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Consider a polygon that represents the area of New York State enter image description here

I made a 125KM buffer from this layer enter image description here

My problem is that the new buffer layer is covering the source polygon and I need only the outer borderlines of the 125KM radius. I need it as a new line layer for future work, so just changing the fill of the polygon into 100% transparent will not work enter image description here.

How can I display only a line layer that is located in a distance of 125KM from the borderline of the New York State polygon?

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    The easiest way for showing just the outline of the buffered polygon is to edit the rendering styles of that layer and remove the fill. – user30184 Aug 13 '17 at 12:35
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    This is the correct output of a buffer operation. If you want to isolate the "outside" you just need to Erase (or Union Or Intersect) the source from the result. – Vince Aug 13 '17 at 13:00
  • Yes, but I only need the outline border not the all of the area between the source polygon and the new one – Nitzan Matan Aug 13 '17 at 13:26
  • You could convert the buffer polygon to a line with Vector -> Geometry Tools -> Polygons to Lines. – AndreJ Aug 14 '17 at 6:13
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If the aim is to show only the borderline of a polygon there is no need to convert the polygon layer into line layer. It is enough to make polygon fill transparent by adjusting the rendering styles.

Here is a polygon that is created by buffering Colorado

enter image description here

For showing just the borderline make QGIS to use transparent fill

enter image description here

  • ... or change the fill style from Solid to No Brush. – AndreJ Aug 14 '17 at 6:15
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For this specific problem : "My problem is that the new buffer layer is covering the source polygon"

If you need only the area outside, you could use the following tool "Symmetrical difference" in the QGIS geoalgorithms. After that, you could convert this polygon to a line layer

Tool

Configuration :

Toolbox

Result with no overlay :

Result

Then if needed :

Convert tool

And a conversion just for the line you need (delete the other one) :

line

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I spent some time looking into it and found a pretty nice solution.

When I made a new buffer layer in MMQGIS it's making a new layer with an extra area respectively to the radius we defined.

So, if we have only the outer line of the buffer we will get the borderline we needed.

In the Vector tool bar there is an option to transfer a polygon layer into a line layer that represented only the outer line of the polygon enter image description here

We need to be sure that we are choosing the right layer and some other layer from our project enter image description here Once we have done thus steps we will have a temporary layer which is recommended to save as a layer. It's also recommended to measuring the radius, just for being sure that our result makes sense enter image description here

We cannot be one hundred accurate in this kind of hand measurement, so small a deviation of 880 meters, or 0.58%, it is probably as good as it gets.

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