Changes between Version 1 and Version 2 of Processing/LidarCalibrationProcedure

Apr 30, 2009, 12:33:24 PM (15 years ago)



  • Processing/LidarCalibrationProcedure

    v1 v2  
    11= Calibration of the Leica ALS50 II LIDAR =
     2This page describes the procedure for calibrating the ARSF LIDAR.  The procedure is performed in 3 main stages: Boresight, Range correction and validation. For full information on the calibration see [wiki:Sensors/LeicaLIDAR/MashUp#Calibration here].
     4== Boresight ==
     5The first stage of calibrating the LIDAR is to calculate the boresight angles. These are the angles in pitch, roll and heading that describe the pointing direction of the laser scanner with respect to the nadir. Using incorrect boresight parameters will result in georeferencing errors and gross errors will be obvious when comparing overlapping flight lines, especially lines flown in varying directions. The boresight calibration is performed using data taken at 3 altitudes: 750m, 1350m and 2300m. The flight lines are acquired in 'cross formations' and parallel lines with a sizeable overlap.
     7=== Pitch ===
     8To correct pitch we take opposing flight lines.  If there is a pitch error, the lidar will be sensing pulses ahead/behind (along track) the nadir point.  There will be a small overall range error, but the main effect will be that the data is shifted backwards/forwards of where it should be.
     10=== Roll ===
     11If there is a small roll error, surfaces will be tilted up on one side of the swath and down on the other.  In opposing lines, these errors will be on different sides.  Vertical displacements due to roll error will be most noticeable at the edges of the swath.  For small errors, the nadir point will be roughly the same, for large ones, there will be some across-track shift, but that will be much harder to measure than the vertical errors.
     13=== Heading / Yaw ===
     14If there is a heading error, the left side of swath will move backwards while the right side will move forwards (or vice versa).  If you reverse the direction of flight (ie. with opposing flight lines), these errors will overlap (left side down in one direction matches right side up in the other) and not be visible. Instead, take two parallel flight lines with 30-50% overlap
     16=== Pitch error slope ===
     17The mirror will not be mounted exactly flat to the laser so, as the mirror moves, the pitch of the beam will change by a small amount.  The resulting errors are along-track offsets just like the pitch errors, but varying, with no error at nadir and maximum error at the edges of the swath.
     19== Procedure ==
     20The main procedure in the boresight correction is:
     21 * Process Navigation
     22 * Process raw scan to attune files in ALS Post Processor
     23 * Use Attune to calculate boresight parameters
     24 * Analyse in Terrascan - reprocess if necessary
     261. Process the Navigation as per usual using IPAS Pro and Grafnav
     282. In ALS Post Processor:
     29 * uncheck the “average last returns” option in the settings dialog
     30 * Check the output as attune files in the outputs dialog
     31 * Add all the flight data into the ALSPP, including the BIT mode data.
     32 * Run the RangeCardCal program from the utilities menu.  Select the use all SCN files option when prompted.
     33   * Enter range offset A1 as 0 since it is unknown#
     34   * Enter the output Range offsets into the range corrections dialog of ALSPP. Only need R1 for A and B for now.
     35 * Uncheck the BIT mode data (we don't need to process this) and run the ALSPP processing
     36 * Look at the filesizes of the output data.  If the LAS files are over 400MB in size then Attune will not run. If this is the case re-process with only the part of interest of the line.
     37   * Part of interest will be the area where all flight lines overlap.
     383. Ground classification needs to be performed on the files before tie pointing in Attune.
     39 * We do this so that we only use points on the ground in the tie pointing (since we don't want errors due to perspective or shadowing)
     414. Start Attune and tie point the data
     42 * Add ALS data -> add the real ATN.LAS files from the ALSPP output
     43   * The suggested pixel sizes (from top level calibration pdf) are 0.3m for 750m, 0.4m for 1350m and 0.8m for 2300m.
     44 * View the image pngs to make sure no data holes. (Some are OK. Re-load using a different resolution if too many holes. e.g. Try 0.35m for 750m)
     45   * 'a' centres the image
     46   * '+/-' to zoom in/zoom out
     47 * Now select tie points. We use streets for tie points since these should be on the ground so no (little) effects due to look angles and also has slow varying topography
     48   * Open tie point editor in Attune.
     49   * Press 'm'  to measure a point and then click where you want it. Do this for each image (selecting the same point). Then press 'sample all' to record the locations.
     50   * To change one, re-measure it and then just click 'sample point'.
     51 * Save the project after selecting 50-100 points. (Attune is very crash-able, save often)
     52 * Select points on roads with uniform intensity. Use lines of roads and field boundaries to get the same place in each image. Don't use buildings as markers due to perspective/shadow.
     53 * Edit project file properties.
     54   * Torsion constant =-100,000
     55   * Image observation Weights 0.2, 0.2, 0.1
     56   * Atmosphere same parameters as in the ALSPP processing
     57   * Set class as ground -> 2 (ground is usually class 2)
     58   * Adjustment criteria  maxiter=50, angular (?)=1.00e-08
     59 * Click Solve Calibration parameters and unselect Torsion, just want to solve roll, pitch and heading.
     60 * Analyse adjustment
     61   * A good result would have aposteriori reference of between 1 and 2.
     62   * No. of observations of 300 is good
     63   * Standard deviations of around 0.00001 for Roll and Pitch, 0.00005 for Heading.
     64   * Average residuals of 25cm for X and Y, 5cm for Z is good.
     65 * If the above criteria are not met (roughly...they are only guidelines)
     66   * Analyse results and remove or adjust any points with large residuals
     67     * Open all the images + point editor and then can select a point from the solution list and zoom to it on the images
     68     * You could just remove the point from one or two images (especially the high altitude images) or from all images.  Then recalculate the network adjustment and iterate until Aposteriori reference is approx 1-2 and average residuals are 0.25,0.25 and 0.05.
     69     * Can remove the points from the lower resolution images if it will help. We want more measurements in the higher resolution images than the lower ones if they are unclear to measure in (so don't just measure in them for the sake of it)
     70   * When happy save the solution
     715. Return to ALS Post Processor
     72 * Add the Pitch, Roll and Heading values into the boresight calibration dialog.
     73 * Change output to LAS (not attune)
     74 * Change output directory
     75 * Re-run the processing
     766. Now analyse the results in Terrascan
     77 * Check profiles etc for misaligning flightlines.
     78 * To load all flightlines will probably need to use a fence (this is like a region of interest)
     79 * To check the Roll accuracy – check opposing flights and look at cross sections at either end of the swath for a tilt (across track).
     80 * To check the Pitch accuracy – check along track on slopes for offsets
     81 * To check the Heading accuracy – check the parallel lines for offsets
     82 * Also possible to check for errors by re-ordering points by elevation
     837. The boresight parameters can be manually twiddled (rather than using Attune) by reprocessing in ALS, changing the R,P,H values one at time.  Only process parts of the flightlines for speed benefits.