Monday, May 21, 2018

Perform a 2D geo-registration of a LiDAR LAS file to a GeoTiff image with Cloud Compare

Recently I received LiDAR LAS point clouds generated without using accurate GPS positioning. As a result, the data is not placed at the correct geographic location in the world. To rectify this, I used Cloud Compare to so-called geo-register the point clouds to the world using GeoTiff images as reference.

The following illustrates how it was done:

Load the GeoTiff image as reference
  1. Start Cloud Compare.


  2. Select File | Open.

    The Open file(s) dialog box appears.

  3. Browse and select the reference GeoTiff raster file, e.g. autzen.tif. Click Open.

    The Result type prompt appears.

  4. Click No to load as a texture map instead of converting to a point cloud.

    The Global shift/scale dialog box appears.

  5. Accept the defaults. Click Yes.

    The GeoTiff image is loaded.
Load the LAS file
  1. Select File | Open.


  2. Browse and choose the LAS file, e.g. autzen.las. Click Open.

    The Open LAS File dialog box appears.
  3. Click Apply.

    The Global shift/scale dialog box appears.

  4. Click Yes to accept the defaults.

    The LAS file is loaded.
 Setup the display
  1. In the vertical tool bar on the left, click the Set Current View Mode icon
    and choose Orthographic projection.

    Note: we do not want the perspective projection display to mislead us as to the location of the features
  2. Optional. Select Edit | Colors | Height Ramp. Click OK in the Gradient color dialog box to display the point cloud with a height color gradient for better visualization.


Perform the rectification
  1.  In the DB Tree pane, select the point cloud to rectify, e.g. autzen.las.
  2. In the top tool bar, click the Translate/Rotate icon .

    The Translate/Rotate widget appears in the top right corner.
  3. In the Rotation field, select Z. Toggle off Tz.

    Note: we only want to move horizontally in the 2D plane.
  4. Look for matching features in the point cloud and the GeoTiff image.



  5. To translate the point cloud, press down the mouse right button and drag the point cloud to the new location. To rotate the point cloud, press down the mouse left button and rotate the point cloud.

    Note 1: To toggle the display of the point cloud, toggle on/off the Visible property in the Properties pane on the left.
    Note 2: To zoom in/out/pane around, pause the Translate/Rotate widget by clicking the widget's Pause icon.
  6. In the Translate/Rotate widget, click the Tick icon to save the changes.

    The point cloud is rectified.


2 comments:

Daniel Girardeau-Montaut said...

Thanks for the information. Actually it would be a little bit more accurate to use the 'Align' tool (http://www.cloudcompare.org/doc/wiki/index.php?title=Align)

dominoc925 said...

@Daniel, it is very inefficient with the Align tool. Every time I zoom in to locate an object of interest, toggling on/off the reference/to align will inevitably zoom the view to display the entire extent of the point clouds. Then I will have to try and locate the object of interest again. Since the Align tool works only with a single View, toggling on/off is a necessity to see which objects to click.

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