TERRAIN INDICES

This page explains the indices that are derived from terrain data and the maps that arise from them.

MESO-SCALE ELEVATION RESIDUAL (MSER)

McRae (1992) showed how lightning ignitions tend to occur where the actual elevation surfaces and the macro-scale elevation surfaces intersect. The difference is generated in the grid calculator. The more rugged landforms are more prone to these ignitions, and this is factored in with the following equation, expressed in the terms used by the grid calculator:
Power(Sqr( MSER -30 )+1,-0.5)* MACRO-SLOPE /22
and it is called the Natural Ignition Index (NII).


A: B:

Figure. Grids examples.
  • Panel A is the MSER. Red areas are highly exposed, blue areas are highly incised.
  • Panel B is a map of areas prone to lightning ignitions, the NII




Figure. The cross-section through Wadbilliga National Park in the south coast of NSW, showing the actual elevation as the black line. The colour that the black line is over at any point shows the MSER category:
  • Red is very exposed.
  • Blue is very incised.
  • Green is roughly average and is where lightning ignitions tend to occur.


WIND REGIME

Measured winds on rugged terrain has shown how complex the wind regimes can be (Sharples et al 2009). We have provided a relatively simple classification of wind regime that makes shows primarily how complex the regime is.




Figure. The cross-section, showing the actual elevation as the black line. The colour that the black line is over at any point shows the wind regime:
  • Orange is exposed to the prevailling winds.
  • Green is also prone to lee-slope eddies forming, and becoming dominant when the prevailling winds exceed 25km/hr. This permits fires to leap-frog across the landscape when fuels are very dry, as all slope are uphill runs with the wind.
  • Blue is additionally prone to valley channelling. These sites are so incised that air flow in the valleys can be driven either upvalley or downvalley by pressure gradients or by friction.


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