Topic 13: Terrain mapping and analysis

Question 1

Acquiring data for terrain mapping

Answer 1

3 common ways:
- Ground-based methods, such as GPS and total station surveys (not practical for broad extents, but can be very effective over small areas)
- Airborne direct measurement using technology like LiDAR
- Photogrammetric methods using imagery (eg., from air photos or drones)

Question 2

Representing terrain: DEMs, and TINs

Answer 2

Digital elevations models (or digital terrain models or DEMs)
- a most common form of terrain data
- Digital elevation model: bare earth model
- Digital surface model: the surface of the earth (trees, buildings, etc.)

Triangulated Irregular Networks
- surface model created from vector data

Question 3

Visualizing terrain data

Answer 3

Contouring
- usually relate to elevation, but can be done for nearly any feature at interval or ratio scale
- isolines
- important to visualize terrain, particularly for static maps

Hill shading
- improve the visualization of terrain data from DEMs
- unidirectional vs multidirectional
- Hypsometric - assigns colour to values and can be overlayed hillshade

Eyton (1990) - Colour stereoscopic effect
- Importance of colour sequence, hillshade, and contour for depth perception
- blue always looks more far away

Vertical Profiling
- interpret changes in elevation for line or path overlaid on a terrain map
easily view the changes in elevation from a “side-looking” view

3D rendering of digital surfaces
- have become very sophisticated in recent years, provide realistic views of terrain surfaces and features

Question 4

Measurements using terrain data and derivative terrain products
- slope and aspect

Answer 4

Slope computations
- computation is the ratio of two components: vertical component (Essentially a modified Pythagoras formula) and window distance (ie., 2 x the cell dimension)
- convert to percentage by multiplying by 100
- convert to degrees by taking arctangent of the ratio

Aspect computations
- Aspect is expressed using angles on a unit circle (circular data)
- based on an algorithm that compares difference in slopes in x and y directions

Question 5

Precision of slope and aspect maps

Answer 5

• the derivation of slope and aspect from terrain models are very sensitive to precision/accuracy of the input DEM or TIN

Question 6

Measurement using terrain data and derivative terrain products
- surface curvature

Answer 6

• if we take the spatial change in slope or aspect, we get curvature
• we compute higher-order derivative products from terrain models (eg., input map: digital terrain model; First order product: slope angle, aspect; second order products: slope profile and plan curvature)