18: Terrain Analysis

Question 1

LiDAR & SfM

Answer 1

LiDAR

• uses laser to scan earth’s surface and creates dense point cloud
• drones, airplanes, terrestrial equipment
• very expensive devices

SfM

• take photos of a place/object from lots of angles
• can use a normal camera
• inexpensive but time consuming to process and large quantities of photos take up a lot of space/memory

Question 2

USES OF 3D DATA IN GIS (2)

Answer 2

Geovisualisation:
uses 3D to display data
interactive, visualisation, animations

Spatial modelling:
uses 3D to create data and analyse
Examples:
-site suitability
-environmental planning
-geology
-viewshed analysis

Question 3

DATA FORMATS

Answer 3

can be viewed in ArcGIS as raw points

LAS Toolset or convert to:

• raster (DEM/DTM)
• vector topology (TIN)
• vector contours (polyline features)

Question 4

TERRAIN ANALYSIS TOOLS

Answer 4

Spatial Analyst tools:

• Contour
• Hillshade
• Slope
• Aspect
• Hydrological tools
• Viewshed
• Profiles

Question 5

CASE STUDIES

Answer 5

viewshed study used by DOC for environmental impact assessment (changes in landuse)

sniper modelling to plan and protect public officials/events

Question 6

DEM vs DTM

Answer 6

Digital elevation model:

• bare ground surface
• the last return from the LiDAR

Digital terrain model:

• includes features on the surface like buildings and vegetation
• includes multiple returns from LiDAR

Question 7

Raster DEM/DTM

Answer 7

grid of pixels, in each pixel the value represents the height above sea level

Question 8

Raster DEM/DTM

Answer 8

grid of pixels, in each pixel the value represents the height above sea level

resolution based on pixel size

Question 9

Vector TIN

Answer 9

points from the point cloud are connected with a network of lines to create a surface

• polygon mesh
• arc-node topology

Question 10

Contour tool

Answer 10

Creates vector contour from a DEM input

-select the contour interval

Question 11

Hillshade tool

Answer 11

Creates a grayscale 3D representation of input DEM

• takes into account sun position
• shaded relief map
• gives a 3D effect to the map

Question 12

Slope tool

Answer 12

Per cell estimate of surface angle

• calculated using difference in elevation over distance between cells
• rise/run=slope (degrees or radians)

Question 13

Aspect tool

Answer 13

Calculates direction of slope

-azimuth: N,S,E,W and flat

Question 14

Aspect tool

Answer 14

Calculates direction of slope

• azimuth: N,S,E,W and flat
• useful for solar orientation studies

Examples:

• how much sun does a glacier get
• where to position solar panels

Question 15

Hydrology tools

Answer 15

• modelling hydrographic processes
• runoff processed depend on form and structure of surfaces and surface cover
• water courses (tracing flow lines)
• amount of runoff
• catchments (upstream identification of contributing areas)

Question 16

Viewshed

Answer 16

calculate viewable area within DEM using 1 or more input features

• determination of line of sight
• binary result 0 or 1
• useful for environmental impact assessment

Question 17

how does viewshed analysis work?

Answer 17

• created from grid-based or TIN models of terrain
• from each cell in the grid a line of sight is constructed between viewpoint and feature
• if a line of sight intersects part of the DEM then the feature is not visible
• if a line of doesn’t intersect then the feature is visible

Question 18

issues with viewshed analysis

Answer 18

problems with accuracy

• vegetation/buildings not taken into account
• atmospheric limitations: haze, weather, etc
• error in elevation measurements (data)
• effect of DEM errors
• curve of earth

Question 19

Profile graph tool

Answer 19

can be used with TIN or DEM

• draw a cross-sectional line
• arc generates a profile based on that line
• great with hi-res LiDAR derived data