Airborne Laser Scanner (ALS) processing

General Processing with a settings (.reg) file

1. Start ALS PP (possibly using Linux)
2. Load in the reg file to set the specific settings for the project.
   • File > Load Settings
   • Reg files under ~arsf/calibration/lidar. Usually, pick the calibration that's the most recent prior to your project day. However, sometimes calibration flights are flown AFTER data is collected. Check with someone if unsure.
3. For Wine, Go to Inputs > Range Correction and for Intensity Based Range Correction browse to \users\rsg\arsf\calibration\lidar\... and select the relevant SN089_IBRC*txt file for the flight day you are processing
4. Set the Goto Folder to the main project directory
   • File > Set Goto Folder
5. Check the settings are appropriate for the project before proceeding (see below). This is important if using a reg file for a different project.
6. Select POS filename
   • Click Browse and add the .sol file (posatt/ipas_honeywell/proc/)
7. Select the Output directory
   • Click Select > Goto Folder and select <projname>/processing/als50/las. You should create directory if it doesn't exist.
8. You will need to change the projection (File -> Output Options -> Change output format) Change: zone, horizontal datum, vertical datum.
9. Save settings for future re-processing purposes.
   • File > Save Settings (e.g. processing/als50/2009280b.reg)
10. Add the raw SCN data
    • Click Add and browse to the parent raw data directory. This will add all flight lines in the immediate child directories. lidar->als50->RawLaser
11. Run the processing
    • Run > Run

Setup of the system

• Hardware options. Once set these should never change (unless the system is modified etc) Should be supplied when the system is delivered. Can view/set these from File > Processing Options.
• Output file options. Set the projection details (usually OSGB36), LAS file format, and whether want GeoTIFF or ASCII output awswell. Also contains options for debugging.

• Input options
  • Atmospheric Correction. Need values of the altitude, temperature and pressure for a low altitude and high altitude. With these values a correction gradient will be produced.
  • Range Correction. A time-distance delay in metres of the ALS system, in the measurement of range data. (This is complicated...see user manual for details)
  • Scanner Correction. These do not need to be changed after the calibration/boresighting of the system.
  • POS errors. Angular difference between the IMU and the ALS optics reference frames. Needs to be known accurately.
  • Elevation offset. This needs to be set if we have ground control points: is normally non-zero. It is identified using the TScan Control Report (eh?). The LIDAR is then reprocessed using the opposite of the DZ offset.
  • Intensity. Use the Raw intensity.

Possible errors

• ALSPP.EXE on Linux

1. If a Windows crash message appears when trying to 'Run', ensure the POS file is pointed to on the Linux filesystem and not the Windows hard drives (D:,.. etc.).
2. If an error message claims an "invalid IBRC file" was specified when trying to Run, this can happen because the intensity based range correction file was not correctly specified under Input > Range Correction. Make sure the correct .txt file is being used, or try any others in the same directory.

ALSPP Known Bugs

Two known issues with LAS files produced using ALSPP (using Ver 2.74#8):

• The EPSG codes are not populated correctly (using WGS84 UTM). Under the key 3076, ProjLinearUnitsGeoKey, it should give the linear units instead it gives the projection code. The key 3072 ProjectedCSTypeGeoKey is missing, which should contain the projection code. Thus a program that tries to out projection information and follows geoTIFF specification will fail because ProjLinearUnitsGeoKey holds incorrect information and ProjectedCSTypeGeoKey doesn't exist.

• The max/min values in the header file are incorrect so there are usually a couple of points in the file that are outside the bounding box. Some programs then exclude these points as they only read those point within the max/min values given in the header.

These have been reported to Leica Support 08/07/2012, who have forwarded to their development team.