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I have created a polygon in QGIS (in this case it describes a country's maritime baseline).

I need to create a second polygon whose boundary is a fixed distance from the original polygon. (in this case I need to create the territorial sea boundary, which lies 12 nautical miles outside the baseline).

I think geoprocessing tools>buffer is correct tool for this job? But I can't figure out what units QGIS uses for the buffer distance.

What should I enter in buffer distance to generate a 12nm buffer?

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what is your data's coordinate system? – vinayan Aug 7 '12 at 8:09
up vote 5 down vote accepted

If your data is in Geographic Coordinate System(WGS84 etc..), the units will be in decimal degrees(angular). So I would project the polygon shapefile first to a Coordinate System with Meter as unit(like NAD83 UTM Zones..). Then the buffer distance will be in meters.

With a Quick google Search, find the conversion between Nautical mile and Meter.

12 Nm = 22224 meters

Now Run Vector > Geoprocessing > Buffer give 22224 as buffer distance

An excellent answer is here

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This was exactly the issue. I assumed buffer would use the units of the layer, but it seems to use the units of the map. So, I saved the polygon using a global mercator projection, added it to a new blank map which also uses a mercator projection, then added the buffer in meters. Perfect, thanks! – Andy Harvey Aug 7 '12 at 18:21
I'm sorry to revive an old post, but I'm running into a similar problem (I want to create a buffer in km) but I was wondering if using NAD83 UTM zones would cause problems if I was working with a map that covers a very large area, such as the entire continental US? I wanted to create 500km buffers around points throughout the US, and was wondering if reprojecting to NAD83 was still the right choice, and what would be the best one to choose? Thanks! – J. Taylor Dec 7 '15 at 8:51

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