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I've been playing with Mapbox's blog post about converting heat maps to contour lines, and I'm stuck at the extraction part of this process.

I used the CSV plugin to create a vector layer of points, then a heat map raster layer based on that, but I can't seem to be able to see the contour lines based on that raster layer after that. When I looked at the properties and selected categorized, it didn't seem to have any symbols listed.

I'm guessing this is probably some kind of type error since I had to create the raster layer based off of map units and not meters, but I don't know how to correct it. What am I missing?

Here's a screenshot. It's a map of racially charged geographic locations based on their name. The main difference being that I've stripped any instances of Arroyo Negro, but left in more explicit cases.

qgis screenshot

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How did you create the contour lines? Did you use the gdal tool in the "Raster->Extraction" menu? –  Micha Nov 18 '12 at 7:54
    
@Micha Yes I did. Was that a bad thing? –  geraldarthur Nov 18 '12 at 16:47
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No, that's the correct procedure. What did not work as you expected? What do you mean "didn't seem to have any symbols listed" ?? –  Micha Nov 20 '12 at 13:41
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could you provide a screenshot so we don't have to guess what's going on? which attribute did you chose for categorization? –  underdark Nov 20 '12 at 20:25
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You may have better results using kriging or interpolation. –  Aaron Nov 20 '12 at 22:35
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Okey dokey. After the holiday weekend I figured it out.

It was basically a type error with the CRS. Although it's not explicitly stated, I got in touch with MapBox and found out they were working in Google Mercator. It's also a perfectly acceptable CRS to use when you eventually have to import the .shp file into MapBox/Tile Mill.

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