Monday, January 12, 2015

Elevation and Vegetation at a Glance

Long distance hiking leaves plenty of time for reflection, and an accelerated JMT trip I did this fall was no exception.  Despite being armed with maps and elevation profiles, I realized that I still wasn't aware whether a given day would be spent in shady forests or on barren, south-facing slopes.  I'd also been pondering a better way to size up search areas for SAR, and decided to solve both those problems with a new stats dialog.  It took a couple months to put the idea into production, but it's finally here.

Like with elevation profiles, there are two ways to bring up a stats dialog.  One is to right click on a line or polygon, and choose Stats.  The second is to right-click on the map background, choose Measure, and then either Line Stats or Area Stats.

Either option will probably involve a short delay as CalTopo pulls up the relevant elevation and vegetation data, followed by a dialog.  Unlike with elevation profiles, polygons will bring up data for the polygon's interior rather than just its perimeter.

The first two charts are elevation and slope histograms, along with min, max and average values; color-coding on the slope histogram matches CalTopo's slope shading layers.  The third chart is a circular histogram tracking aspect across 45 degree slices.  The pie slices are area-proportional rather than radius-proportional, i.e. if N aspects are 50% as frequent as W aspects, then the N pie slice will cover half the area of the W pie slice, with a radius that's 70% as large.

The tree cover histogram shows tree canopy coverage - 100% means that trees completely block all views of the sky.  Land cover shows land coverage as listed in the National Land Cover Dataset (NLCD).

In addition to other benefits, this provides another quick sanity check on planned routes for backcountry skiing.


  1. Amazing feature! Thanks for all your work.

  2. Hmm, right click / ctrl click on Mac works the 1st time on a polygon for me, but then won't respond a 2nd time.

  3. Hate to be such a newbie but where can I find out what the symbols are on the different layers are?

    1. Like this, or something else?

  4. Hi Matt,

    I am trying to create an interface similar to yours, but I would like to be able to see the high resolution contour lines (on the national scale) all through the Google Earth interface. Is there a way I could download a large kmz of the contour maps for the entire US? (

  5. Hi Matt,

    I would like to alter the histogram which is created for slope angles in the terrain analysis.

    I want to create my own x- axis value (0-20, 21-25, 26-30 31-35, 36-40, 41-45, 46-50, 51-90) as well as create my own color for each of these values.

    This level of detail would be similar to how we can alter DEM shading. How possible would it be to change the displayed angle histogram to my needs?


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