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Swath Separation Images Update

The requirements for Swath Separation Images (SSI) are being updated to allow only last returns, instead of all returns, in image creation. The resolution requirements are also being relaxed in order to provide more flexibility in SSI creation. 

Status of this revision: Published in Lidar Base Specification 2021 rev. A

Please Note: This is a clarification to allow another option for image creation, no other part of the SSI requirement is changing. 

Relevant requirement in Lidar Base Specification 2020 rev. A Deliverables:
First change: 
  • Image creation:

    1. All returns shall be used to create the images.

Second change: 
  • Spatial extent and coordinate reference system: 

    • Spatial resolution (pixel dimension) of the images shall be between 2 and 4 times the Nominal Pulse Spacing (2-4 x NPS) in the project’s linear unit (meters or feet).

Updated requirement (bolded text is addition):
First change:
  • Image creation:

    1. All returns, single returns, or last returns shall be used to create the images.

Second change:
  • Spatial extent and coordinate reference system: 

    • Spatial resolution (pixel dimension) of the images shall be no greater than 4 times the Nominal Pulse Spacing (2-4 x NPS) in the project’s linear unit (meters or feet).

Additional clarification

An additional clarification will be added to the Swath Separation Images coloration scheme. The color breaks are dependent on the swath overlap difference, RMSDz, for the QL (table 2). A QL0 dataset requires different breaks than QL1 or QL2 datasets. More clearly defining the breaks will avoid any confusion. 

Addition is bolded:

7. Where two or more swaths overlap within a pixel (based on point source ID),  

  • pixel color shall be based on vertical difference of swaths using the following breaks (based on multiples of the Swath Overlap Difference for the QL, table 2). 

    • For QL1 or QL2 data the breaks are: 

      • 0-8 cm: GREEN;  

      • 8-16 cm: YELLOW;  

      • > 16 cm or > last additional color ramp bin value: RED (for example, addition of ORANGE pixels for the range of 16-24 cm would require red pixels to represent > 24 cm). 

    • For QL0 data the breaks are:

      • 0-4 cm: GREEN;  

      • 4-8 cm: YELLOW;  

      • > 8 cm or > last additional color ramp bin value: RED (for example, addition of ORANGE pixels for the range of 8-12 cm would require red pixels to represent > 12 cm). 

  • color choice of GREEN, YELLOW, and RED is suggested but not required.

  • no pixel shall remain uncolored (transparent) in the overlap areas.