3d and surface/terrain analysis - gep.frec.vt.edu · digital terrain models • a digital terrain...

VCCS Geospatial Institute Day 5: Spatial Analysis with ArcGIS Extensions

Blacksburg, VAMay 21st – May 26th, 2011 3D Surface Analysis |

3D and Surface/Terrain Analysis

Prepared by:

in partnership with:

With support from:

NSF DUE‐0903270

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

φ

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 modelsPolyhedral terrain models

• Contour Maps – aka “topo” maps

The Data…

DEM

LIDAR (LIght Detection And Ranging) TIN



• Terrain mapping

• Land surface is 3‐D

• Elevation data or z‐data is treated as a

Introduction to the Data

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

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 h l d f hthe 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( 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

• Needs larger storage capacity • Needs smaller storage capacity

DEM TIN

g g p y

• Computationally difficult

• Flexibility of data sources

• Can add points

• Better display

• More efficient

g p y

• Computationally simpler

• Fixed with a given cell size

• Cannot add sample points

• Raster display

• Less efficient

Contour Mapping• Contouring is most common

method for terrain mapping

• Contour lines connect points of equal elevation (isolines)

• Contour intervals represent the pvertical 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 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.



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

i f i h f h i h li i ibili f b f• 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 toproblems, but by identifying trends based ongeography, GIS can reveal patterns that canhelp us make informed decisionshelp us make informed decisions.

• A GIS can improve decision‐making; it cannotmake decisions for us.

3d and surface/terrain analysis - gep.frec.vt.edu · digital terrain models • a digital terrain...

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