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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.

underdark
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user2105469
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2 Answers2

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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.

artwork21
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  • I've looked for a Projected Coordinate System suitable for the continental US, found one at http://gothos.info/tag/map-projection/ (+proj=lcc +lat_1=20 +lat_2=60 +lat_0=40 +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs,North America Lambert Conformal Conic), went to Settings>Custom CRS and created a new customer CRS, save the new projection, exited & reloaded QGIS, made sure OTF was on -just in case- and that both inland polygons & coast line were set to the new CRS, but looking at "Set Layer CRS" for each layer I see: "This layer appears to have no projection spec" – user2105469 Mar 30 '15 at 15:47
  • It appears that CRS exists in Q already EPSG = 2163, +proj=laea +lat_0=45 +lon_0=-100 +x_0=0 +y_0=0 +a=6370997 +b=6370997 +units=m +no_defs – artwork21 Mar 30 '15 at 15:52
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    Don't use Set CRS for Layer, but Save As ... to another file name and target CRS. – AndreJ Mar 30 '15 at 15:53
  • @AndreJ: I looked at Set CRS for Layer on the new PCS after running Save As, to confirm the new PCS was indeed using the CRS I expected (it did). – user2105469 Mar 30 '15 at 16:09
  • @artwork21 , I'm trying Q's built-in Projected Coordinate System. – user2105469 Mar 30 '15 at 16:11
  • Inland polygons & coast line & Project CRS are all set to EPSG:2163 (OTF still on). However, defining a coast line buffer leads to a "Feature Geometry Error". I'll look for a working example online, and work back from there. Thanks. – user2105469 Mar 30 '15 at 16:29
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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.

AndreJ
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  • OTF is off, all layers are in the same CRS (saved as EPSG:2163, then reloaded in new project with EPSG:2163 as project CRS). The problem is that the layers are located in separate areas of the canvas (according to the coordinates, after panning each layer separately). Getting closer... – user2105469 Mar 30 '15 at 18:11
  • Can you add the extent of the layers, as reported by Rightclick, Properties, metadata tab? And the buffer size you have chosen? – AndreJ Mar 30 '15 at 19:44
  • This is more practical than select/panning each layer, thanks: Coast: xMin,yMin 2321544.64,258328.06 : xMax,yMax 2333464.74,277553.12 and Cont_US xMin,yMin -176.685,17.9108 : xMax,yMax -65.2246,71.3413 – user2105469 Mar 30 '15 at 20:05
  • Buffer size is 500m (I'm working off MA coastal lines until I figure out the appropriate intersect procedure). – user2105469 Mar 30 '15 at 20:06
  • The extent of Cont_US is definitely in degrees. If you have set the CRS of the layer to anything projected, it is definitely wrong. And Intersecting can not work. – AndreJ Mar 31 '15 at 06:15