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We are interested at the "Pole of inaccessibility" (https://en.wikipedia.org/wiki/Pole_of_inaccessibility) of our country. Our approach is to use QGIS, and download coastline ([ngdc.noaa.gov]), and repeatedly create "contour lines of equal distance" until there is a point which is the farthest point away from the coastline. Do you know how to generate a polygon which is an inner polygon and has equal distance the outer one? An example of contour lines of equal distance on the Wikipedia page:
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You could use negative buffer values on your polygons. However, in your case, I would suggest the you compute the distance from your coastlines (Raster > Analysis > Proximity (Raster Distance)) in order to get a continuous distance raster. Then you can compute the local maxima with some SAGA GIS tools or you can derive your contour lines (raster > extraction > Contour). Warning: measuring distances globally is tricky. Ideally you should probably work in geodesic distances in order to avoid artefacts caused by the coordinate system. For instance, using r.grow.distance with metric=geodesic. |
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A possible alternative method is to calculate the pole of inaccessibility directly. Vladimir Agafonkin recently published a quick (though somewhat imprecise) method for this that is easy to implement. There are also a number of papers with precise but more complex methods available. |
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You should take a look at using "Raster > Analysis > Proximity" in QGIS which creates a raster with a value of its distance from a location in each pixel (in this case your coast line). Then just use "Raster > Extraction > Contour" and set a reasonable distance between contours. Because the pixels are all the same size, the incremented distance will increase linearly so your contours that are 10, 100 or 1000 metres apart will be equidistant. The "Pole of Inaccessibility" furthest point would also have the highest value. To note: you need to use a closed polygon, which is fine if you are usign it on an island, however you'll need to create a polygon if you are searching for somewhere out to sea. |
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