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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). . D. Digital Terrain Models. - PowerPoint PPT Presentation
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3D and Surface/Terrain Analysis
in partnership with:
With support from:
NSF DUE-0903270
Prepared by:
George McLeod
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…
LIDAR (LIght Detection And Ranging)
DEM
TIN
• 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.
Introduction to the Data
• Digital Elevation Model (DEM)– Gridded array of elevation points obtained from a
variety sampling methods
DEMsA 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
• Needs larger storage capacity
• Computationally difficult
• Flexibility of data sources
• Can add points
• Better display
• More efficient
• Needs smaller storage capacity
• Computationally simpler
• Fixed with a given cell size
• Cannot add sample points
• Raster display
• Less efficient
DEM TIN
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 waysExamples: 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 complexautomated 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 hill-shading– 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
• 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.