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I have a shapefile of polylines for trails, and I need to convert them all to be polygons that are 8 ft wide (4 ft from the center of each side of the polyline). I tried a following a few other posts that I thought were doing the same thing, but I always just ended up a big blob of color.

  • Can you clarify what you have tried and/or post a screenshot of your results? Buffering your lines at 4ft should be generating the output you want. – Chris W May 28 '14 at 19:26
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I can't tell if "blob of color" means all the trails are now joined together, or that it takes a lot of screen area.

If the former, see if you have dissolve buffer results checked, as it will dissolve all the 8 ft wide polygons representing individual trails into one.

If the latter, it might be a symbology problem, where you have set the outline too thick, try thinning it, or changing the border of the polygons from millimeters to map units.

  • That certainly helped me narrow down my problem. It seems I was using far to large of a buffer distance and everything looked like one big blog. When I changed the buffer from 1 to .0001 I could at least see the different areas. However, what units is this scale using? I tried the directions in gis.stackexchange.com/questions/30987/… but I don't fully understand projections and it but me in a completely different part of the country with my converted file to NAD83-UTM16. I am current in WGS84. – Scott May 28 '14 at 21:49
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    Yes, it appears you're using degrees for your units. 1 degree is 111 km (69 miles) at the equator, so by setting buffer distance of 4, you buffered your trails by 444 Km. NAD83 is a meter grid, which means QGIS will do the buffer calculations in meters. Define your project CRS as NAD83, Then export your trail data - which is in EPSG:4326, aka WGS84 - to EPSG:26916, aka NAD83. Now the buffer will work in meters, which is easier to convert to from feet (4 ft = 1.2192 m). – HDunn May 29 '14 at 6:10
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    @user1098504 I think it's important to point out here that NAD83 is a datum. It can be used in both GCS and projected coordinate systems, so just because you see NAD83 doesn't mean you're in a CRS that isn't using degrees. UTM on the other hand is a projection that does use meters for units. By default it typically uses the WGS84 datum, but it can use the NAD83 datum as well (the two are very similar). The important thing to know/remember is that for area and distance work you need to be in a projected coordinate system. – Chris W May 29 '14 at 18:15
  • @Chris W, so when QGIS gives me the option of WGS 84/UTM zone 16N (EPSG:32616), would that allow me to measure in meters without leaving the WGS? Thanks for bearing with me on this as my understanding of projections and datums probably could use some improving. – Scott May 29 '14 at 23:17
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    @user1098504 That's correct - there is no real need to change datums. You just have to be in a Projected Coordinate System (PCS) and not a Geographic Coordinate System (GCS). Also just to note there are other projections you could use if you want to work in feet. As far as learning about projections and datums, I find this pdf a good resource. – Chris W May 30 '14 at 2:31

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