- Digital Charts of Lough Corrib


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Creating Islands and Holes

How the Chart is made

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Create your shoreline – right click and “close polyline”

Right click and copy the shoreline to the clipboard, then convert the shoreline polyline to a land polygon and paste back the shoreline - as detailed in the shoreline tutorial

Let us assume that we have some dangerous shallows around the island, and some fishable shallows of 4-10ft – we’ve already been and mapped them out and have the contours available (see depth mapping tutorial). So we take our 4ft contour, and our 10ft contours and put them onto the chart

Starting from the deepest contour, select, copy it to the clipboard, convert to a polygon (if using the corribskin.txt as a skin select depth polygon “<10”, and paste the contour back – just as if creating an island

Now do exactly the same for the 4ft contour, and convert it to a “shallow” polygon

Next click anywhere in the “<10” area, hold down the ctrl key, and click in the “shallow” area, release the ctrl key then right click in the “shallow” area and select “subtract polygons”. This is where a lot of folks think something has gone wrong and give up – bear with me…. you now have something looking like this

Now go to Tools – “Merge inner polygons"

Now do the same process with the “shallow” polygon and the “land” polygon, don’t forget to “merge inner polygons” when you have subtracted

To create a hole simply subtract one polygon from another, then delete the first polygon. “Merge inner polygons” when you have done it

You will notice that the polygon takes on the attributes (label and any extras) of the polyline you created it from, so if you want to you can simply delete the label on the polygons by double clicking and editing the properties. It keeps the chart tidier

Always start from the outside and work inwards when subtracting polygons

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