I have a coastal line layer (
us-medium-shoreline from http://shoreline.noaa.gov/data/datasheets/medres.html ), which is loaded into a project with On-The-Fly reprojection ( WGS 84 / World Mercator CRS with
+proj=merc +lon_0=0 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs) and I'd like to create a 500m buffer around this shoreline with the ultimate goal of intersecting the shoreline with inland polygons.
I'm expecting a polygon with the coastal line as its center, but I keep getting a circular region as resulting layer (with empty attributes table). Clearly I'm missing something as a GIS newbie.
I'm guessing it is b/c your layer is in geographic coordinate system (Nad83) not projected coordinate system. Perform a save as on the layer and change the CRS to a projected coordinate system then try the buffer on the new projected layer.
You have not mentioned what buffer size you entered.
If the source data is in degrees, the buffer size will be in degrees too, while a source in meters results in a buffer size in meters.
To avoid confusion, turn OTF off and see if both layers still align.