TERRAIN RANGE

This page explains the mathatical analyses of terrain data and the maps that arise from them.

RANGE

The first step in the analysis of terrain is to manipulate the data set to the following parameters: The work was done within Vertical Mapper in MapInfo Professional.
As a normal desktop computer system cannot cope with analysis the entire grid, it was then subsampled back into degree-by-degree tiles but with an additional 4 kilometres of data around that tile. Most of this buffer was later trimmed off, taking with it any edge effects.
The key steps were, for each tile:

Figure. A screen shot of the user interface for the Custome Point Estimator. The output grid is set to 250m resolution, the search radius is set to 1500m and the output is the minimum elevation value found within the 1500m search radius for each point. Identical runs are then done for the maximum and median values - the output file names ending in turn with MIN, MAX and MACRO.


The relationship between the actual, minimum, maximum and median values is explained in the figure below.


A: B: C: D:

Figure. Examples of the new grids, along a cross-section through Wadbilliga National Park in the south coast of NSW.
  • Panel A is the actual elevation data.
  • Panel B is the minumum elevation within a 1500m radius of any point. Note how it has the appearance of being etched out.
  • Panel C is the maximum elevation within a 1500m radius of any point. Note how it has the appearance of being puffed out.
  • Panel D is the average (median) elevation within a 1500m radius of any point. Note how it has the appearance of being smoothed out.


After these layers have been created, the remaining layers are much more straightforward. Firstly we need to generate a slope grid for the MACRO grid. Then the rest are generated in Vertical Mapper's grid calculator. They are shown in the figure below.


A: B:

Figure. Examples of calculated grids, along the cross-section through Wadbilliga National Park in the south coast of NSW.
  • Panel A is the raw difference between the maximum and minimum grids.
  • Panel B is the same but classified into three ranges: beige indicates a range of up to 150m, light green a range of 150m to 300m, and dark green a range of over 300m. The largest ranges seen are around 1200m.


The relationship of these layers may be expalined more clearly by looking along the cross-section. The figure below shows this.






Figure. The cross-section, showing the terrain elements discussed above.
  • The actual elevation is shown by the heavy black line.
  • The mimimum and maximum elevations are shown by heavy orange lines. Note how they bracket the actual value. As this is a one-dimensional cross-section, it may not pick up the actual values that determine the extremes seen in the orange lines.
  • The median elevation, called the macro-scale elevation, is shown by the blue line.
  • The range is shown by the orange points.


Note that the median elelvation is termed the macro-scale elevation is recognition of the fact that this is how elevation appears on a macro-scale topographic map.

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