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Topography

topo sample

Understanding topography is a critical part of understanding the intensity, rate, and directions of a fire for firefighters.

Topographic maps are representations of a three dimensional surface on a flat piece of paper. To the left you can see a portion of a topographic map with shaded relief to see how the contours represent elevation. Most maps are laid out in rectangular blocks with sides parallel to latitude and longitude lines. Fires are often reported by latitude and longitude. Topographic quadrangle maps west of the Mississippi River are laid out in township, range, and section as well as latitude and longitude.

In the field, firefighters use compasses along with topographic maps to calculate sighting lines to determine fire direction of travel via a mountain top, rock formation, etc. Understanding topography is a critical part of understanding the intensity, rate, and direction of a fire. Slope and aspect are topographic features that are calculated to determine fire risk and behavior.

Slope can be defined in terms of ratios, percentages, or per mil (formulas). Slope is a ratio of rise over run or another way to think of it is height over distance, and then expressed in percentage.

On a topographic map (or a contour map) the rise represents a change vertically. The scale tells the distance (the horizontal value or x-axis or run). Remember to watch units (are they feet per mile? meters per km?). Make sure all the units plugged into the formula are the same.

slope

Slopes can range from slight to steep but the influence on wildland fire is substantial. The steeper the slope the faster a fire moves uphill. Flames are closer to the fuel source, radiation heat increases the dehydration and preheats the vegetation, resulting in ignition sooner than on a slight slope or level ground.

Aspect is the direction the slope faces (north, east, south, west). The aspect determines the effect of solar heating, air temperature, and moisture. In the Northern Hemisphere, south facing slopes receive more solar heating which results in lower humidity, rapid moisture loss, and lighter fuels such as grasses. Seasonal directions of solar heating should be taken into consideration when analyzing a slope's aspect.