probabilistic topographic maps from raw, full …lsiit-miv.u-strasbg.fr/paseo/slides/agu11.pdf ·...
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PROBABILISTIC TOPOGRAPHIC MAPS FROM RAW, FULL-WAVEFORM AIRBORNE
LIDAR DATA
André Jalobeanu1, Gil R. Gonçalves2
FCTUC, INESC - University of Coimbra, PortugalCGE - University of Évora, Portugal
AGU FM’11 EP51EAutoProbaDTM project
PTDC/EIA-CCO/102669/2008, FCOMP-01-0124-FEDER-010039
Outline
Objectives
LiDAR principles
Bayesian waveform processing
Probabilistic DEM
Study area in Portugal
Recommendations
Perspectives USGS
Scientific objectivesAutoProbaDTM project
Automated Probabilistic Digital Terrain Model generation from raw LiDAR data
Generate gridded elevation models for topographic mapping and environmental studies
Robustly estimate the bare ground topography
Provide predictive elevation accuracy maps
Minimize the user interaction: automation
Process large datasets (1G waveforms)
Principles of LiDARlight detection and ranging
1. Time of flight measurement(waveform acquisition)
2. Scanning:aircraft motion + laser rotation3. Direct georeferencingIMU: DGPS+INS
Riegl
Bayesian inference
p(θ | observations) =p(observations | θ)× p(θ)
p(observations)
evidence
(useful for model comparison)
likelihood
image formation model
prior model
(a priori knowledge
about the observed object)
parameters of interest
(unknown solution)
OBJECTIVE:
posterior probability
density function (pdf)
Bayes’ rule:
Zk
elevationpdf
Waveforms and ground extraction
Complex Point Spread Function (PSF) - ringing
Ground peak contamination by low vegetation
Variability: amplitude, width, noise...
PSF modeling - 3 peaks (ringing)
Full deconvolution:artifacts (noisy/oversmooth)
Partial deconvolution (Diracs)Target = single Gaussian PSFRinging removal without artifacts, no regularization required
1. Partial waveform deconvolution
artifacts!
original
deconvolved (partial)
original
deconvolved (full)
–t (ns)
–t (ns)–t (ns)
ampl
itud
eam
plit
ude
PSF
2. Bayesian ground range inference
wav
efor
m
full estimation window
Ground peak = Gaussian (position, width, amplitude)
Background = Gaussian noise (mean, variance)
Full estimation window: classical (Bayesian) Gaussian fitting
Partial estimation window: contaminated peak modelingModel 0 (Gaussian peak) & Model 1 (no peak)
Extra parameters:- model 0 width- mean, variance for model 1
Bayesian inference:unwanted parameters areintegrated out automatically
mod
el 1
wei
ghts
model 0
weights
full
win
dow
wei
ghts
Define search intervals using Bayesian model selection1. Model 0 (Gaussian peak) / Model 0 (no peak) > Significance Level2. Peak amplitude > Threshold
Ground: compute only the first interval
Compute the ground position pdfSum over model 0 & peak width, or use joint pdf and Gauss approx.
Real waveforms: robust inference!
–t (ns)–t (ns)
ampl
itud
e
ampl
itud
e
search interval
ground pos. pdf
search interval
ground pos. pdf
σt = 0.1 ns (1.5 cm)σt = 0.3 ns (4.5 cm)
Bayesian ground range inference
–t (ns)
–t (ns)
Simulations: ground peak + 2nd peak + noise
Ful
l win
dow
bias
!O
ptim
al w
indo
wco
nsis
tent
/ gr
ound
trut
h
ampl
itud
eam
plit
ude
wid
enin
gfa
ctor
–t (ns)
–t (ns)
wid
enin
gfa
ctor
search interval
ground pos. pdf
search interval
ground pos. pdf
Towards a probabilistic DEM
Wave
waveformprocessing
Georef
directgeoreferencing
Grid
jointgridding & filteringra
nge,
σra
nge
X, Y
, Z, σ
Z
Raster: z, σz
POS file:trajectory, attitudewaveforms y(t) timestamps, θ
raw LiDAR data
DEM + error map
Calibrateboresight alignment Analyze
DEM-derived products
internalparameters
Study area and data acquisition
Elevation range: 150-350 mLand cover: fields, shrubs, isolated trees, forests, hills, lakes,small rivers, villages and isolated housesGeomorphology: fault scarps, knickpointsTotal area: 216 km2 (150 km2 specified point density)Flight altitude: 1500m AGL (normal), 3000m AGL (experimental)Point density: 3.8 pts/m2 (total >600M waveforms), 60% overlap, 100 GB raw data
NW of Arraiolos, Portugal
10 km
Flight trajectory (June 2, 2011) and densityFlight planning and data acquisition: IMAO (France) www.imao-fr.com
LiDAR: IGI LiteMapper 6800 (Riegl LMS-Q680i)
Double density
Simple density
5 km
GPS Field work (control points)
Over 5700 points: isolated, tracks, x-sectionsDGPS, GPRS, RTK... from 1 to 5cm vertical accuracyMostly elevation control points, a few features (buildings, sports fields)
Use raw waveform data
Sample the topography at a sufficient densityApply Shannon’s sampling theorem to avoid aliasing point spacing ≈ footprint size➔ consistent change detection, ground motion, deformation...
Make sure the calibration is done right
Fly as low as the budget permits, allow sufficient overlap
Have a good reference GPS station
Acquire some control points, just in case...
Recommendationsfor accurate, consistent and useful probabilistic DEMs
Filtering and classification- Vegetation filtering and bare earth gridding (simultaneous)- Classification, segmentation of the DEM
Computational efficiency- Process large volumes of raw data, roughly 100 GB- Complexity of the gridding and filtering algorithms
Full automation- Unsupervised gridding and classification: parameter estimation- Automatic calibration (boresight) from normal flight lines
Full uncertainty map computation- Inversion of sparse matrices for variance/covariance computation
Work in progress...
sites.google.com/site/autoprobadtm