Radiometric calibration of ALS intensity

Transcription

1 Radiometric calibration of ALS intensity Sanna Kaasalainen a, Juha Hyyppä a, Paula Litkey a, Hannu Hyyppä b, Eero Ahokas a, Antero Kukko a, and Harri Kaartinen a a) Department of Remote Sensing and Photogrammetry b) Helsinki University of Technology

2 Contents - Background - The FGI calibration concept for ALS - Overview and results of flight campaigns - Laboratory validation & results - Discussion, applications, future developments 2

3 Brightness measurement with ALS: background The usage of ALS intensity data has been limited (due to, e.g. calibration problems) Systematic calibration method would provide more precise surface & target characterization + simultaneous/ weatherproof reflectance measurement Sometimes a major or only source of (brightness) information from a particular area Development of sensors could improve the accuracy 3

4 FGI calibration concept for ALS Started 2006 using portable reference targets Set of eight 5x5m tarps of 5%-70% reflectance New innovation 2006: use of industrial gravels Laboratory brightness calibration with laser instruments and broadband BRDF spectro-goniometer Reflectance relative to Spectralon 1,2 1 0,8 0,6 0,4 0,2 0 Relative reflectance Target nominal intensity Tarps at Espoonlahti boat harbour (2006 ALS campaign) Tarp reflectances with 1064 nm laser/ccd detector (99% =Spectralon) 4

5 Overview of previous campaigns using test tarps Location & Date Instrument Wavelength (nm) Altitude (m) Sjökulla Jul 05 Optech Nuuksio May 06 Optech Espoonlahti Aug 06 Topeye Espoonlahti Dec 06 Topeye Scaled intensity 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0, Flight Altitude (m) Tarp 5 Tarp 20 Tarp 26 Left: Comparison of the intensities of some tarps in Espoonlahti (Dec 06), scaled with 40% tarp. Above: tarps being set up in Espoonlahti (Dec 06 campaign). 5

6 Use of industrial gravels in ALS intensity calibration Preliminary test campaign for using standard industrial gravels (first flight campaign Dec 2006 in Espoonlahti) In addition to or instead of portable tarps Laboratory calibration similarly to the test tarps: brightness (gray)scale Espoonlahti Apr 2007 (left) and Dec 2006 (right) 6

7 Preliminary results from the TopEye campaign Dec ,2 Relative reflectance 1 0,8 Sand Quartz 0,6 Diabase LECA 0,4 0, Flight altitude (m) 1,2 Relative reflectance 1 0,8 Sand Quartz 0,6 Diabase LECA 0,4 0, Flight altitude (m) Gravels (from top): sand, quartz, diabase, and LECA (Light Expanded Clay Aggregate). Results: scaled to the brightest (top), scaled with the 45% test tarp (bottom). 7

8 Latest flight campaigns 2007: Espoonlahti and Nuuksio Leica/FMI International Testing of those gravels that were investigated in the laboratory to be most suitable (flat spectra/low incidence angle dependency/ uniform colour etc.) Results still being processed... Top: Espoonlahti Apr 2007 Bottom: Nuuksio Jul

9 Laboratory validation measurements CCD-based laboratory laser measurement: 1064 (Nd: YAG) and nm (He-Ne) Terrestrial (FARO) laser scanner: surface topography and intensity at 785 nm Konica-Minolta laser digitizer: high resolution (50µm) 3D-surface models Supercontinuum ( white ) laser source: laser-based hyperspectral measurement at nm 9

10 Laboratory laser measurement 1064 nm Nd:YAG laser (wavelength similar to most airborne scanners) SuperK: spectral measurement (similar setup + spectrometer) Backscatter measurement: similar measurement geometry as in laser scanning. CCD detector for laser intensity measurement; imaging spectral measurement with Specim ImSpector spectrometer A sketch and photos of the laser laboratory instrument 10

11 Laser backscatter measurement Reference measurement of laser intensity at 1064 and other wavelengths Study of brightness effects at backscatter: increase in intensity (related to the physical backscatter effects) Brightness effects take place in ALS because of the backscatter geometry Relative Reflectance 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0 Gabro Sand Sand Redbrick Polystyrene Refl. at 0 Refl. at 5.5 Target reflectance (1064 nm) at backscatter (0 ) compared to that at 5 viewing angle (relative to 99% Spectralon) 11

12 Terrestrial laser scanner Faro LS HE80 terrestrial laser scanner, 785nm Applied also in: construction quality, tree growth/extraction, etc. Not optmimized for intensity measurement, requires: Brightness calibration measurements (test tarps and calibrated 4-step Spectralon) Also: calibration for distance effects Left: The FARO-scanner measuring the gravels. Right: FARO and the 4-step Spectralon standard (12, 25, 50, and 99%). 12

13 Konica-Minolta Vi-9i laser digitizer Effect of surface properties on brightness measurement The sample is illuminated with a laser line scanning over the target, and the topography is reproduced by triangulation using the CCD image taken simultaneously Currently used for smallscale targets (lichen etc.) and, e.g., construction quality measurements Top: the Konica-Minolta digitizer. Bottom: 3D-point presentation of a gravel sample. 13

14 Supercontinuum laser New technology based on nonlinear optical fibers Continuous spectrum over a wide optical range FGI: Koheras SuperK Red, nm (most stable data at about nm), 20kHz, 100mW, 1-2 ns pulse length Laser-based hyperspectral measurement -> Development of an hyperspectral lidar Left: the SuperK beam reflected from a quartz gravel sample. Below: the spectrum of crushed redbrick. 14

15 Laboratory reference studies Effect of the incidence angle: mostly negligible up to 20, after which a considerable decrease in intensity Effect of moisture/wetness: clear decrease in brightness for wet samples. More systematic study in progress 0,6 0,5 Relative Reflectance 0,4 0,3 0,2 0,1 Dry Wet 0 Black Gabro Crushed Redbrick 1064 nm backscatter measurement for dry and wet gravel samples 15

16 Strengths and weaknesses Gravels + Available in standard hw stores; reasonable price + Disposable; no dirt problem - Uneven surface: a large bulk is needed (logistics problem) Tarps - Custom made; expensive + Easier to get desired charcteristics - Dirt affects spectral/ radiometric properties - Also need an even surface (logistics problem) 16

17 Scientific applications of ALS brightness measurement Snowmelt at the FGI backyard Feb-March Laser images with the FARO scanner. Monitoring and mapping of forests (e.g. tree growth), construction, agriculture Environmental change detection, e.g. snowmelt, snow/glacier albedo, hydrolgical implications, climate change 17

18 Discussion of future developments Changes in target intensity (e.g. due to snowmelt) are clearly observable with laser instruments; change detection Systematic measurement campaign going on for more accurate calibration scheme and further search of most suitable targets Use of natural targets also under investigation: Uniform and stable targets from surroundings, which could be calibrated during flight campaigns with terrestrial instruments Further test measurememts are in great demand, especially flight data 18

19 References Ahokas, E., Kaasalainen, S., Hyyppä, J., and Suomalainen, J. Calibration of the Optech ALTM 3100 laser scanner intensity data using brightness targets. ISPRS Commission I Symposium, July 3-6, 2006, Marne-la-Vallee, France, International Archives of Photogrammetry, Remote Sensing and Spatial Information Sciences, 36(A1), CD-ROM. Kaasalainen, S., Lindroos, T., Hyyppä, J. Towards hyperspectral lidar - measurement of spectral backscatter intensity with supercontinuum laser source. IEEE GRSL 4 (2007), Kaasalainen, S., Kukko, A., Lindroos, T., Litkey, P., Kaartinen, H., Hyyppä, J., and Ahokas, E. Brightness Measurements and Calibration with Airborne and Terrestrial Laser Scanners. IEEE Trans. Geosci. Remote Sensing (2007), submitted for publication. Kukko, A., Kaasalainen, S. and Litkey,P. Effect of incidence angle on laser scanner intensity and surface data. Appl. Opt. (2007), submitted for publication. Thank you! 19