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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

1 Answer 1

up vote 4 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 http://gis.stackexchange.com/a/23863/5850

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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

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