# How to produce outline of a coastline?

I have polygon dataset containing quite a complicated coastline. How can I produce an outline of that coastline?

Please see the image: my dataset is in green and the coastline I need to get is in red.

• Is the divergence at the bottom intentional? If so, can you say why? Also, what is the source data format, and can you make it available? Even a representative sample? If you have a particular toolset, that would be worth adding to the question too. Commented Oct 19, 2015 at 10:01
• Hi, @BradHards. The divergence is not intentional, it is just my shaky hand . Commented Oct 19, 2015 at 10:10
• @BradHards, I am using CRESTA world boundaries (work requirement) but you could use any coastline from ngdc.noaa.gov/mgg/shorelines for testing purposes. I work in ArcGIS and so far I tried Simplify Polygons and Minimum Bounding, Geometry. I wonder if Aggregate Polygons would help. Thank you for helping. Commented Oct 19, 2015 at 10:21
• @GeoMeteoMe what went wrong with "Simplify Polygons"? This should do what it says on the tin, and simplify that coastline once you get the parameters right. It might follow those inland seas/bays unless you really up the smoothing. Another possibility is "alpha shapes" aka "concave hulls". Commented Oct 19, 2015 at 12:05
• @Spacedman I am aiming to "merge" all bays and islands with the main land as in the next step I have to generate 25km and 50km coastal zones (buffers) - it is a part of cyclone modelling. If the buffers follow the coastline of every bay and island the results are not reasonable. Simplifying polygons did not solve the problem of coastal islands. Commented Oct 19, 2015 at 14:17

Here's something.

Procedure was:

• extract the states of that bit of coast
• dissolve to one feature
• convert polygon nodes to points as a new layer (with 300k points)
• subsample those 300k points to a manageable 9,000
• compute a concave hull (alpha shape). Try parameters 0.3, 0.2 and then 0.1
• resulting polygon drawn in red over the state boundaries

The line does a pretty good job of mimicing your hand-drawn line. The smaller the concave hull parameter, the tighter the line sticks to the coastline.

The other red line, running round the back, is the rest of the polygon generalised. You won't get this if you are working on a complete set of east coast polygons, or you could clip it out later.

This was all done using QGIS and built-in processing functions.

• thank you @Spacedman. Your workflow is working well for me. Commented Oct 21, 2015 at 10:17