6. ASET
Terrain
• Terrain is a derivative of the word “terra”
meaning earth.
• Refers to the horizontal and vertical
versions of the land surface.
• To describe the terrain of the land, one
uses factors such as the slope, elevation,
and orientation of the land.
7. ASET
Terrain
• Geomorphology - study of the formation of terrains.
• Three major processes involved in the formation of terrains:
geological process, erosional process, and meteorite impacts.
– Geological process consists of activities such as river
formations, volcanic eruptions, faulting and folding, and
tectonic plates’ movements.
– Erosional process involves weathering processes on land
such as wind erosion, water erosion, and landslides.
– Meteorite impacts - when meteorites fall to earth, they form
craters filled with meteorite ores.
8. ASET
Terrain: Relief, Topography
• Relief is related to terrain and is the quantitative
measurement of the changes in the elevation of
the landscape
• Topography is the measure of terrain or relief of
land. Several techniques may be used in
topography such as direct survey, remote
sensing, photogrammetry, and active sensor
methodologies.
9. ASET
Terrain
• Common ones are plateau, mountain,
plain, and valley terrains.
• Other types of terrains include open,
tundra, oasis, steppe, desert, swamp,
forest, marsh, river, and hill.
10. ASET
Terrain study?
• Terrain of land is responsible for determining its
suitability for human settlement
• Understanding terrains is significant for agricultural
purposes
• Description of the terrain allows for soil
conservation efforts such as contour plowing
• Terrains have military importance
• Terrain is related to the weather patterns
• Terrains affect aviation especially for aircrafts
which have low-flying routes and altitudes of
airports
11. ASET
Terrain Study – Military Importance
• Assist military forces in developing
strategies related to their defensive and
offensive attacks while at war.
• Military Battlefield Simulation
• Combat Vehicle Modeling
13. ASET
Modelling?
• Modelling is about building representations of
things in the ‘real world’ and allowing ideas to be
investigated; it is central to all activities in the
process for building or creating an artefact of
some form or other.
• https://www.youtube.com/watch?v=RK9m4OmFAb
Y
14. ASET
Model?
• A model is an abstraction, which allows people to concentrate on the
essentials of a (complex) problem by keeping out non-essential
details.
• Models are, in one respect, idealisations in the sense that they are less
complicated than reality; they are simplifications of reality.
– For example, a road map is a model of a particular part of the earth's surface. We do
not show things like vegetation or birds' nests as they are not relevant to the map's
purpose. We use a road map to plan our journeys from one place to another and so
the map should only contain those aspects of the real world that serve the purpose
of planning journeys.
15. ASET
Model?
• A model and the real world are alike in some ways, but different in
others.
– For example, road maps are helpful because they represent the distance between,
and relative positions of, a set of places and the routes between them. They use the
relevant properties of the real thing with just a change in scale; one centimetre on
the road map, for instance, may be equivalent to one kilometre on the ground. A
map may be unhelpful if it shows only major roads.
• Quite often, a property of the real world may be represented by a
different kind of property in a model.
– In the case of the road map, different colours are normally used to represent
different classes or types of road. Such a road map should have a key or legend so
that those who read the map can understand what the different coloured lines are
intended to represent. In effect, an analogy is being used to exploit the similarity
between two different properties: one in the real world and one in the model.
16. ASET
Model?
• Models of a problem situation are only an approximate representation
of that situation.
• The real world situation will have a complexity that tends to reduce
your chances of achieving an exact representation.
• So, you need to find some way of achieving an acceptable balance
between accuracy and manageability.
• As a project unfolds, there will be a number of practical considerations
that result in some compromise.
• It is for this reason that several different models are built, each one
representing different aspects (views) of the real world.
18. ASET
Digital Terrain Models
• Digital Terrain Models (DTM) sometimes called Digital
Elevation Models (DEM) is a topographic model of the
bare Earth that can be manipulated by computer
programs.
• The data files contain the elevation data of the terrain in
a digital format which relates to a rectangular grid.
• Vegetation, buildings and other cultural features are
removed digitally - leaving just the underlying terrain.
• DTMs are used especially in civil engineering, geodesy
& surveying, geophysics, geography and remote
sensing.
21. ASET
– A DEM (Digital Elevation Model) Represents the bare-Earth
surface, removing all natural and built features;
– A DSM (Digital Surface Model) captures both the natural and
built/artificial features of the environment, as shown below;
– A DTM (Digital Terrain Model) typically augments a DEM, by
including vector features of the natural terrain, such as rivers and
ridges. A DTM may be interpolated to generate a DEM, but not
vice versa.
https://geodetics.com/dem-dsm-dtm-digital-elevation-models/
29. ASET
x
dx
)
y
,
x
(
f
)
y
(
f
Two fundamental ways of representing geography are
discrete objects and fields.
The discrete object view represents the real world as objects with well
defined boundaries in empty space.
The field view represents the real world as a finite number of variables,
each one defined at each possible position.
(x1,y1)
Points Lines Polygons
Continuous surface
30. ASET
Raster and Vector Data
Point
Line
Polygon
Vector Raster
Raster data are described by a cell grid, one value per cell
Zone of cells
31. ASET
Raster and Vector are two
methods of representing
geographic data in GIS
• Both represent different ways to encode
and generalize geographic phenomena
• Both can be used to code both fields
and discrete objects
• In practice a strong association between
raster and fields and vector and
discrete objects
33. ASET
Six approximate representations of a field used in GIS
Regularly spaced sample pointsIrregularly spaced sample points Rectangular Cells
Irregularly shaped polygons Triangulated Irregular Network (TIN) Polylines/Contours
from Longley, P. A., M. F. Goodchild, D. J. Maguire and D. W. Rind, (2001), Geographic Information
Systems and Science, Wiley, 454 p.
34. ASET
A grid defines geographic space as a matrix of
identically-sized square cells. Each cell holds a
numeric value that measures a geographic
attribute (like elevation) for that unit of space.
35. ASET
The grid data structure
• Grid size is defined by extent, spacing
and no data value information
– Number of rows, number of column
– Cell sizes (X and Y)
– Top, left , bottom and right coordinates
• Grid values
– Real (floating decimal point)
– Integer (may have associated attribute
table)
36. ASET
Definition of a Grid
Number
of
rows
Number of Columns
(X,Y)
Cell size
NODATA cell
45. ASET
Raster Sampling
from Michael F. Goodchild. (1997) Rasters, NCGIA Core Curriculum in
GIScience, http://www.ncgia.ucsb.edu/giscc/units/u055/u055.html, posted
October 23, 1997
48. ASET
Topographic Slope
• Defined or represented by one of the following
– Surface derivative z (dz/dx, dz/dy)
– Vector with x and y components (Sx, Sy)
– Vector with magnitude (slope) and direction (aspect) (S,
)
49. ASET
Calculation of slope
a b c
d e f
g h i
cing
x_mesh_spa
*
8
i)
2f
(c
-
g)
2d
(a
dx
dz
acing
y_mesh_sp
*
8
c)
2b
(a
-
i)
2h
(g
dy
dz
2
2
dy
dz
dx
dz
run
rise
run
rise
atan
deg
50. ASET
Aspect – the steepest downslope direction
dx
dz
dy
dz
dy
/
dz
dx
/
dz
atan
51. ASET
Example
30
80 74 63
69 67 56
60 52 48
a b c
d e f
g h i
229
.
0
30
*
8
)
48
56
*
2
63
(
)
60
69
*
2
80
(
dx
dz
cing
x_mesh_spa
*
8
i)
2f
(c
-
g)
2d
(a
329
.
0
30
*
8
)
63
74
*
2
80
(
)
48
52
*
2
60
(
acing
y_mesh_sp
*
8
c)
2b
(a
-
i)
2h
(g
dy
dz
o
8
.
21
)
401
.
0
(
atan
o
8
.
34
329
.
0
229
.
0
atan
Aspect
o
o
2
.
145
180
145.2o
401
.
0
329
.
0
229
.
0
Slope 2
2
56. ASET
Flow
direction.
Steepest direction
downslope
1
2
1
2
3
4
5
6
7
8
Proportion flowing to
neighboring grid cell 3
is 2/(1+2)
Proportion
flowing to
neighboring
grid cell 4 is
1/(1+2)
The D Algorithm
Tarboton, D. G., (1997), "A New Method for the Determination of Flow Directions and
Contributing Areas in Grid Digital Elevation Models," Water Resources Research,
33(2): 309-319.) (http://www.engineering.usu.edu/cee/faculty/dtarb/dinf.pdf)
57. ASET
Steepest direction
downslope
1
2
1
2
3
4
5
6
7
8
0
The D Algorithm
If 1 does not fit within the triangle the angle is chosen along the steepest edge or
diagonal resulting in a slope and direction equivalent to D8
1
0
2
1
1
e
e
e
e
atan
2
1
0
2
2
1 e
e
e
e
S
59. ASET
Summary Concepts
• Grid (raster) data structures represent
surfaces as an array of grid cells
• Raster calculation involves algebraic like
operations on grids
• Interpolation and Generalization is an
inherent part of the raster data
representation
60. ASET
Summary Concepts (2)
• The elevation surface represented by a grid
digital elevation model is used to derive
surfaces representing other hydrologic
variables of interest such as
– Slope
– Drainage area (more details in later classes)
– Watersheds and channel networks (more
details in later classes)
61. ASET
Summary Concepts (3)
• The eight direction pour point model
approximates the surface flow using eight
discrete grid directions.
• The D vector surface flow model
approximates the surface flow as a flow
vector from each grid cell apportioned
between down slope grid cells.
