COMPARATIVE ACCURACY EVALUATION OF FINE-SCALE …

COMPARATIVE 3D EVALUATION OF FINE-SCALEGLOBAL AND LOCAL NORMALISED DEMS:

THE TSHWANE CASE STUDY IV

Compiled and presented byAndré Breytenbach

Outline

1. Introduction

2. Study Rationale

3. Area of Interest

4. Data Components

5. Methodology

6. Results

7. Conclusions

37th ISRSE, 8-12 May 2017, Tshwane, South Africa Slide 1

37th ISRSE, 8-12 May 2017, Tshwane, South Africa

Intoduction

DIGITAL ELEVATION MODEL

Slide 2

DIGITAL SURFACE MODEL DIGITAL TERRAIN MODEL


Rationale

A high quality, consistent (seamless) DEM offers large potential benefits not only to those involved in geomorphometry, but also to a variety of other Earth Observation or space related research fields and industries.Often the need exits to reliably demarcate and quantify existing above ground manmade or natural features or objects, e.g.:• built-up structures including building, reservoir or mine dump dimensions• vegetation types e.g. tree structure determination and species identification or

above ground biomass calculations (Deng et al., 2014)• monitoring glacier disposition in artic regions (Giroda et al., 2016)

Global positional accuracy figures normally accompanying many DEM products, global or commercial versions in particular, are however only an indication of the true potential of the DEM towards the uses and needs and are often untested locally.Also needs an thorough understanding of the impact that the initial support size of topographic source data normally has on complex 3D spatial computations and resulting data quality.The study therefore involved the collection and assessment of the vertical and volumetric accuracy of three normalised DEMs available locally and gauges their usefulness across several primary LCLU types in a predominantly urban environment.

Slide 3


Area of Interest

3.2 km2


Data Components (1/2)

Airbus DS Elevation4™ DEM (Pléiades tri-stereo)

Airbus DS WorldDEM™ DSM (Tandem-X InSAR)

Available commercial DEM products

2011 – 2013

Product at 12 m postings

September 2013

Product at 4 m postings

Slide 5


Data Components (2/2)

Leica ALS50-II

(average 8 points/m2)

DMC

(50 cm RGBI pixels)

Local DEM product Reference Data

Sep-Oct 2013

Reference grids produced at matching spatial resolutions

May 2012

CSIR prototype at 2 m postings

+Kodak KAI-11002 dual CCD that produced RGB aerial

ortho-photos (10 cm GSD).

Slide 6


Methodology

Slide 7

• Commercial products re-projected and co-registered to common reference framework (Lo29 /Transverse Mercator projection)

• Bias removal and DSM correction (if necessary)• Calculate sample nDEM and subtract from co-registered

reference nDEM• Calculate and record relative vertical accuracy metrics (MEAN,

STDEV, MAE and RMSE) for each sample across nine LCLU classes

• Inspect and compare relative height errors within LCLU types• Calculate volume difference between sample and reference

surface across nine LCLU classes• Inspect and compare volume differences within LCLU types


Results (1/4)

Land Cover / Use MEAN STDEV MAE RMSEz

VEG – Trees -3.17 2.71 3.39 4.18

VEG – Tall Shrub/Grass -0.95 1.69 1.61 1.94

VEG – Grass 0.82 1.62 0.96 1.82

VEG – Grass & Bare Soil 0.15 1.42 0.72 1.43

BLT – Artificial/Bare/Paved 0.23 1.85 0.96 1.86

BLT – Single Storey -0.05 1.81 1.28 1.81

BLT – Multi Storey -0.67 2.80 1.72 2.88

NAT – Bare Rock/Gravel/Soil -0.91 1.40 1.17 1.67

WTB – Artificial Waterbody 0.29 1.58 0.78 1.60

Relative vertical accuracy computed (meters) within each of the nine primary LCLU types of the

experimental 2 m nDEM as measured againsta co-registered reference surface


Results (2/4)


VEG – Trees -3.47 3.28 3.81 4.77


VEG – Grass 1.05 1.64 1.57 1.95




BLT – Multi Storey -0.43 2.58 1.63 2.62

NAT – Bare Rock/Gravel/Soil 0.19 1.24 1.02 1.25


Relative vertical accuracy computed (meters) within each of the nine primary LCLU types of the nDEM

constructed from the Elevation4™ DEM products as measured against a co-registered reference surface


Results (3/4)


VEG – Trees -4.16 3.19 4.36 5.24


VEG – Grass 0.62 1.67 1.23 1.78




BLT – Multi Storey -4.15 3.54 4.33 5.46

NAT – Bare Rock/Gravel/Soil -0.92 1.50 1.42 1.76


Relative vertical accuracy computed (meters) within each of the nine primary LCLU types of the produced by

the WorldDEM™ samples as measured against a co-registered reference surface


Results (4/4)

Volumetric comparisons between the three nDEM samples as measured against the co-registered reference surface within

the nine primary LCLU classes

AP DEM 2m E4 DEM 4m WDEM 12m

VEGETATION – Trees -3245100 -3550440 -4905460

VEGETATION – Tall Shrub/ Grass -357060 -278968 -354948

VEGETATION – Grass 479429 614181 330292

VEGETATION – Grass & Bare Soil 26325 214715 447

BUILTUP – Artificial/ Bare/ Paved 100576 523735 373531

BUILTUP – Single Storey -12797 -45411 -151102

BUILTUP – Multi Storey -146180 -94980 -898651

NATURAL – Bare Rock/ Gravel/ Soil -93835 19233 -80537

WATERBODY – Artificial 2044 3903 6316

Volume Difference (m3)

LCLU


Conclusions

• When using these three DEM samples significant vertical errors and volume under-estimations can be expected in the VEGETATION classes, particularly the Trees class, followed by bushes/Tall Shrub/Grass class. Also true for BUILTUP Multi-storey class, but less severe.

• The experimental 2 m DSM performed very well, particularly in the vegetative classes, but less so in the built-up classes. It demonstrated that it is feasible to consider its realisation over large areas to obtain a good quality baseline DEM.

• Elevation4™ tri-stereo product also performed very well throughout in terms of relative vertical accuracy and volume, particularly in the BUILTUP classes.

• The quality of the WorldDEM™ remains remarkable when considering its well preserved scale characteristics and terrain continuity as a global coverage product.

• The impact that these very useful 3D data sources and topographic representations will make in the local geomorphometry domain is almost evident (improved standard of orthorectification, advance most image processing, classification or segmentation and change detection techniques, etc.).

• Both space borne commercial DSM alternatives unfortunately remain high cost options at present for South Africa.

Slide 12


Thank you!

For more information:

André [email protected]

Council for Industrial and Scientific ResearchSouth Africa

www.csir.co.za

"Without 3D visualisation you're just another person with a 2 dimensional opinion"

Adopted from a quote by – W. Edwards Deming

Slide 13

mailto:[email protected]

http://www.csir.co.za/

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