W6 Flashcards
What is a DTM? Provide an example
A digital terrain model.
Any digital representation of a continuous surface
ex. Elevation, contours
What is DEM?
Digital Elevation Model, is a digital representation of the Earth’s surface topography.
- each cell contains a value representing the elevation or altitude of the corresponding location on the Earth’s surface.
What is digital terrain modeling?
A simple representation of a portions of Earth’s surface
- encompasses many general tasks
What are contours helpful for?
Visualizing steepness, identifying areas within certain elevation zones
What is aspect?
Aspect in GIS refers the horizontal angle of the steepest slope.
How is aspect calculated?
By finding the slope (%) one is able to determine aspect. Slope is equal to the elevation difference/resolution of cells. Then take that percentage to find the aspect.
How does slope correspond to aspect?
North = 0
East = 90
South = 180
West = 270
How do you compute a slope from a DEM?
Many different ways. Neighbourhood cells can cause many different values. The easiest option is maximum downward gradient.
How does maximum downward gradient work?
- First establish which of the neighbouring cells has the steepest gradient
- then I am not sure
What are some other slope methods
- second order finite difference
- third order finite difference
- multiple regression fit to 9 points in a window
What is a z-factor
It is a scaling factor used to convert the units of elevation in a Digital Elevation Model (DEM) to the same units as the horizontal coordinates.
Why do we use the z-factor conversion?
it adjusts for the difference in scale between the horizontal (x and y) and vertical (z) units
- In many cases, the horizontal units are different from the vertical units. Without proper scaling, the elevation data might appear exaggerated or flattened compared to the horizontal coordinates.
What is a slope map?
A slope map is a type of thematic map that visualizes the steepness or gradient of the terrain in a geographic area. It represents the rate of change of elevation over a given distance, typically expressed as a percentage or angle.
What other focal features are used to characterize terrain?
- Curvature
- roving window analysis
- aspect
- viewshed
- hillshade
- cut and fill
- 3D visualization
How to calculate curvature simply?
From the elevation at a location, subtract the mean of the surrounding values
- positive deviation values indicate convex and negative indicate concave surface
What is aspect useful for?
- Identifying the direction that a hill faces
- Identifying areas that are (or are not) exposed to strong solar illumination
What is viewshed
Shows the observer visibility
- Identifies areas that are visible from a certain location
What is viewshed useful for?
Useful for identifying how visible a new construction may be or identifying suitable locations
How is a viewshed made?
It is created from a DEM by using an algorithm that estimates the differences of elevation from one cell to the next
- to determine the visibility of a target cell, each cell between the viewpoint cell and the target cell is examined for line of sight. If there is a cell of higher value between viewpoint and target cell, then the target cell is not included in the viewshed.
What does hillshade do?
Calculates the hypothetical illumination or shading for a given sun position
What is hillshade useful for>
- might be important for vegetation
- makes maps look nicer
What is cut and fill
Summarizes areas and volumes of change between two surfaces
