacent feature, but this is a bit more complicated. I can imagine a method of breaking one line up into segments and then creating an index of those segments compared to the other line acting as an anchor. The index might then be used to specify the offset, however this is a sort one time run solution. Rather just looking for the elegant cartographic solutions, if they exist.

ta

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I think what I mean, is how to avoid colliding parallel lines - edit title – Willy Aug 10 '12 at 10:24
4  
Colliding parallel lines? Not possible in a euclidean geometry! :) A picture speaks a thousand words - any chance of one? – Spacedman Aug 10 '12 at 10:53
OK, fat parallel lines.... – Willy Aug 11 '12 at 5:42
I have gone to finer scale and gone hard with Atlas, more paper, ink etc. – Willy Aug 11 '12 at 6:02

1 Answer

up vote 2 down vote accepted

If the point is just to show it in map layout - what about line buffers? If you create lets say 200m buffers for both lines and then erase both from each other you will get approx 20m thick "lines" (polygons) 200m far from each other.

enter image description here

Btw in QGIS:

  1. select one line
  2. Vector - Geoprocessing Tools - Buffer(s)
  3. select second line, repeat step 2
  4. Vector - Geoprocessing Tools - Difference buffer1 - buffer2
  5. repeat step 4 - buffer2 - buffer1

Done.

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how do you mean, erase both from each other? – Willy Jan 11 at 9:46
Well do 2 copies of both. Than erase in first pair first with second and in second pair second with first. – Miro Jan 11 at 9:53
I will try this, thanks – Willy Jan 11 at 9:56
I added image with example to show what I mean. At the and with scale you mentioned it should be more apart (I used small buffer, you should go for something bigger) and you can change outline of polygons to get desired "line width". – Miro Jan 11 at 10:54

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