I have a polygon file of area boundaries and I want to measure part of the polygon lines in each area which next to the sea.

Is there a way to do this?

In theory I guess I could edit and make individual polylines between each edge and then measure them.....it's a bit time consuming though.

  • 1
    What software are you using? As a general principle you could degenerate your polygons to lines (being sure to keep a unique index), buffer your sea, clip the lines with the sea buffer then calculate the lengths of the lines that remain after the clip.. but I can't be any more specific without knowing what software you have to achieve your goal. – Michael Stimson Nov 26 '19 at 7:47
  • Hi, thanks, I am using QGIS. – Tessa Bell Nov 26 '19 at 9:44
  • There are many ways to measure polygons, can you add a screenshot showing what you want to measure? – BERA Dec 29 '20 at 15:23

As indicated in my comment this is the workflow that I would follow:

  1. First buffer your sea by a small amount, the value of small depends on the spatial reference units you're working in and how well the polygon data follows your coast. Try to use the smallest number you can.
  2. If you don't already have a unique polygon ID then add a unique polygon ID field.
  3. Degenerate your polygons into lines, the generated lines will have all the attributes of the contributing polygon (which includes the unique ID).

  4. Clip the polygon lines with the sea buffer to extract the part that is adjacent to the sea only.

  5. Calculate the lengths of lines into the attribute table, there may be multiple sections from a the same source polygon within the clip but that's ok for now.
  6. Generate statistics from the clipped lines using the unique ID as a classification field and length as a statistic field, this will give you the SUM of all the lengths adjacent to the sea for each polygon unique ID. This table can then be joined back to the original polygons if that is your requirement.

I would use the same process in Esri or any other GIS software, all that would change is the links to the instructions.

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