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LAS withheld bit flag proof of performance

By NGP Standards and Specifications

Proper use of the withheld bit flag is of significant importance in point cloud processing and the creation of derivative products such as a digital elevation model. This change requires proof of performance that the withheld bit flag was used properly. This change will be incorporated with "Restrict use of the LAS Withheld bit flag" revision and also includes the Maximum Surface Height

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

Current requirement (including pending change in bold):

  • Use of the LAS Withheld Bit Flag

    • The withheld bit flag, as defined in LAS specification version 1.4–R15 (ASPRS, 2011), shall only be used to identify points that cannot be reasonably interpreted as valid surface returns. Examples include outliers, blunders, geometrically unreliable points, aerosol back-scatter, laser multi-path, airborne objects, and sensor anomalies.

    • The withheld flag may be used in conjunction with other classification codes (low/high noise for example), but it should be used in all cases where the previously mentioned criteria are met. 

Change: 

The following will added to the requirement:

  • The usage of the LAS Withheld Bit Flag is of such importance that proof of performance is required. This proof shall be provided as 

    • Preferred: Maximum Surface Height Rasters as detailed in the appendix (link).  

    • Other test or metadata as agreed to by the USGS in advance and documented in the project Task Order.   

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