NGP Standards and Specifications

Swath Separation Images

This is a new requirement for delivery of Swath Separation Images. Swath separation images use color-coding to illustrate differences in elevation (z-) values where swaths overlap. They are ancillary metadata used as visual aids to more easily identify regions within point cloud datasets that may have suspect interswath alignment or other geometric issues.

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Status of this revision: Approved by the ESRB

Current requirement in LBS v.2.1: None, this is a new requirement.

Background: 

Swath separation images are a useful tool to visualize interswath alignment and positional quality of the lidar point cloud. These are used as qualitative checks. 

ESRB approved addition:

Description: Swath separation images use color-coding to illustrate differences in elevation (z-) values where swaths overlap. The color-coded images are semi-transparent and overlay the lidar intensity image. They are ancillary metadata used as visual aids to more easily identify regions within point cloud datasets that may have suspect interswath alignment or other geometric issues. These images may be produced by a variety of methods; however, their usefulness will be ensured by following the specification requirements and documenting the processes used to create the images in the lidar acquisition and processing report.  

Swath Separation Images
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Image creation:
  a.  All returns shall be used to create the images.
  b. All point classes and flags shall be enabled when creating the images and points flagged as withheld or classified as noise shall be excluded.  
  c. Elevation values and differences shall not be subjected to a threshold or otherwise clipped so all differences are represented. 
  d. The images will be derived from TINs to reduce the number of false difference values on slopes; however, other algorithms are acceptable. 
  e. The images shall consist of a 50 percent transparent RGB layer overlaying the lidar intensity image.  
  f. The images shall use at least three color levels wherever two or more swaths overlap within a pixel. 
  g.  Where two or more swaths overlap within a pixel (based on point source ID),  
    i. 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):  
      1. 0-8 cm: GREEN;  
      2. 8-16 cm: YELLOW;  
      3. > 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). 
    ii. color choice of green, yellow, and red is suggested but not required.
    iii. no pixel shall remain uncolored (transparent) in the overlap areas.
  h. Where swaths do not overlap, pixel values shall be intensity alone.
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Image file formats and version control: 
  a. Swath difference image format may be delivered as GeoTIFF or JPEG (with world file) by tile or as a single compressed JPEG 2000 (JP2) image mosaic. 
  b. The point cloud geometry and intensity data delivered shall be identical to the point cloud geometry and intensity data used to create the difference images. Changes in the point cloud geometry or intensity requires recreation of the difference images.  
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Spatial extent and coordinate reference system: 
  a. 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).
  b. The difference images must be representative of the associated data delivery.
  c. The images shall be in the same CRS as the point cloud data to ensure alignment with the point cloud.

Example image:

Swath Separation Image

Example of a Swath Separation Image. 

(Public domain.)