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I have to regularly create KML polygons of map area outlines (white outline, QGis width 1.754, Google Earth width 5). I tend to have map image source files. I know how to georeference a source image to a raster, and would like to create area outlines from raster sections. Example: Current swordfish map

This is the existing map of SW Pacific swordfish stocks. I'm looking to update this.

Swordfish management

This is the management area image which will be the source file. There are two possible routes for me to create maps from this:

  1. Just do the subarea boxes (1N, 1C etc).

1.1 Do this by hand in google earth. Time consuming but gets it done.

1.2 Make a csv of WKT points, import into QGis with QuickWKT. Firstly: lots of work to manually enter it all. Secondly: since subarea 2 boxes cross over the +/- 180 degrees line, this is handled wrong by QGis, splaying all the way around the world rather than across the 180 line.

1.3 georeference the image then trace the subarea borders with the polygon creation tool, save as individual polygons, etc. But might as well do in Google Earth in that case.

  1. Do the high-seas areas only. This would be ideal. These are the bits outside of the 200 nautical mile buffer zones around land masses. Does anyone know if it's possible to create a polygon outline from an area of georeferenced raster, e.g. if I first colour the map image in Gimp/Paint/etc, and flood fill the high seas areas in 6 different colours (1 per subarea), then georeference, could I create polygon by colour, convert polygon to outline, save outline as KML? Coloured areas example below, ignore the green dots.

Coloured high seas

Will this all be confounded by the +/- 180 degree line problem?


I've found this plugin which claims to convert coloured areas to polygons.... but it doesn't work well.

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    How about manually creating the subarea boxes, then taking the difference between the buffer zones and the subarea boxes? – csk Sep 6 '17 at 15:53
  • thanks for the idea. manually create subarea boxes would be by hand or with manually written WKT which isn't impossible but more work than anticipated. Nonetheless, should work. Take the difference = vector>geoprocessing tools>difference? Just checking, haven't used these tools much. Thanks! – dez93_2000 Sep 6 '17 at 19:28
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    I will post my suggestion as an answer with more detailed instructions. – csk Sep 6 '17 at 21:17
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Manually create the subarea boxes, as described in the question. enter image description here Then take the difference between the buffer zones and the subarea boxes, as follows:

Vector > Geoprocessing tools > Difference

Input Layer: subarea boxes

Difference Layer: 200 nautical mile land buffer

enter image description here

enter image description here In the example, the Difference layer is generated as a temporary scratch layer. Be sure to right click > save as... before closing the project.

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