Module 11

Question 1

Terrain Analysis

Answer 1

terrain analysis is the process of using topographic data to derive terrain
attributes

is the analysis and interpretation of topographic features through geographic information systems. Such features include slope, aspect, viewshed, elevation, contour lines, flow, upslope flowlines and dowslope flowlines

Question 2

DEM’s and Terrain Analysis

Answer 2

▪ digital elevation models emerged in
photogrammetry in the 1950s and since then, digital
point elevation data in the form of DEMs have
gradually replaced contours on topographic maps
and have become the main source of topographic
data
▪ supported by DEMs and other forms of digital
topographic data, digital terrain analysis takes the
place of manual terrain analysis – it is now one of the
most powerful tools in GIS for environmental
modeling and landscape visualization

Question 3

__________ are the most widely used digital terrain model due to their simplicity and ease of computer implementation

Answer 3

digital elevation matrices

Question 4

TINs

Answer 4

TINs use the vector model to represent terrain by using contiguous, nonoverlapping triangle facets that consist of planes linking the three adjacent elevation points in the network

Question 5

Breaklines in TIN

Answer 5

▪ breaklines are line features or boundaries of
area features with or without height
measurements
▪ hard breaklines represent a discontinuity in the
slope of the surface, capturing abrupt changes in
the surface (eg road cuts, streams)
▪ soft breaklines do not change the local slope of a
terrain surface, but mark an important feature of
the terrain (eg, coastlines, study area boundaries)

Question 6

DEM vs TIN

Answer 6

DEM=RASTER
-▪ DEMs can be readily integrated with remotely sensed environmental data – there
are well-established algorithms for digital image processing that can be used to
process DEMs
▪ however, DEMs can have erroneous sinks or depressions generated during their
production as a result of data input errors, insufficient input data, interpolation
effects, or by rounding and averaging errors

TIN=VECTOR
-▪ vector contours are simply digitized contour lines and are only used in GIS for data
capture and visualization, not for analysis
-Triangular irregular networks (TIN) have been used by the GIS community for many years and are a digital means to represent surface morphology. TINs are a form of vector-based digital geographic data and are constructed by triangulating a set of vertices (points). The vertices are connected with a series of edges to form a network of triangles.

Question 7

DEM sink

Answer 7

▪ a sink is a cell or a set of spatially connected cells in a DEM that are surrounded by
higher elevation values
-▪ while there are real sinks in nature
(sinkholes, quarries), they tend to be
rare
-▪ sinks are particular problems for
hydrologic research, creating false
drainage patterns – a DEM that has its
sinks removed is known as a
hydrologically correct DEM

Question 8

Removing Sinks in DEMs(2)

Answer 8

1.▪ sinks can be removed or ‘filled’ by raising the values of cells in depressions to the value of the depression’s spill point, the cell with the lowest value surrounding the
sink
▪ a smoothing filter (eg, moving average) is a common method of removing sinks on a large scale, but it may also remove real sinks, as well as raise the average
elevation of the terrain and create artificial flat areas

2.▪ an alternative approach to sink filling is
using the ‘Topo to Raster’ tool in ArcGIS,
which couples the DEM to a watershed
model, showing where the lakes and rivers
are located
▪ since rivers and lakes are natural sinks, the
algorithm knows to keep them, while DEM
sinks found far from the lakes or rivers are
assumed to be error sinks and are filled

MOST DEMs are not hydrologically correct and therefore contain spurious sinks

Question 9

the ____ instrument onboard the Terra satellite is capable of generating DEM quality elevation data using stereo pairs of imagery
– the product has a horizontal resolution of 30m and a vertical resolution of 15m

Answer 9

ASTER

Question 10

InSAR

Answer 10

▪ InSAR acquires multiple radar images of the terrain to extract 3-D information, with
accuracies as good as traditional photogrammetric techniques

Question 11

LiDAR

Answer 11

LiDAR produces masspoints (an x,y location with an elevation) – the space between
points can then be interpolated to produce a continuous elevation surface

Question 12

Terrain Data Sources(3)

Answer 12

1. ASTER
2. INSAR
3. LIDAR

Question 13

____ ____ _____(DEMs) provide a basis for extracting terrain-related attributes or features, and can be divided into primary and secondary attributes (2)

Answer 13

Digital Terrain Models

▪ primary attributes: are mainly used to describe the size, shape, and other surface attributes of hillslopes, catchment boundaries, and stream channels – ie, elevation, slope, aspect, profile and plan curvature, flow accumulation, drainage network, flow length, upslope area…

.Secondary attributes:
▪ secondary attributes are used to characterize the spatial variability of specific environmental processes occurring in the landscape
– they tend to be applied to quantify the influence of topography on the spatial distribution and abundance of
water, soil, fauna, and flora

Question 14

for both DEMs and TINs, _____
is calculated using the normal
vector approach

Answer 14

slope

Question 15

Aspect

Answer 15

– an important factor that controls the spatial distribution of solar irradiance (thus affecting evapotranspiration), and flora and fauna distribution and abundance

• ▪ like slope, aspect is calculated for each DEM grid cell or every triangle facet in a TIN based on the normal vector
• ▪ aspect is usually described as a compass direction, measured clockwise in degrees from 0 (due north) to 360 (due north), using the following rule

Question 16

Aspect: Give me direction= N,E,S,W

0-22.5=

157-202=

247-292=

67.5-112=

Answer 16

North

South

West

East

Question 17

Terrain Profile

Answer 17

a terrain profile is a 2D diagram that depicts the landscape in vertical cross-section
-Terrain point attribute grouped with unique symbol, Terrain point attribute with graduated color ramp, and Terrain point elevation with graduated color ramp.
-▪ terrain profiles typically include a vertical
exaggeration, a scaling factor that emphasizes
vertical features which might be too small to
identify relative to the horizontal scale

Question 18

Vertical Exaggeration

Answer 18

a scaling factor that emphasizes
vertical features which might be too small to
identify relative to the horizontal scale

Question 19

in GIS, the method for producing a terrain profile based on a digital terrain model generally involves:(5)

Answer 19

1. finding all intersections of a specified profile line with sides of the triangles in a TIN or with grid cells in a DEM
2. calculating the horizontal position along the profile line of every intersection and its elevation through interpolation
3. drawing the x-axis with its length equal to the horizontal distance of the profile line at the scale of the digital terrain model
4. determining the height of the vertical axis according to the specified vertical exaggeration factor, and drawing the y-axis
5. plotting the intersections in the diagram using their horizontal distances to the starting point of the profile line as x coordinates and their elevation x vertical exaggeration as y coordinates

Question 20

surface curvature

Answer 20

is the amount by which a surface deviates from being flat
▪ there are many measures of surface curvature – GIS typically uses plan and profile
curvature

Question 21

▪ there are many measures of surface curvature – GIS typically uses ___ and ____
curvature

Answer 21

▪ while slope influences the overall rate of movement downslope and aspect determines the direction of flow

1.plan: plan curvature has an effect on convergence
and divergence of flow
-curviture to the middle on sides like a flat paper vs paper bent at centre into a tube
—-__—-

2.profile:profile curvature affects flow acceleration and
erosion/deposition rates
-curviture to the middle from top and bottom increasing slope
—–_____

Question 22

Positive curvature indicates

Negative curvature indicates

Answer 22

the surface is upwardly convex (eg, hills and ridges)
__—__

the surface is upwardly concave (eg, depressions and
valleys)
—__—

—-a flat surface has 0 curvature

Question 23

Convex

Concave

Answer 23

hills

depressions/valleys

Question 24

Hydrological Terrain Analysis:

Flow Direction

Flow Accumulation

Flow Length

Answer 24

Flow Direction: is fundamental in hydrological analysis because in order to determine where a landscape drains, it is necessary to determine the direction of flow for each location in the landscape

flow accumulation: refers to how much water accumulates from upstream areas and contributes to overland flow – basically calculated as the number of upstream cells that flow into each cell based on their flow direction
▪ locations having high flow accumulation values are the areas of concentrated flow (usually stream channels), while locations having 0 accumulation correspond
to the pattern of ridges

Flow length: is the distance or weighted distance along a flow path, either upstream or downstream, and used to calculate the length of the longest flow path
▪ flow length is an important input to surface runoff models, primarily through the calculation of time of flow concentration within a catchment

Question 25

when multiplied by the average rainfall in each cell during a given storm, the flow accumulation value represents the amount

Answer 25

of rain that flows through each cell or the | amount of rain that fell in the upslope area of each cell (assuming that all rain becomes runoff)

Question 26

Drainage Network

Answer 26

▪ by selecting cells with a flow accumulation value greater than some threshold value, a network of high-flow cells can be generated, known as the drainage network ▪ the threshold is important – a higher threshold value will only choose the largest flow accumulation zones, resulting in a low drainage density; a lower threshold will include smaller flow accumulation zones, resulting in a higher drainage density

Question 27

Topographic Wetness Index(TWI) is derived through the following procedure on a depressionless DEM(6)

Answer 27

1. derive the flow direction from the DEM 2. derive the flow accumulation (𝑓𝑎𝑐𝑐) from the flow direction 3. calculate the upstream catchment area: 𝐴 = 𝑓𝑎𝑐𝑐 + 1 × 𝑑^2 4. calculate the specific catchment area as: 𝐴𝑠 = 𝑓𝑎𝑐𝑐 + 1 × 𝑑 5. calculate the slope (𝜃) from the DEM 6. calculate the topographic wetness index

Question 28

Describe the Topographic Wetness Index(TWI)

Answer 28

▪ TWI serves as a physically-based index quantifying and estimating the location of zones of surface saturation ▪ in a catchment, areas having similar TWI values are considered to have a similar hydrologic response to rainfall when their other environmental conditions are the same

Question 29

Stream Power Index(SPI)

Answer 29

is a measure of the erosive power of overland flow ▪ as the specific catchment area and slope increase, the amount of water contributed by upslope areas and the velocity of water flow increase, hence stream power index and erosion risk increase ▪ averaging (eg, focal/neighbourhood smoothing) is commonly used to reduce the high spatial variability seen in both data sets

Question 30

Sediment Transport Index(STI)

Answer 30

accounts for the effect of topography on soil erosion ▪ STI can be used to characterize the processes of erosion and deposition, and to present the effect of topography on soil loss

Question 31

Visibility Analysis

Answer 31

▪ visibility analysis involves determining what is visible from a location or a set of locations along given lines of sight or across an entire area – useful in landscape planning for assessing scenic quality and the impact of new developments on the landscape ▪ “which areas can be seen from a forest fire observation station that is 20m high?” ▪ “which land areas are visible from a planned scenic road, and how often can those areas be seen from the road?”

Question 32

View Shed analysis

Answer 32

▪ viewshed analysis expands the line-of-sight analysis to cover every possible point in the study area as a target, which is useful to define the areas of the terrain surface that are visible from a viewpoint or a set of viewpoints, thus allowing users to calculate what they can see from a given set of vantage points ▪ through viewshed analysis, it is possible to determine what would be visible from planned developments, how visible a particular spot on the landscape is,