Exact Geo- Referencing of Airborne Lidar Data

Exact Geo- Referencing of Airborne Lidar Data

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H. Kager
Vienna University of Technology,
Institute of Photogrammetry and Remote Sensing,
Gusshausstr. 27-29 / 122, A-1040 Wien
[email protected]

ABSTRACT:
This paper deals about the importance of exact geo-referencing of airborne LIDAR data. Flying companies usually deliver their point cloud data (and even derived products as DTMs or DSMs) in a co-ordinate system specified by the client. For that purpose they use transformation elements between e.g. WGS-84 datum and the appropriate national surveying system. Nevertheless, the original navigation data (GPS, IMU) are not free from errors, esp. small systematic errors. Moreover, the calibration of the scanning system’s components is not perfect. These imperfections together yield discrepancies between overlapping laser-scanner strips which can be easily visualized by colour-coded difference-DEMs created from overlapping strips. We assume, these discrepancies are stemming from non-sufficient system calibration. They are unsatisfactory phenomena for end-users of the ground data – appearing in height as well as in ground plan.

Nevertheless, these gaps can be eliminated to a great portion doing a simultaneous 3D adjustment by least squares. An operative adjustment strategy for doing that is outlined: correcting exterior orientation elements, as well as interior orientation elements concerning the system components (misalignments, eccentricities, etc.). The method applied reminds of photogrammetric block adjustment with self-calibration.

The distribution of control-features (instead of control-points) is discussed.
Final quality checking also uses colour-coded difference-DEMs.

Introduction
Abbreviations used and their definition in short
ALS … Air borne Laser-Scanning or Air borne Laser-scanning System; the latter contains LSU + dGPS + IMU
LIDAR … LIght Detection And Ranging; an often used synonym for ALS; came up as analogy to “radar”
LSU … Laser Scanning Unit;
GPS … Global Positioning System
dGPS … differential Global Positioning System; a refinement of GPS using a reference-station to increase accuracy
IMU … Inertial Measurement Unit; is capable to “know” about its attitude in space whatever motion it is undertaken
INS … Inertial Navigation System; contains an IMU; in this context the combination of dGPS and IMU
XYZ … in the context of a national ground-survey co-ordinate system: X … easting, Y … northing, Z … elevation
r.m.s.e. … Root-Mean-Square-Error; a coarse measure of a mean discrepancy, but easy to calculate
DTM … Digital Terrain Model: a description of the earth’s ground surface; see: DEM
DSM … Digital Surface Model: a description of the surface inclusive man-made objects; see: DEM
DEM … Digital Elevation Model; a generic term (superordinate concept) for DSM and DTM;
describes Z=Z(X,Y), i.e. the elevation as bi-variate function above ground plan (X,Y);
there exist different variants as raster-DEM or TIN-DEM (triangulated irregular network) with or without break-lines
dDEM … difference DEM; a DEM defined as difference of two DEMs; dDEM:=DEM2-DEM1 with respect to Z

General
Laser-scanners are mounted in aircrafts for collecting 3D-data of the surface of the earth. One of the components of an air-borne laser-scanning (ALS) system is the laser scanning unit (LSU) containing a laser emitter, a deflection unit, and a receiver for the laser pulse’s echo. Progressing the flight path, the laser beam sent downwards is deflected rhythmically aside by the deflection unit and scans the ground surface in a meandric or quasi-parallel pattern with a high pulse rate (2 kHz earlier up to 100 kHz now (Friess, 2006)). Most such devices use the technique of run-time measurement: the distance to a ground point then is a function of the time gap between the pulse was sent and its echo received assuming the velocity of light known. For digital terrain models (DTM) we are interested in the last echo, only. For digital surface models (DSM) we are interested in the first echo. When vegetation studies are of interest, the potential of full-wave-scanners will be fully exploited.

So, the LSU records polar co-ordinates of ground points in its own local co-ordinate system. Like a motorized ranging theodolite, the zenith-angle of which is restricted to 100gon. The LSU (more exactly: its antenna eccentricity) follows the flight path and its movement can be measured with dGPS very precisely. Since coupled to the aircraft, the attitude of the LSU changes also during the flight and can be recorded with an IMU attached to it rigidly.

The components GPS, IMU and LSU have to be synchronised using a precise clock. Moreover, their relative – but constant – displacements have to be determined (eccentricity calibration of the LSU: a shift-offset of the GPS-antenna; a mounting rotation against the IMU).

For transforming laser-scanner strips from the world co-ordinate system into the national ground-survey co-ordinate system using dGPS and INS, we principally need only one ground reference-station with known ground-survey co-ordinates; provided, these global transformation parameters (usually given as 7-parameter transformation for the datum) are known and published by the national surveying agency.

But, in practice, we should not be satisfied with that minimal solution above because, nowadays, ALS has a potential precision of about ±5cm! By the flying companies a typical ground accuracy of ±15cm in height is promised; they are using heights on football-fields for calibration.

On-the-fly-initialisation for solving the GPS phase ambiguities nowadays is possible for aircrafts with a r.m.s.e. of about 10cm; this might result in errors of some dm shift. Usually, neighbouring precision of dGPS is better by one order of magnitude. Errors increase with strip length (Cramer, 2000).

Attitudes as delivered from IMUs in use are prone to errors of about 0.01gon resulting in 16cm on the ground assuming 1000m relative flying height. Errors of IMU attitude also introduce some torsion of laser-scanner strips inducing errors in ground co-ordinates. Alike, IMU attitudes have a high neighbouring precision based on the gyros used; nevertheless, they show drifting phenomena. Resulting error effects might reach again some dm in the positions of ground points. (Cramer, 2000)