Given a polyline is it possible to determine where to close it (to form a polygon) using a single line segment that's no more than 1 nautical mile long while ensuring a maximum polygon area using QGIS?
I am trying to use a geoprocessing operation and parameter to get results similar to the US Census's Inland Water geography. I have some coastlines that I want to apply the same definition of the Inland Water used by the US Census (see GARM Chapter 15, specifically Figure 15-1 @ https://www2.census.gov/geo/pdfs/reference/GARM/Ch15GARM.pdf), but I can't figure out how to do it, specifically how to derive that ≤1 nautical mile line between two points on a given continuous line. In other words basically have the program figure out where should be an inlet and ensure that inlet has the maximum area possible, considering that for the entire length of the inlet there could be multiple location where it could be narrow to ≤1 nml and you'd want the one "furthest out".
If this problem is complicated enough to require any programming/coding I may consider aborting this as I'm not up to my stuff in any CompSci matters; likewise, as I only have 1 college course worth of GIS skill, I realize this question could be more advanced than I hoped and out of my league, so I might not be able to verify/replicate any answer.
But for starter perhaps the coastline should be a polygon object (fake the other line segments crudely?) since there might be a problem using line object such that the GIS might end up working the wrong side of the line so maybe a coastline in polygon format could preempt that.
This is GARM's Fig15-1 with my highlight of what I'm seeking.
so if this was a real coastline how to find the lines highlighted; so finding if this stretch of the "river" have any where that I can close it off with a line of 1 nml maximum and achieve a maximum area (the "maximum area" criteria is there in consideration that there could be, say, a larger-than-1-nml lake upstream)
Note: This question is not about obtaining open/restricted data, and I am not trying to ask US Census to supply details of their methodology; this is about developing a GIS technique to obtain a result similar to the example mentioned (i.e. US Census' GARM Ch15).