Saturday, December 10, 2011

Introducing the 40' Contour Layer

I've just added a contour line layer to CalTopo.  Current coverage is California only, although it will be expanded to the western US during the next month or two.  California provides its elevation data online; for the rest of the country, you actually have to mail a hard drive to the USGS.  What is this, the 90s?

The contour lines are the first in a series of what I'll call "transparent overlay" layers; these are vector data drawn on a transparent background that show up as checkboxes in the overlay selection box.  The contour layer is intended to sit on top of aerial imagery; in an environment without trails or roads, combining it with shaded relief can provide better terrain visualization than a traditional topo.  The other benefit is that by printing an aerial+contour map and a traditional topo, it becomes easy to match locations between the two.

One difficulty in creating the contour layer was choosing a line color.  Traditional USGS brown works great on topo maps because it only has to mix with a couple colors, all light.  Aerial images have a much wider range, and a noisier background.  The USGS has decided to stick with brown for its "US Topo" aerial-based maps, but I find the lines difficult to follow, especially on prints.

The other issue was line weight.  Too thin, and they're hard to follow; too thick, and you start obscuring details in the underlying map.  Red seemed like a good bright, unnatural color, but it didn't mix well with CalTopo's SAR origins: autogenerated assignment and debriefing maps use red for the line color.  I also found that my brain was accustomed to brown, and that bright colors were harder for the back of my mind to turn into a mental terrain map.

Parts of Europe use orange for their contour lines, and in the end I settled for an orange-brown color.  Is it perfect?  Probably not.  Does it get the job done?  I think so.  Hopefully you agree.

Top of Avalanche Gulch, Mt Shasta

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