buffers and polygons: merge, slice, chop/notch?

caver456's Avatar


13 Jul, 2018 01:42 PM

When we create a buffer and then realize it should really be part of another buffer (akin to 'join' or 'extend' for a line), it would be handy to be able to merge them into one polygon. Similarly for any arbitrary polygons.

Slicing (by drawing a cut line completely across an existing polygon/buffer) would be very handy in case we realize we drew the buffer too long (akin to 'split here' for a line).

Chop/notch (by drawing a closed polygon which intersects the existing polygon) would remove the intersecting area from the existing polygon. (If the drawn chop shape is entirely within the existing polygon, it would create a donut, really a 'closed horseshoe' in order to have a donut with just one boundary line.)

One software package calls their command 'notch' instead of 'chop' to kill several of these birds with one stone, meaning that if the first vertex of the notch shape is inside the existing polygon and at least one other vertex is outside the existing polygon, the existing polygon will be grown, and vice-versa: if the initial vertex is outside the existing polygon, and at least one other vertex is inside the existing polygon, the existing polygon will be notched/reduced. Either way this is still a different beast than 'slice' which just separates the existing polygon into two (or more) separate polygons.

Do you foresee any of these next-level polygon editing tools becoming a part of caltopo/sartopo?


  1. Support Staff 1 Posted by matt on 15 Jul, 2018 08:59 PM

    matt's Avatar

    At some point, yes. The java topology suite (JTS) that I use for the buffer command looks like it supports those operations, which means that I'd (mostly) have to just add the UI, not figure out how to actually bisect a polygon. However I'm not sure how well it would handle various corner cases like a polygon getting sliced multiple times (picture a W buffer with a horizontal line through it).

    I couldn't put a timeframe on it though.

  2. System closed this discussion on 02 Oct, 2019 04:24 PM.

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