This odd question comes from believing that such an analysis is plausible in a GIS system.

Only, I don't how to accomplish the various discrete steps implied from the Title, nor where to get a topo map that already has the iso-elevation contour lines developed in some GIS format, nor how to color in areas which face SW-to-SE, nor how to decipher the physical slope from the proximity of iso-elevation lines.

All these things, sensible to the engineer in me, but wondering what set of stackexchange questions (or other resources) might even begin to answer my question....

...in my ideal world, I scan in a USGS Topo map for the region of my interest, and out the other end of my QGIS black-box procedure comes a map with polygons lit up as my areas of interest.

What tools and techniques....? Learning resources....?

[and, if anyone wants to suggest 4 other appropriate tags, let me know....]

1 Answer 1


The following is a rough outline of what you might do. I won't include a great deal of detail, you can research further using these terms and/or ask new more specific questions.

Note: you will need to careful of coordinate systems. Firstly that they are the same for your datasets, and second that they use metric (metres) horizontal units (not actually essential, but there are a number of "gotchas" that will creep in otherwise). You may need to reproject one or more datasets.

  1. Get a Digital Elevation Model (DEM) in raster format from the NED (alternatives are SRTM or Aster, both downloadable free from earthexplorer.usgs.gov).
  2. Get roadway data in vector format. I don't know the best source for this, I just googled and came up with the NHPN dataset for U.S. Roadways.
  3. Use QGIS to calculate Slope and Aspect from your DEM (QGIS Toolbar->Raster->Terrain Analysis->Slope/Aspect...)
  4. Convert your roadway data to raster with the same dimensions and pixel size as your DEM (QGIS Toolbar->Raster->Conversion->Rasterize).
    • Hint - the Rasterize tool can create a new raster or burn values into an existing one. The easiest way to ensure your roadway/highway/interstate raster has the same dimensions as your DEM is to rasterize into an existing raster with those dimensions. To create an empty raster based on your DEM dimensions, in the Raster Calculator (QGIS Toolbar->Raster->Raster Calculator) create an expression that will ensure all output values are zero, something like "DEM@1" = 99999
  5. Use QGIS to calculate distance to interstates raster (QGIS Toolbar->Raster->Analysis->Proximity)
  6. Use the QGIS Toolbar->Raster->Raster Calculator to create an expression that specifies your criteria, something like: ("dem@1" > 1000) AND (("aspect@1" >= 135) AND ( "aspect@1" <= 225)) AND (("slope@1" >= 10) AND ("slope@1" <= 35)) AND ("proximity@1" <= 10000)
  7. Convert to polygon (vector) format if desired - QGIS Toolbar->Raster->Conversion->Polygonize
  • If you're really looking for polygons, you can start with the data Luke has identified. At step 4 buffer your interstates by the desired distance. New step 5 is to use the raster calculator to create a new binary layer that is 1 where the slope/aspect criteria are satisfied. Then use the raster polygonize tool to create polygons, which you can then intersect with the buffered layers. Be warned that you might get lots of tiny little polygons corresponding to single pixels or small groups of pixels that meet the slope/aspect criteria.
    – Llaves
    Commented Jan 5, 2015 at 5:21
  • Thinking polygons, as I would want to lay them on top of a real ('useful') map. I can't think of another endpoint, but if I'm missing something, let me know. Your efficiency schema, @Llaves, is appreciated
    – John
    Commented Jan 5, 2015 at 23:09
  • @Luke or anyone else: Does anyone have any thoughts on the physical scope of land for which one can realistically (take that word for your own meaning) operate on? I am thinking of the consequences of the size of data sets, processing time, crash potential, and more. County? One quadrangle Topo Map at a time? The entire state of (Choose one small state, like W.Va)
    – John
    Commented Jan 5, 2015 at 23:19
  • @John, I have no idea how big your US quads/counties are. I tested in QGIS with a 100Mb section of an SRTM DEM over Australia that was 6500x4000 pixels and each calculation took only a second or so to run. Roughly an area covering Washington DC to Philadelphia with 30m pixel resolution.
    – user2856
    Commented Jan 6, 2015 at 0:29
  • @Llaves: After creating polygons, how (generally) would I delete polygons smaller than a given area? [SQL or area testing using something or other in QGIS...?] My "area" would be some useful minimal size, bigger than the area of a pixel.
    – John
    Commented Jan 7, 2015 at 14:05

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