Continuing from last week’s post, I will show you how to use terrain analysis to calculate:-
- ruggedness index
Slope is calculated by comparing the pixel value at a particular location relative to the surrounding 8 pixel values. This gives the steepness of the slope.
The Slope dialogue box is very simple:-
- Select the elevation layer (this will be the DTM raster)
- Select the output layer
- I have left the Ouput format and Z factor as default. If the ground is very flat, then exaggerating the z factor might make the slopes easier to visualise.
The aspect shows the compass bearing of the slope
The raster has been given values from 0-360 depending on the slope aspect. The darker areas with the lower values are the north to north east facing slopes; the lightest areas with the highest values are the west to north west facing slopes.
This calculates the amount of sun or shade for a 3D surface. The dialogue box is similar to the previous ones, however there are new options for the sun angle:-
This analysis uses a fixed location of the sun to accurately simulate the effects of bare hillside and shaded valleys. I positioned the sun to the south west (200 degrees), the east facing slopes around the River Medina estuary in the north of the island are very shaded, in contrast to the brightly lit west facing slopes on the other side of the river.
This is the most visually appealing and easily understood result and so it is often used as a backdrop for maps with other layers added.
The ruggedness index value is calculated for every location, by summarizing the change in elevation within the 3×3 pixel grid. Ruggedness index values are grouped into categories to describe the different types of terrain. The classifications are as follows:
Ruggedness Index Value
|Level||0 – 80m|
|Nearly Level||81 – 116m|
|Slightly Rugged||117 – 161m|
|Intermediately Rugged||162 – 239m|
|Moderately Rugged||240 – 497m|
|Highly Rugged||498 – 958m|
|Extremely Rugged||959 – 4397m|
The dialogue box for the ruggedness Index is the same as it is for the other types of analysis mentioned above. The IOW is all categorized as level or nearly level in the ruggedness index. This is despite it being quite hilly! I used the Stretch to MinMx contrast enhancement on the layer properties box:-
The result is quite different to the relief and hill shade raster’s. This is because, it doesn’t attempt to show actual slopes, rather it shows the change in elevation categorised as shown in the ruggedness index table. It is still easy to see the line of hills that cross east to west across the island.