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MashUp

Timestamp:: Sep 24, 2008, 5:16:16 PM (17 years ago)
Author:: mark1
Comment:: --

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Sensors/LeicaLIDAR/MashUp

-                      v2
+                      v3
  * altitude (kinda a parameter ;) ) - minmium of ~650 up to ~2000m (after 2km, you start getting poor returns on forests, etc, the real limit is up to about 6km in ideal conditions)
  * ... pulse frequency, scan angle, etc [TBD]
 …
 Items required in general:
  * calibration (see elsewhere)
  * lever arm measurements
 …
 == General processing ==
+''Mark covered this better''
+The general processing procedure is as follows:
+ * Trajectory processing
+   * see the pages on GrafNav and IPAS processing
+ * ALS post processor to convert raw LIDAR scan data to binary LAS point cloud
+   * see the page on the ALS post processor
+ * QC of the processed data
+   * check % of first returns - should be >95% in most cases
+   * check intensity images look ok (like real world, no streaking etc)
+   * check neighbouring flight lines match up well
+   * basic classification or filtering to remove noisy points
 -----------------------
 …
 Poor accuracy at edges - consider trimming the swath width (reduce processed angle) to cut off bad edges.
+Initial QC for when data first arrives for processing - raw scan data files should all be the same size, except for the last file.
 === Other error sources ===
 …
    * find altitude over ground (measured GPS alt - geoid-spheroid height) and see if it's within the min & max ranges
  * check the speed they were flying at is not too fast
+ * check the images in TerraScan
+ * look at start/stop times of points at end of flight line – do these match the flight logsheet
 Laser power too high - if the intensity overflows (reaches 255), the intensity based range correction will probably be wrong.  For example, a freshly asphalted road with bright white reflective (overflowing) strips may appear with the strips appearing to float up to 20cm above the road surface.
 …
  * This measures how much the mirror flexes at the edges of the swath, when under high acceleration, putting it out of position.
  * Later Leica mirrors (including ours) are much stiffer and apparently do not flex.
  * Fixed very high (negative) value (-100,000 N?) meaning "no measurable flex".
  * Can be measured during the calibration procedure, but should never be necessary.
 …
 Second, we need to establish the timing differences between bank A and bank B.  As with the first step, we need the timing cards to measure exactly the same instant.  To do this, we use BIT mode data, where R1 in both bank receives the same electronically generated pulse at the same instant and are thus measuring the same event.  Averaging these numbers gives the timing offsets between the R1 cards, which can be combined with the first measurements to establish timing between all cards.
+Procedure:
+''PROCEDURE:''
 Can do range correction if the boresight results are good. This is a 2 stage process:
 …
 '''Note that Attune apparently crashes if you use >400MB files, and crashes plenty anyway - save often!  Do an initial run with just a few tie points to make sure the lines will work.'''
+PROCEDURE:
+''PROCEDURE:''
  * In ALSPP uncheck the “average last returns” option in the settings dialog.
  * Check the output as attune files in the outputs dialog
 …
    * Try to include some on slopes(?)
+=== Final validation and fine tuning ===
+ * check the torsion and pitch error slope
+ * To check the Torsion look at cross sections of high altitude cross flight. Flat roads and grassy areas are good. Look for “smiles” in the data. This can be done by looking at the centres of the edges of the overlapping area (Figure 3).
+ * We could create a surface model using one of the opposing flight lines, and then difference the other flight line against this to show if there are any roll effects [or maybe torsion etc]. Do this as well as the cross sections.
+When finished save all the results to a reg file in ALSPP
 === Ground classification ===
 …
 Run as a macro.  Terra->tools->macro->new.
+''PROCEDURE:''
+ * This is an iterative procedure to take points from default classification to ground classification.
+ * Classify from default classification to ground.
+ * Max terrain slope – if lots of man made structures then use 88-90 degrees, else estimate from the natural terrain and add 10-15 degrees.
+ * To classify, both the vertical distance and angle between points are tested against.
+ * Its better to classify too few points to ground than too many.
+ * Its good to preclassify more difficult areas first. [i.e. classify large buildings and steep hill tops].
+ * In TerraScan load in a LAS file.
+ * Classify->Routine->ground
+   * use measurement tool to get max building size in area. Also for terrain slope can view elevation and use cross section tool to measure/estimate it.
+   * Initial points = aerial low and ground
+   * iteration angle = 6 degree to start with
+   * iteration distance = 1m to start with
+ * Before ground classification we should remove noisy points.
+   * Default -> low points classification.
+   * Do this for group of points and then single points.
+   *  We can set up a macro to do this.
+ * Set up the Macro
+   * Tools -> Macro
+   * any -> default
+   * default -> low (group)
+   * default -> low (single)
+   * isolated points -> any -> low
+   * To run on large projects use “selected files” instead of reading all into memory.
+ * Run the macro and check result.  Update distances etc used in the classifications if needed.
+ * Then add ground classification.
+   * Default -> ground.
+ * The Add point to ground tool can be used to add points to the classification that have been missed.
+''Aside:''
+ * Can get Model Key Points
+   * Routine -> Model key points
+   * from ground -> model key points
+   * These are the points which can be used as a model (?)
 --------------
 = Random snippets =
+SCN file, DC=delta counter (time in ms since last GPS-second tick), ANG=angle in ticks, RI = return? (in m?).
+SCN file, DC=delta counter (time in ms since last GPS-second tick), ANG=angle in ticks.
+R1-R4 are the ranges in metres of the returns. R4 is always the last return. There is no intensity values for R4
+If there are less than 4 returns then R4 is an INDEPENDENT measurement of the last pulse received
+ * This allows the “average last return” option to be used to give a better measurement (in the ALSPP)
+minimal detectable distance between R1-R2, R2-R3, R3-R4 is 2.7m.
+--------------
+= TerraScan stuff =
+''This should probably be moved to a separate TerraScan page when we get chance''
+ * It is possible to synchronise more than one viewer in TerraScan (as in Envi/ERDAS) so can look at intensity and elevation at the same point.
+ * We can use TerraScan for creating projects, showing trajectories, splitting and combining LAS files.
+ * Create new project – “first button” -> “offset stacked rectangle button”
+ * file -> new
+   * select LAS storage
+   * scanner airborne
+   * give description.
+ * Place block – allows to draw rectangles to split the data up into sections.
+   * Block -> add by boundary
+     * will allow to create new LAS files from existing ones.
+   * File -> import points into project
+     * This does the “splitting” of the data.
+ * Can then do things as normal to these project points.
+ * Can also recombine all the points to a single LAS file if required.
+ * May want trajectory loaded up in TerraScan
+   * Setup ALSPP as if to process data.
+   * ALSPP -> utilities -> generate trj files.
+   * Import into TerraScan and draw trajectory [use the “3 child squares” -> “parallel lines button”to import]
+   * The trajectory is important for removing overlaps of data or trimming data and other things.
+   * When cutting overlapping points we can use the “by quality” option – low flights take precedence but can also add a quality factor to points which will be used to weight. Alternatively can use the “by offset” option – defines an angle of flightline to cut by.