3D and Surface/Terrain
Analysis
Prepared by:
George McLeod
With support from:
NSF DUE-0903270
in partnership with:
Geospatial Technician Education Through Virginia’s Community Colleges (GTEVCC)
Digital Terrain Models
• A digital terrain model is a model providing a
representation of a terrain relief on the basis
of a finite set of sampled data
• Terrain data refers to measures of elevation at
a set of points V of the domain plus possibly a
set E of non-crossing line segments with
endpoints in V

D
Data Sampling
• Regular
• Irregular
Sampling effects resolution
Our three Primary terrain Models
• Digital Elevation Models (DEMS) – aka Regular
Square Grids (RSGs)
• Triangulated Irregular Networks (TINS) – aka
Polyhedral terrain models
• Contour Maps – aka “topo” maps
The Data…
DEM
LIDAR
(LIght Detection And Ranging)
TIN
Introduction to the Data
• Terrain mapping
• Land surface is 3-D
• Elevation data or
z-data is treated as a
cell value or a point
data attribute rather
than as a coordinate.
• Digital Elevation Model (DEM)
– Gridded array of elevation points obtained from a
variety sampling methods
DEMs
A constant function can be associated with each
square (i.e., a constant elevation value). This is called
a stepped model (it presents discontinuity steps
along the edges of the squares)
D
• The function defined on each square can also be
a bilinear function interpolating all four elevation
points corresponding to the vertices of the
square
• Triangulated Irregular Network (TIN)
– Series of non-overlapping triangles
– Elevation values are stored at nodes
– Sources: DEMs, surveyed elevation points, contour
lines, and breaklines
– Breaklines are line features
that represent changes of
the land surface such as
streams, shorelines, ridges,
and roads
TINs
• Example of a TIN based on irregularly
distributed data
Data for Terrain Mapping and Analysis
• Triangulated Irregular Network (TIN)
– Not every point in DEM is used
– Only points most important
– VIP (Very Important Points) algorithm
– Maximum z-tolerance algorithm
– Delaunay triangulation: all nodes are connected to
their nearest neighbor to form triangles which are
as equi-angular as possible.
Tins vs. Grids
DEM
TIN
• Needs larger storage capacity
• Needs smaller storage capacity
• Computationally difficult
• Computationally simpler
• Flexibility of data sources
• Fixed with a given cell size
• Can add points
• Cannot add sample points
• Better display
• Raster display
• More efficient
• Less efficient
Contour Mapping
• Contouring is most common
method for terrain mapping
• Contour lines connect points
of equal elevation (isolines)
• Contour intervals represent the
vertical distance between
contour lines.
• Arrangement of contour lines
reflect topography
Digital Contour Maps
Contours are usually available as sequences of points
A line interpolating points of a contour can be obtained in different ways
Examples: polygonal chains, or lines described by higher order equations
Digital Contour Maps: properties
They are easily drawn on paper
They are very intuitive for humans
They are not good for complex
automated terrain analysis
Contour Profile Mapping
• Vertical profile shows changes in elevation along a line, such as a hiking
trail, road or stream.
Cartographic Terrain Mapping
•
•
•
Hill shading is also known as a shaded relief or simply shading
Attempts to simulate how the terrain looks with the interaction between sunlight
and surface features.
Helps viewers recognize the shape of land-form features on a map.
Hillshading
• Four factors control the visual effect of hillshading
– Sun’s azimuth is direction of incoming light (0 to
360°)
– The sun’s altitude from horizon (0-90°)
– Surface slope (0-90°)
– Surface aspect (0 to 360°)
Hypsometric Tinting
•
Hypsometric tinting
– Applies different color symbols to represent elevation or depth zones.
Methods of Analysis
•
Slope measures the rate of change of elevation at a surface location
•
Aspect is the directional measure of the slope (degrees- 4 or 8 directions)
•
Hillshade, refer to previous slides
•
Line of sight refers to the straight line visibility from an observer to a feature
•
Viewshed analysis refers to the areas of the land surface that are visible
from an observation point or points.
•
Watershed analysis refers to an area that drains water and other substances
to a common outlet.
•
Area and volume calculations
Connectivity Function Example:
Viewshed Analysis
Image Source: Chrisman, Nicholas.(2002). 2nd Ed. Exploring Geographic Information Systems. p 198. fig. 8-14
.
Line of Sight Analysis
Slope
Aspect
Hillshade
Setting a hypothetical light source and calculating the illumination values for each
cell in relation to neighboring cells. It can greatly enhance the visualization of a
surface for analysis or graphical display.
Azimuth 315°, altitude 45°
Viewshed
•
•
Viewshed identifies the cells in an input raster that can be seen from one
or more observation points or lines.
It is useful for finding the visibility. For instance, finding a well-exposed
places for communication towers
hillshaded DEM as background
Surface Area and Volume
Application: Environmental Impact Analysis
3D landscape model impact on natural beauty
Application - Flood Risk
3D height data changing water levels-danger areas
The 3rd Dimension: Height Analysis – combining
several methods together
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•
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Contours
Hill shading
Spot height symbols
Cliff & slope symbols
Viewpoint symbols
3D Terrain Analysis: Summation
• GIS does not always provide exact answers to
problems, but by identifying trends based on
geography, GIS can reveal patterns that can
help us make informed decisions.
• A GIS can improve decision-making; it cannot
make decisions for us.