I have been looking around to find a way to calculate the longest line within a polygon.

Determining longest line segment within polygon that passes through its centroid? and Calculating maximum distance within polygon in x-direction (east-west direction) in PostGIS? is what I found on here however these are quite advanced solutions. Also they are pretty old (5 and 6 years).

Is there by now a pre made library or github repository that I can use to do this or is there no other way then to write it myself?

Secondly, if I have to do it myself, could anybody post some kind of pseudo code or plan that I can follow.

I am trying to understand the second link at the moment but i'm having a hard time.

closed as too broad by John Powell, whyzar, lynxlynxlynx, mgri, HDunn Sep 26 '17 at 20:13

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • In answer to your first question, I don't think the algorithm will have been improved on, so it being a few years old isn't relevant. So, is your question can someone convert this R code to some other (unspecified) language or point me to a github repository of the same or is there another third party tool, such as in whyzar's answer. – John Powell Sep 26 '17 at 19:13
  • I understand that the method hasn't changed. The question is, is there an pre-build option right now, such as whyzar's answer. I believe GME is dependent on Arc? I would prefer an answer not being dependent on Arc, but will look into his answer. – ArnJac Sep 26 '17 at 19:15
  • You should state this preference for not using anyting ESRI-based as an edit to the question, as there are many potential answers to such a broad question. I have never heard of GME before, though apparently it might mean Geospatial modelling environment. – John Powell Sep 26 '17 at 19:17
  • 1
    I think this is too broad for focused Q&A because the first question seeks a software recommendation whereas the second is more like seeking an algorithm. It is only tagged with the latter and the only answer seems to target the former. – PolyGeo Sep 26 '17 at 19:38
  • You are right. I will mark the answer as correct and try to make GME work. If necessary I will ask another question with exclusion of Arc. – ArnJac Sep 26 '17 at 19:47

This option, from GME, the will allow you to:

Determines the longest line that can be position within the bounds of a polygon, crossing no edges

geom.polygonfetch (Calculate Fetch In Polygons)

This tool estimates the longest straight line that can be positioned within a polygon, without crossing any edges. It corresponds to the 'fetch' of a lake: the longest stretch of water over which waves can build up as a function of wind action. The calculation is based on evaluating the lines created by connecting all pairs of non-neighbouring vertices and retaining the longest line that does not cross any interior or exterior boundaries of the polygons. This is a brute force algorithm that can take a long time for very complex polygons (e.g. polygons with more than 1000 vertices).

The 'where' clause can be used to define a subset of polygons to process. See the 'where' section for further details on how to formulate a where clause.

Here is the syntax:

geom.polygonfetch(in, uidfield, out, [where]);

in the input polygon data source
uidfield the unique ID field of the input feature data source
out the output line data source
[where] the selection statement that will be applied to the line feature data source to identify a subset of lines to process (see full Help documentation for further details)

Here is an example input:

geom.polygonfetch(in="C:\data\lakes.shp", out="C:\data\fetch.shp", uidfield="LAKEID");

geom.polygonfetch(in="C:\data\lakes.shp", out="C:\data\fetch.shp", uidfield="LAKEID", where="AREA > 10000");

  • Nice and complete! This seems to be what I need. However it is dependent on Arc right? I'm trying to run it from ubuntu. There might be a workaround but I would prefer a non Arc solution. – ArnJac Sep 26 '17 at 19:16
  • Perhaps edit your post, it doesn't say that you are after straight line. If so, mention of centroid is not relevant, because centroid might very well sit outside polygon. – FelixIP Sep 26 '17 at 19:56

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