TitleName name name

Introduction to Digital Elevation Models (DEMs)

Harry West

Terminology - DEMs

DTM DSM

History of DEM Development

• More traditional methods of DEM creation: interpolation of printed contour lines and manual photogrammetry

• RS approaches started to develop during the Cold War Period (1980’s)• Driven by a military requirement

• Missile guidance systems required rapid high quality elevation data

• SPOT-1 (1986) first commercial digital stereo imagery

History of DEM Development

• Turn of the Century saw technology continue to improve enabling a range of elevation data solutions

NASA TERRA Satellite (1999)

ASTER SensorAdvanced SpaceborneThermal Emission and Reflection Radiometer

IKONOS (1999)

Stereo Imaging

• The primary advantage of stereo imagery is the ability to extract geographic features in 3D such as buildings, roads, manmade structures and other terrain features

• Terrain slope conditions of <20%

Space Shuttle Radar Topography Mission (SRTM)

• One of the biggest milestones in RS Elevation Data – Feb 2000

• SRTM mapped all land surfaces between +/- 60 degrees over 10 days

• Driven by military requirements

• First global scale elevation model (90m –30m resolution)

• Only active C-, L-, X-Band sensor

TanDEM-X (2010)

• Add on mission to TerraSAR-X (2007)

• 12 m gridded model ‘WorldDEM’

• Processed over about 3 years, this data was designed to provide a step up from SRTM, covering the high latitudes, increasing the detail and filling SRTM voids

• Based on Synthetic Aperture Radar (SAR) principle

• One satellite acts as a transmitter to illuminate a common radar footprint

• The scattered signal is recorded by both satellites simultaneously

• This simultaneous data acquisition makes dual use of the available transmit power and helps avoid potential errors

SR

TM

vs

Tan

DE

M-X SRTM = 30m Res WorldDEM = 12m Res

SR

TM

vs

Tan

DE

M-X

SRTM = 30m Res WorldDEM = 12m Res

Light Imaging Detection & Radar (LiDAR)

• Measuring the location of objects from a known point

• The known point is mounted on a moving aircraft or helicopter (platform)

• The measuring device is the ‘reflection’ of a beam of light off the target feature

• You need to be able to measure and capture location extremely quickly and efficiently

Raw

LiD

AR

: P

oin

t C

lou

d • A large data set of points

• Elevation information

• Intensity information

• Return information

Higher Resolution = “Better” Data?

LiDAR DEMs 25cm-2m

Just because they are higher resolution

doesn't mean they are perfect

Need to manage expectations of

realism (Dottori et al., 2013)

Hyper-resolution Ignorance ! (Beven et al., 2015)

Dig

ital

Ele

vati

on

Mo

dels

&

‘U

ncert

ain

ty’

This can lead to inaccurate representations of reality

Impact on management decisions?

There is inherent error which constitutes uncertainty in the data

When working with DEMs the accuracy with which the topography was mapped with affect the output of any results

Satellite accuracy? Aircraft accuracy?

Vert

ical

Accu

racy /

Err

or

4.0maOD

5.0maOD

6.0maOD

RMSE = 1m

Vis

uali

sin

gU

ncert

ain

ty

West, H. , Horswell, M. and Quinn, N. (2018) Exploring the sensitivity of coastal inundation

modelling to DEM vertical error. International Journal of Geographical Information Science, 32 (6).

pp. 1172-1193. ISSN 1365-8816

DEM Hydrological Functions (ArcMap)

Original DTM

FillFlow

DirectionFlow

AccumulationDrainage

Area

Flo

w D

irecti

on

Flo

w A

ccu

mu

lati

on

DEM Hydrological Functions (ArcMap)

Original DTM

FillFlow

DirectionFlow

AccumulationDrainage

Area