W6

Question 1

What is a DTM? Provide an example

Answer 1

A digital terrain model.
Any digital representation of a continuous surface
ex. Elevation, contours

Question 2

What is DEM?

Answer 2

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.

Question 3

What is digital terrain modeling?

Answer 3

A simple representation of a portions of Earth’s surface
- encompasses many general tasks

Question 4

What are contours helpful for?

Answer 4

Visualizing steepness, identifying areas within certain elevation zones

Question 5

What is aspect?

Answer 5

Aspect in GIS refers the horizontal angle of the steepest slope.

Question 6

How is aspect calculated?

Answer 6

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.

Question 7

How does slope correspond to aspect?

Answer 7

North = 0
East = 90
South = 180
West = 270

Question 8

How do you compute a slope from a DEM?

Answer 8

Many different ways. Neighbourhood cells can cause many different values. The easiest option is maximum downward gradient.

Question 9

How does maximum downward gradient work?

Answer 9

• First establish which of the neighbouring cells has the steepest gradient
• then I am not sure

Question 10

What are some other slope methods

Answer 10

• second order finite difference
• third order finite difference
• multiple regression fit to 9 points in a window

Question 11

What is a z-factor

Answer 11

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.

Question 12

Why do we use the z-factor conversion?

Answer 12

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.

Question 13

What is a slope map?

Answer 13

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.

Question 14

What other focal features are used to characterize terrain?

Answer 14

• Curvature
• roving window analysis
• aspect
• viewshed
• hillshade
• cut and fill
• 3D visualization

Question 15

How to calculate curvature simply?

Answer 15

From the elevation at a location, subtract the mean of the surrounding values
- positive deviation values indicate convex and negative indicate concave surface

Question 16

What is aspect useful for?

Answer 16

• Identifying the direction that a hill faces
• Identifying areas that are (or are not) exposed to strong solar illumination

Question 17

What is viewshed

Answer 17

Shows the observer visibility
- Identifies areas that are visible from a certain location

Question 18

What is viewshed useful for?

Answer 18

Useful for identifying how visible a new construction may be or identifying suitable locations

Question 19

How is a viewshed made?

Answer 19

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.

Question 20

What does hillshade do?

Answer 20

Calculates the hypothetical illumination or shading for a given sun position

Question 21

What is hillshade useful for>

Answer 21

• might be important for vegetation
• makes maps look nicer

Question 22

What is cut and fill

Answer 22

Summarizes areas and volumes of change between two surfaces