Ask A Cartographer

offset polylines

June 04 2008 | 0 comments
Categories: Map Data, Symbology

I have a feature class of polylines, however, when I attempt to offset the lines I get lines shooting off to some imaginary point. What is the best way to offset? This also happens with cartographic representations.

Mapping Center Answer:

Offsetting line symbology unfortunately depends on some fairly complex algorithms that are subject to fail when there is complex geometry (i.e., very curvy lines with sharp angles). The cartographic representations "Offset" rule is a good deal better than the older symbology methods, but it si still by no means perfect. Basically, there is no good on-the-fly way to do this.

That said, you still do have a couple of options. One is to clean the underlying data up a bit to reduce the sharpness of the angles. Using the Simplify Line tool with a relatively gentle tolerance (i.e., small number) along with the Point Remove method -- this will eliminate many of the offshoot instances you are seeing. But if your data legitimately has sharp andgles, such as switchbacks, in it, then even this won't solve all your problems.  For the remaining probelm areas, you would have to either manually edit the data or use a different solution. 

The next thing you could try is the ArcGIS Schematics Extension.  This will allow you to produce offset data that can be easily symbolized. We wrote a blog entry on Mapping Center that describes the basics of this with a transit map in mind.

Given your image, I would start with teh first solution--if your data is too detailed for the scale you're depicting it at, you can expect our line offset algorithm not to do very well. For instance, if you change the map scale so that you are zoomed way in on some of those features, you should much better results.  I would not expect perfect results for the data you have and the map scale you are currently using for your map.

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