3D and Surface/Terrain Analysis - PowerPoint Presentation

Slide1

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)Slide2

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

Slide3

Data Sampling

Regular

IrregularSlide4

Sampling effects resolutionSlide5

Our three Primary terrain Models

Digital Elevation Models (DEMS) – aka Regular Square Grids (RSGs)Triangulated Irregular Networks (TINS) – aka Polyhedral terrain modelsContour Maps – aka “topo

” mapsSlide6

The Data…

LIDAR

(

LI

ght Detection A

nd Ranging)

DEM

TINSlide7

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 DataSlide8

Digital Elevation Model (DEM)Gridded array of elevation

points obtained from a variety sampling methodsSlide9

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)

DSlide10

The function defined on each square can also be a bilinear function interpolating all four elevation points corresponding to the vertices of the squareSlide11

Triangulated Irregular Network (TIN)Series of non-overlapping triangles

Elevation values are stored at nodesSources: 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 roadsSlide12

TINs

Example of a TIN based on irregularly distributed dataSlide13

Data for Terrain Mapping and Analysis

Triangulated Irregular Network (TIN)Not every point in DEM is usedOnly points most important

VIP (Very Important Points) algorithmMaximum z-tolerance algorithmDelaunay triangulation: all nodes are connected to their nearest neighbor to form triangles which are as

equi-angular as possible.Slide14

Tins vs. Grids

Needs larger storage capacityComputationally difficult

Flexibility of data sourcesCan add pointsBetter display

More efficient

Needs smaller storage capacityComputationally simplerFixed with a given cell size

Cannot add sample pointsRaster displayLess efficient

DEM

TINSlide15

Contour

Mapping

Contouring is most common

method for terrain mappingContour lines connect points of equal elevation (

isolines)Contour intervals represent the vertical

distance between contour lines.Arrangement of contour lines

reflect topographySlide16

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 Slide17

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

Slide18

Contour Profile Mapping

Vertical profile shows changes in elevation along a line, such as a hiking trail, road or stream.Slide19

Cartographic Terrain Mapping

Hill shading is also known as a shaded relief or simply shadingAttempts 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.Slide20

Hillshading

Four factors control the visual effect of hill-shadingSun’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°)Slide21

Hypsometric Tinting

Hypsometric tintingApplies different color symbols to represent

elevation or depth zones.Slide22

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 calculationsSlide23

Connectivity Function Example:Viewshed Analysis

Image Source: Chrisman, Nicholas.(2002). 2

nd

Ed. Exploring Geographic Information Systems

. p 198. fig. 8-14.Slide24

Line of Sight AnalysisSlide25

SlopeSlide26

AspectSlide27

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

°Slide28

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 backgroundSlide29

Surface Area and VolumeSlide30

Application: Environmental Impact Analysis

3D landscape model impact on natural beautySlide31

Application - Flood Risk

3D height data changing water levels-danger areasSlide32

The 3rd Dimension: Height Analysis – combining several methods together

Contours Hill shading Spot height symbols Cliff & slope symbols Viewpoint symbolsSlide33

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.