Rendering overlapping lines

Question

I am making a map containing information about public transportation: busses, trams, etc. The map will have a layer containing for example the tram lines. Each line has its own colour, and is represented by a line string geometry.

The problem is, many of the lines have overlapping parts, where more than one tram line cover the same section. To show this to the user, I'd rather want the lines to run parallel to each other instead of being drawn on top of each other. For an example, see how Google Maps show the New York subway lines.

I suspect this is a quite common problem in cartography, but don't know what terminology I should search for.

I am using PostGIS/GeoServer/OpenLayers as my stack, but any open source solution would be acceptable.

Answer 1

If you're interested in learning more about this area, the problem is named cartographic displacement, and its one aspect of cartographic generalization.

A couple of articles discussing displacement and approaches for handling the problem:

• Bader, Matthias. 2001. Energy Minimization Methods for Feature Displacement in Map Generalization.

• Steiniger, S Tefan S, and S Iegfried M Eier. Snakes: a technique for line smoothing and displacement in map generalisation: 1-11.

• Ware, Mark J, and Christopher B Jones. 1998. Conflict Resolution in Map Generalization Using Iterative Improvement. GeoInformatica 2, no. 4: 383-407+.

Answer 2

This problem is a very typical one in cartographic generalisation. Automated methods exist for that, but no implementations are available yet.

Methods based on "Beams" and "Snakes" give efficient results to solve these cartographic conflicts of network data (see also the references given by scw). Here are some results of the beams algorithms on road data:

Before:

After:

See also this paper and this presentation that explain how to do this transformation.

EDIT: I have never tested it, but it seems there is something related to snakes in GRASS. See here.

Answer 3

I don't know of a solution, but I think the term you are looking for is "conflict resolution" - a topic of map generalization. A Google search on "map generalization conflicts" shows some interesting info - but I don't know there's much of practical use.

Answer 4

In current implementations, you can often find either "Offset" or "Dislocation". Using you're current stack, it looks a bit difficult to achieve parallel lines automatically:

• Geoserver's SLD Geometry Transformations function "Offset" seems to not be an equal to the "Offset" you can specify in a UMN Mapserver mapfile.
• PostGIS doesn't provide parallel lines - by default - either. (See related question: How to create one sided buffers or parallel lines in PostGIS?)
• I'm not sure if you can achieve anything decent messing around with OpenLayers JavaScript.

To sum it up: The easiest way I know of would be to use UMN Mapserver and the Offset value there. Example style for your mapfile:

STYLE
 SYMBOL 7
 OUTLINECOLOR 160 160 160
 SIZE 5
 OFFSET 2 -99 # <-- This will offset the line to the right.
 ANTIALIAS FALSE
END # STYLE

Answer 5

If you are drawing line layers that overlap, you have several options to allow the viewer to see all of the lines.

1. Width - you vary the line width between layers and put the wider lines on the bottom.
2. Opacity/Transparency - you can make individual line layers partially opaque, so you can 'see through' individual layers.
3. Offset - you can offset the line symbol representing the actual line feature to one side of the actual geometry of the line. Individual layers can be offset by different amounts and in different directions to allow all layers to be seen.
4. Lines can be represented by a series of point symbols with a defined gap between the symbols. By varying the symbol, symbol color, symbol size, and gap between the different line layers, you should make each individual line layer distinguishable.

I don't use GeoServer, but I know that MapServer has the functionality to do all of these things. It is likely that GeoServer does as well.