How to Calculate Raster Slope using Angle and Aspect

How to Calculate Raster Slope using Angle and Aspect


Raster slopes can be measured in two ways,
by using the slope angle or by using the slope aspect. In FME slope angle is measured with
the raster slope calculator and the slope aspect is measured with the raster aspect
calculator. In this tutorial we will use both of these transformers to see what they do. Let’s open FME workbench and begin with an
empty canvas. Add a reader and set the data format to Canadian
digital elevation data or CDED for short. Use the attached data set found on the corresponding
FME knowledge-based tutorial. Place a raster aspect calculator and a raster
slope calculator on the canvas. Draw a connection from the reader to each
of the transformers. Connect the output from each transformer to
an inspector. Go ahead and run the workspace. The results will be two raster features that
appear in the FME data inspector. One raster surface representing slope and the other representing
aspect. Let’s select the slope layer. Query the raster cells on the slope surface by clicking
on them; each cell represents the slope at that location. The areas with brighter spots
have a steeper slope. The value of the slope for each cell location is displayed in the
feature information panel. Now let’s select the aspect layer. Query the rasters again
notice that each cell represents the direction of the slope at that location from 0 to 360 degrees.
FME doesn’t visualize this very well so let’s improve how slope aspect is displayed by modifying
the raster surface using the raster expression evaluator transformer. Let’s return to our previous work space and
place a raster expression evaluator on the canvas. Open its parameters dialog, set interpretation
to preserve. double-click the expression field. Open the arithmetic editor. We will enter
an expression to calculate the slope aspect results from 0 to 180, both clockwise and
counterclockwise – this will greatly improve the display. The expression is included on
the tutorial webpage. Disable the raster slope calculator connection
and run the workspace. The newly calculated raster aspect surface
will be displayed in the FME data inspector. Notice how the coloring is now more consistent
making it easier to see the dark areas that face North. Our next exercise will use raster data and
the raster slope calculator to calculate the suitability of land for development. A building
development usually prefers land that is both flat and low-lying. Let’s run a quick analysis to find the best
areas for future development. Start FME workbench and begin with an empty
canvas. Add a reader for CDED and select the dataset. Place a raster slope calculator into the workspace
and draw a connection from the reader to the transformer. Next place a raster cell value
calculator. Notice how this transformer has both an A and B input port. Connect the raster slope calculator output
port to the raster cell value calculator B input port. Draw a second connection from the reader to
the raster cell value calculator A input port. Open the parameters dialog for the raster
cell value calculator. Simply change the operation parameter to the + option; this will add the
original data, representing elevation, to the transformed data, representing slope. Let’s run the workspace. The results will be areas for building development.
Notice how lighter colors represent cells that have both a steep slope and a higher
elevation. Darker colors are cells that have both a shallow slope and a low elevation. For the purposes of our example, darker colored
cells would be good candidates for building development.

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