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Outline

Project objectives

Why do we need raw data?

Decoding the information

Format description

Modeling the instrument

Waveform processing

Study area & first results

Perspectives USGS

Project scientific objectives AutoProbaDTM (3-year project, FCT Portugal)

Automated Probabilistic Digital Terrain Model generation from raw airborne LiDAR data

Generate gridded elevation models

Robustly estimate the bare ground topography

Lower the acquisition costs by extending the range through processing

Provide consistent, predictive spatial accuracy mapsrather than classical uniform error measures using control points

Minimize the user interaction: automatic parameter estimation

Process large datasets >1G waveforms

Bonus: extract canopy height and vegetation parameters

... but first read and understand the raw data!

Towards a probabilistic raster 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

Principles of airborne LiDAR

1. Time of flight measurement(waveform acquisition)

2. Scanning:aircraft motion + laser rotation3. Direct georeferencingIMU: GPS+INS

Riegl

waveforms

look angles

timestampswhat do we need?

Decoding the Riegl LMS-Q680i binary file format

6 GB binary files (.sdf)

The proprietary software RiAnalyzefrom Riegl can display a wave and some related information, and provides the processing results as a text file (timestamp, range, theta, etc.)

➔ Reverse engineering

Update: RiWaveLib first released in 07/2011, but still has issues and limitations... such as radiometric modeling!

+ processing byproducts, e.g. sdh text files (header constants)

The sdf file general structure

Header

Pulserecord ...

Sync Rec Data

Hdr EmPulse RecWave RecWave... chk

Sync Rec Ctrl

Controlrecord

Pulserecord

Pulserecord

Pulserecord

Pulserecord

Pulserecord

1st row

2nd row

Controlrecord

...

...

Control record (GPS sync)

Pulse record (timing, orientation, waveform)otherrows

General file structure

DataHdr

DataHdr

00 22 00 1C 83 B8 C8 D6 01 66 01 05 09 25 01 67 01 56 25 59 1F FF FE 02 00 6B C0 06 02 02 02 02 02 07 14 3B 78 B1 C8 AE 76 3F 1C 0B 05 03 03 05 05 03 00 00 02 73 00 14 03 02 02 02 02 03 03 04 02 03 03 04 08 10 1A 1F 1C 13 0A 06 02 02 01 02 02 04 03 03 03 02 02 03 02 02 02 03 01 02 01 01 02 03 02 03 03 02 02 02 03 03 02 03 02 02 02 03 02 02 02 03 03 03 03 02 02 02 02 01 02 03 02 02 02 01 03 03 03 03 02 02 A8 BB AD 82

angleencoder

recordtimestamp

datalength

numberof blocks

data timestamp

received waves checksum

Record structure(big endian)

angleencoder

emittedpulse

The sdf fileformat description

Rec (24 bytes)Record parameters:- Data/Control switchIf data block:- Raw timestamp- Theta (4 encoders)- Number of waveform blocks

struct Q680RecBlockBuf { char dum0 [2]; // -- char ctrl [1]; // u1 control block flag char tc [5]; // i5 record timestamp (4ns) char dum1 [2]; // -- char C1f [2]; // u2 fine angle encoder 1 char C1 [2]; // u2 coarse angle encoder 1 char dum2 [2]; // -- char C2f [2]; // u2 fine angle encoder 2 char C2 [2]; // u2 coarse angle encoder 2 char dum3 [3]; // -- char nb [1]; // u1 number of data blocks};

DataHdr (4 bytes)Waveform block header:- Relative timestamp- Type, length

struct Q680DataBlockHdrBuf { char tc [2]; // u2 data timestamp (4ns) char type [1]; // u1 type {EM=192, LO=0, HI=64} char l [1]; // u1 length (4 samples)};

Ctrl (68 bytes)GPS synchronization:- Sync timestamp- GPS weekseconds

struct Q680CtrlBlockBuf { char dum0 [51]; // -- char sync [5]; // i5 sync timestamp (4ns) char weeksec[4]; // u4 weekseconds (ms) char dum1 [8]; // --};

EmPulse (DataHdr.l bytes)Sampled emitted pulse

char EmPulse[DataHdr.l]; // u emitted pulse (1ns sampling)

RecWave (DataHdr.l bytes)Sampled received wave block

char RecWave[DataHdr.l]; // u received wave block (1ns sampling)

Variables and data blocks:- Rec timestamp / theta / facet / n blocks- Data timestamp / length- GPS sync timestamps- Emitted pulse- Received waves

Header (6640 bytes)sdf file header.

LMS-Q680i instrumentconstants: - File format parameters- Angle encoder constants- Geometry constants- Range correction arrays- Amplitude correction arrays

Variables:- Pulse Repetition Rate (PRR)- Measurements per line- MTA zone (pulses in the air)- MTA PRR bounds

struct Q680HeaderBuf { char HeaderSize [4]; // i4 size of this header char ProtocolFlags [4]; // u4 ? char SubheaderIdMain [2]; // u2 ? char SubheaderIdSub [2]; // u2 ? char ParameterIdMain [1]; // u1 ? char ParameterIdSub [1]; // u1 ? char MeasIdMain [1]; // u1 ? char MeasIdSub [1]; // u1 ? char SampleBlockIdMain [1]; // u1 ? char SampleBlockIdSub [1]; // u1 ? char HousekeepingIdMain [1]; // u1 ? char HousekeepingIdSub [1]; // u1 ? char SerialNumber [8]; // s8 serial number char InstrumentType [8]; // s8 type (Q680I) char InstrumentModel [2]; // u2 model number char dum1 [2]; // -- char SyncField [4]; // i4 record synchronization field (=ZZZZ) char BeamAperture [2]; // u2 laser beam aperture (?) char dum2 [2]; // -- char BeamDivergence [2]; // u2 laser beam divergence (?) char dum3 [2]; // -- char BeamFocus [2]; // u2 beam focus (?) char dum4 [2]; // -- char LineAngleCircleCount [4]; // u4 encoder steps (for 360 deg) char NumberFacets [1]; // u1 number of facets (=4) char dum5 [1]; // -- char LineAngleCoarseRes [2]; // u2 coarse angle resolution char LineAngleOffset0 [4]; // u4 coarse angle encoder 1 offset char LineAngleOffset1 [4]; // u4 coarse angle encoder 2 offset char LineEncoderFineCountOffset0[2]; // u2 fine angle encoder 1 offset char LineEncoderFineCountOffset1[2]; // u2 fine angle encoder 2 offset char RangeCorrPPM [4]; // i4 light velocity correction (PPM) char LoChannelRangeOffset [4]; // i4 low channel range offset (mm) char HiChannelRangeOffset [4]; // i4 high channel range offset (mm) char CorrLaserOriginx [4]; // f4 x| char CorrLaserOriginy [4]; // f4 y|laser origin vector (m) char CorrLaserOriginz [4]; // f4 z| char CorrLaserDirx [4]; // f4 x| char CorrLaserDiry [4]; // f4 y|laser direction vector char CorrLaserDirz [4]; // f4 z| char CorrMirrorAxisx [4]; // f4 x| char CorrMirrorAxisy [4]; // f4 y|mirror axis vector char CorrMirrorAxisz [4]; // f4 z| char CorrOffsetx [4]; // f4 x| char CorrOffsety [4]; // f4 y|offset vector (m) char CorrOffsetz [4]; // f4 z| char CorrFacetNormal1x [4]; // f4 x| char CorrFacetNormal1y [4]; // f4 y|facet 1 normal vector char CorrFacetNormal1z [4]; // f4 z| char CorrFacetDistance1 [4]; // f4 facet 1 distance (m) char CorrFacetNormal2x [4]; // f4 x| char CorrFacetNormal2y [4]; // f4 y|facet 2 normal vector char CorrFacetNormal2z [4]; // f4 z| char CorrFacetDistance2 [4]; // f4 facet 2 distance (m) char CorrFacetNormal3x [4]; // f4 x| char CorrFacetNormal3y [4]; // f4 y|facet 3 normal vector char CorrFacetNormal3z [4]; // f4 z| char CorrFacetDistance3 [4]; // f4 facet 3 distance (m) char CorrFacetNormal4x [4]; // f4 x| char CorrFacetNormal4y [4]; // f4 y|facet 4 normal vector char CorrFacetNormal4z [4]; // f4 z| char CorrFacetDistance4 [4]; // f4 facet 4 distance (m) char LinearizeArray [512]; // u2[256] nonlinear amplitude correction (low channel) char DelogArray [512]; // u2[256] nonlinear amplitude correction (high channel) char AmplCorrArrayLO [600]; // i2[300] amplitude-dependent range correction (low channel) (mm) char AmplCorrArrayHI [600]; // i2[300] amplitude-dependent range correction (high channel)(mm) char ADCThresholdArrayLO [2048]; // u1[2048] ADC thresholds (low channel) char ADCThresholdArrayHI [2048]; // u1[2048] ADC thresholds (high channel) char ADCOffset [8]; // u2[4] ADC offsets char LastPulseSuppression [4]; // u1[4] ? char NearRangeSuppression [4]; // u4 ignore nearest return (unit?) char LaserPulseRate [4]; // u4 PRR (Pulse Repetition Rate) (Hz) char MTAPrrConfig [1]; // u1 ? char dum6 [3]; // -- char MTAPrrLow [4]; // u4 PRR lower bound (Hz) char MTAPrrHigh [4]; // u4 PRR higher bound (Hz) char MTAZone [1]; // u1 number of pulses in the air char dum7 [3]; // -- char LineScanStartValue [16]; // u4[4] ? char LineScanIncrement [16]; // u4[4] ? char LineScanMeasNumber [4]; // u4 measurements per line char RangeGate [2]; // u2 ? char PositionStartPulse [2]; // u2 ? char SampleBlockLen [4]; // u1[4] data block lenghts (4ns) char PreQuadCount [4]; // u1[4] ? char ADCThreshold [4]; // u1[4] ADC thresholds char ChannelSwitchLevel [1]; // u1 hi channel recording threshold char dum8 [3]; // -- char LogChannel [1]; // u1 ? char dum9 [3]; // -- char FirstBlocks [1]; // u1 ? char dum10 [3]; // -- char LastBlocks [1]; // u1 ? char dum11 [3]; // -- char GpsStringDecoder [16]; // s16 ?};

Con

stan

tin

stru

men

t pa

ram

eter

s

The sdf format: variable decodingVariable/Constant Unit Decoding function using sdf block fields

r0Lo m (c1/2 / 0.15) (10-3 Header.LoChannelRangeOffset - 0.60)

r0Hi m r0Lo + (c1/2 / 0.15) (10-3 Header.HiChannelRangeOffset)

ΔrLo[256] , ΔrHi[256] m 10-3 Header.AmplCorrArrayLo , 10-3 Header.AmplCorrArrayHi

c1/2 m ns-1 0.15 (1 + 10-6 Header.RangeCorrPPM)

Δn - Header.MTAZone - 1

F-1Lo[256] , F-1Hi[256] - Header.LinearizeArray , Header.DelogArray

Estep deg 360 / Header.LineAngleCircleCount

K deg 1/2 Estep Header.LineAngleCoarseRes

Kf deg 3 10-7 Header.LaserPulseRate / Header.LineScanMeasNumber

θ0 deg 1/2 Estep (Header.LineAngleOffset0 + Header.LineAngleOffset1)+ 3/2 Kf (Header.LineEncoderFineCountOffset0 + Header.LineEncoderFineCountOffset1)

d[4] m Header.CorrFacetDistance

n[4] - Header.CorrFacetNormal

a - Header.CorrMirrorAxis

l - Header.CorrLaserDir

L m Header.CorrLaserOrigin

tRec ns 4 Rec.tc

C1 , C2 , C1f , C2f - Rec.C1 , Rec.C2 , Rec.C1f , Rec.C2f

W s 10-3 Ctrl.weeksec

tsync ns 4 Ctrl.sync

tData ns 4 DataHdr.tc

tEmPulse ns EmPulse waveform processing (Gaussian pulse estimation or other)

tRecWave , A ns , - RecWave waveform processing (research in progress)

Rec (24 bytes)Record parameters:- Data/Control switchIf data block:- Raw timestamp- Theta (4 encoders)- Number of waveform blocks

struct Q680RecBlockBuf { char dum0 [2]; // -- char ctrl [1]; // u1 control block flag char tc [5]; // i5 record timestamp (4ns) char dum1 [2]; // -- char C1f [2]; // u2 fine angle encoder 1 char C1 [2]; // u2 coarse angle encoder 1 char dum2 [2]; // -- char C2f [2]; // u2 fine angle encoder 2 char C2 [2]; // u2 coarse angle encoder 2 char dum3 [3]; // -- char nb [1]; // u1 number of data blocks};

DataHdr (4 bytes)Waveform block header:- Relative timestamp- Type, length

struct Q680DataBlockHdrBuf { char tc [2]; // u2 data timestamp (4ns) char type [1]; // u1 type {EM=192, LO=0, HI=64} char l [1]; // u1 length (4 samples)};

Ctrl (68 bytes)GPS synchronization:- Sync timestamp- GPS weekseconds

struct Q680CtrlBlockBuf { char dum0 [51]; // -- char sync [5]; // i5 sync timestamp (4ns) char weeksec[4]; // u4 weekseconds (ms) char dum1 [8]; // --};

EmPulse (DataHdr.l bytes)Sampled emitted pulse

char EmPulse[DataHdr.l]; // u emitted pulse (1ns sampling)

RecWave (DataHdr.l bytes)Sampled received wave block

char RecWave[DataHdr.l]; // u received wave block (1ns sampling)

Basic variable or constant decoding functions

Modeling the instrument radiometry

IR modelRecWave

0 5-5 10

EmPulseImpulseResponseModeling

sum of Gaussians,amplitudedependent delay,non-linear gain

2 channels: • Lo - low power returns• Hi - high power returns & emitted pulse

Detailed model for each channel:• Short laser pulse (Gaussian 3ns?)• Amplification, non-linear gain function• Amplitude-dependent time delay• Additive offset• Convolution by the system Impulse Response (IR) (sum of 3 ns Gaussians)• 1 ns sampling and 8 bit quantization

LinearizeArray (Lo) DelogArray (Hi)

Radiom.look-up tables

Modeling the instrument radiometry

AmplCorrArray (Lo) AmplCorrArray (Hi)

Rangecorrection tables(mm)

Using the Riegl calibration tables to apply simple corrections

Modeling the instrument geometry

y

x

z

O

L lnθ

u

SOCS

θp0

M

d

Laser origin

Phi difference Theta difference

Rotating squarepyramid mirror

(sensor own coordinate system)

Accurate Timing• Pulse time determination - Bayesian Waveform processing

• GPS synchronization

• MTA processing (multiple pulses in the air) - Automatic MTA adjustment method

Radiometry and range estimation• Radiometric decoding

• Ranging with simple corrections

Geometry• Angle and facet decoding

• Sensor Own Coordinate System (SOCS) coordinates

Decoding, ranging, sensor geometry

tE = tRec + tData + tEmPulse and tR = tRec + tData + tRecWave

T = W + 10-9 (tE - tsync)

Δt = tR(n) - tE(n - Δn)

Acorr = F-1Lo[A] or Acorr = F-1Hi[A]

r = r0Lo + ΔrLo[A] + c1/2 Δt or r = r0Hi + ΔrHi[A] + c1/2 Δt

θfull = (θ0 + K (C1 + C2) + Kf (C1f + C2f) + 180 (C1>C2)) mod 360θ = θfull mod 90 and F = int(θfull/ 90)

p = p0 + r u (see paper)

y

x

z

O

L lnθ

u

SOCS

θp0

M

d

Impulse Response (IR) modeling3 peaks (ringing)

Example: Bayesian waveform processing...

Partial deconvolution (Diracs)Target = single Gaussian PSFRinging removal without artifacts, no regularization required

original

deconvolved (partial)

–t (ns)

ampl

itud

e

–t (ns)

IR

wav

efor

m

full estimation window

mod

el 1

wei

ghts

model 0

weights

Partial estimation window: contaminated peak modelingModel 0 (Gaussian peak) & Model 1 (no peak)

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 range probability density functionSum over model 0 & peak width, or use joint pdf and Gauss approx.

... for robust ground extraction

–t (ns)–t (ns)

ampl

itud

e

ampl

itud

e

search interval

ground range pdf

search interval

ground range pdf

σt = 0.1 ns (1.5 cm)σt = 0.3 ns (4.5 cm)

Study area and data acquisition

Elevation range: 150-350 mLand cover: fields, shrubs, trees, forests, hills, lakes,small rivers, villages and isolated housesGeomorphology: fault scarps, knickpointsTotal area: 200 km2 (150 km2 specified point density)Flight altitude: 1500 m AGL (normal), 3000 m (experimental), 500 m (calibration)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

• Over 12000 control points (+land cover, accuracy): individual, tracks, transects• GPS-RTK Postproc/GPRS, ... from 1 to 5cm vertical accuracy• Mostly elevation control points, a few features (buildings, sports fields)

Control point distribution and flight strip footprints

Filtering & classification- Simultaneous bare earth gridding and outlier filtering

Computational efficiency- Process large volumes of raw data, 100 GB, 1G waves- Complexity of waveform processing, gridding and filtering algorithms

Full automation- Unsupervised parameter estimation for all algorithms- Automatic boresight estimation without extra flight lines- Automatic removal of geometric artifacts (e.g. corduroy)

Efficient accuracy map computation- Avoid matrix inversion and prefer local regression approaches...- Full 3D accuracy prediction, including geometic errors

Validation & Verification

Challenges, work in progress...

sites.google.com/site/autoprobadtm