How to create a polygon (fixed width buffer) around a polygon in QGIS?

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?

Answer

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 https://gis.stackexchange.com/a/23863/5850