VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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 φ
D
Data Sampling • Regular
• Irregular
Blacksburg, VA May 21st – May 26th, 2011
3D Surface Analysis | Page 1
VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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 Polyhedral terrain models • Contour Maps – aka “topo” maps
The Data…
DEM
LIDAR
(LIght Detection And Ranging)
Blacksburg, VA May 21st – May 26th, 2011
TIN
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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)
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Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
• 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 h l d f h streams, shorelines, ridges, and roads
TINs • Example of a TIN based on irregularly distributed data
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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 ( y p ) g – 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 g g p y
• Needs smaller storage capacity g p y
• 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) p • Contour intervals represent the vertical distance between contour lines. • Arrangement of contour lines reflect topography
Blacksburg, VA May 21st – May 26th, 2011
3D Surface Analysis | Page 5
VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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.
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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 ) 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.
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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
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Line of sight i f i h refers to the straight line visibility from an observer to a feature f h i h li i ibili f b f
•
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.
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Area and volume calculations
Connectivity Function Example: Viewshed Analysis
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Image Source: Chrisman, Nicholas.(2002). 2nd Ed. Exploring Geographic Information Systems. p 198. fig. 8-14
Line of Sight Analysis
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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°
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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
Blacksburg, VA May 21st – May 26th, 2011
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VCCS Geospatial Institute
Day 5: Spatial Analysis with ArcGIS Extensions
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. decisions • A GIS can improve decision‐making; it cannot make decisions for us.
Blacksburg, VA May 21st – May 26th, 2011
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