Interswath Consistency check via SSIs

Within Deliverables/Swath Separation Images, suggest the specification be updated as follows: 

Image creation

• In systems that produce multiple returns, only last returns shall be used to create the images.  

• All point classes and flags shall be enabled when creating the images and points flagged as withheld or classified as noise shall be excluded.    

• Elevation values and differences shall not be subjected to a threshold or otherwise clipped so all differences are represented.   

• The images should be derived from TINs to reduce the number of false difference values on slopes; however, other algorithms are acceptable.   

• The images shall consist of a transparent RGB layer overlaying the lidar intensity. USGS recommends a 75% transparency; however, other thresholds will be acceptable. Regardless of the transparency percentage utilized, the images shall utilize sufficient transparency within the RGB layer to allow for visual interpretation of the underlying terrain and landcover as evident in the intensity values.  

• The images shall use at least three-color levels wherever two or more swaths overlap within a pixel.
• 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.
  • Where swaths do not overlap, pixel values shall be intensity alone.
• Image file formats and version control:
  • Swath separation image format shall be delivered as GeoTIFF by tile.
  • The point cloud geometry and intensity data delivered shall be identical to the point cloud geometry and intensity data used to create the swath separation images. Changes in the point cloud geometry, intensity, and/or application of the withheld flag requires recreation and redelivery of the swath separation images.
• Spatial extent and coordinate reference system: NO CHANGES

Within the Collection Requirements, suggest the following new section be added:

Overlap 

• Excluding points flagged as withheld, each individual swath shall have no less than 75 meters of overlap with each surrounding (adjacent, perpendicular, and/or intersecting) swaths within the project. 

Within Positional Accuracy Validation / Interswath (Overlap) Consistency, suggest that all subpoints be removed and the specification be updated as follows: 

Interswath (Overlap) Consistency: 

• Interswath (overlap) consistency shall be assessed across the entirety of the project at the work unit level.
• Interswath (overlap) consistency shall be assessed through the use of swath separation images.
• Due to potential temporal differences, hydrologic features and snow will be excluded from the interswath assessment but should still be included in the swath separation images.
• Areas of overlap within the swath separation images shall be evaluated locally and wholistically. Densely vegetated land cover types, as well as abrupt changes in elevation may require a more wholistic assessment of the swath overlap area versus a more localized assessment.
  • Examples of areas that shall undergo localized assessment include, but are not limited to, the following: improved and unimproved roadways, bare ground open areas with less than 10 degrees of slope, and roofs with less than 10 degrees of slope.
  • Holistic assessments will be conducted across areas of swath overlap that span multiple adjoining terrain types.
• Any portion/percentage area of overlap not included in the temporal polygons, snow polygons, or impacted by hydrology, may be held to the following pass/fail thresholds and will be cause for rejection of the entire point cloud deliverable:
  • For localized assessments:
    • Δz ≤ 8 cm (green) in areas of swath overlap shall be considered acceptable.  
    • Contiguous and prevalent pixels with Δz > 8 cm (yellow, red) in areas of swath overlap shall be considered unacceptable (subject to the underlying terrain)
  • For wholistic assessments:
    • Δz ≤ 8 cm (green) in areas of swath overlap shall be considered acceptable. 
    • If the Δz is greater than 16 cm, is correlated with the collection flightlines, contiguous and prvalent, and covers multiple types of terrain, shall be considered unacceptable.
    • Note: If the Δz is greater than 8 cm and less than or equal to 16 cm, is correlated with the collection flightlines, and covers multiple types of terrain, it may indicate a problem with interswath (overlap) consistency that requires further review, depending on the underlying terrain.