Wednesday, February 8, 2012

Avalanche Slope Analysis

CalTopo has a slope analysis layer that can shade slopes by angle and aspect.  You may or may not find it useful for planning safe travel routes in avalanche-prone terrain, and I make no claims that it is either accurate or useful for this purpose.  However, if you do decide to use it for route selection, it is imperative to understand that this is just one of many tools you should use and that it has serious limitations which can get you killed if you don't understand them.

Although it may look tempting, you cannot transfer an avalanche danger rose into CalTopo and expect that the result will keep you safe.  Remember that an avalanche forecast is a broad assessment covering a large range of terrain, and is not intended to be used with the pinpoint accuracy that CalTopo's aspect shading provides.  I'll list a few known limitations, but there may be others, including gross human error on my part.

Broad v. Local Aspect:  A slope's surroundings are as important as the aspect of an individual point.  Consider the screenshot below; NE and E aspects are shaded orange and the rest yellow.  If a forecast called for wind-loading of E aspects, it might be tempting to consider the yellow areas unaffected, but in reality the entire slope is on the east side of a major ridge.  Local variations in aspect are likely to be less important than the overall orientation of the slope, but CalTopo will not account for this.

Despite local north and south aspects, this entire slope is in the east shadow of a ridgeline

Runout Zones: At best, this shading will only help you determine avalanche starting zones.  Runout zones may extend well beyond the shading onto gentler terrain.

Despite being unshaded, the creek in this image is not a safe travel zone.

Data Limitations: Elevation data is provided by the USGS at approximately 30' intervals, but the original data may have been sampled more coarsely and then interpolated.  Slope and aspect calculations can only be as good as the source data, and may be off.

Snow Changes the Slope Angle:  If snow accumulates unevenly (like on the leeward side of a ridge), the slope angle on bare ground may not match the observed slope angle on top of the snowpack.  CalTopo's slope shading does not account for this effect.