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Vertical Accuracy Assessment Using Ground Points

By NGP Standards and Specifications

This revision modifies the method for assessing absolute vertical accuracy for both NVA and VVA to require the use of ground-classified points. The requirement that unclassified point data shall meet required VVA before further classification and processing has been removed. 

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Status of this revision: Provisional. Published in Lidar Base Specification 2020 rev. A

Requirement in LBS v.2.1:
Absolute Vertical Accuracy
  • Absolute vertical accuracy of the lidar data and the derived DEM will be assessed and reported in accordance with ASPRS (2014).
  • Vegetated and nonvegetated land cover types shall be assessed for absolute vertical accuracy.
  • Federal Emergency Management Agency (2003) identifies seven land cover types; National Digital Elevation Program (2004) and ASPRS (2004) reiterate the first five of those types. The way in which each of the seven classes was reported under the previous standards and how they are reported under the new ASPRS standards and by this specification are shown in table 3.
  • Three absolute accuracy values shall be assessed and reported:
    1. NVA for the point data
    2. NVA for the DEM
    3. VVA for the DEM
  • The minimum NVA and VVA requirements for all data, using the ASPRS methodology, are listed in table 4. Both the NVA and VVA required values shall be met.
  • The unclassified point data shall meet the required NVA before further classification and processing.
  • NVA for the point data is assessed by comparing check points surveyed in clear, open, nonvegetated areas (which typically produce only single lidar returns) to a triangulated irregular network (TIN) constructed from the single return lidar points in those areas.
  • NVA and VVA for the DEM are assessed by comparing check points to the final bare-earth surface.
  • The minimum required thresholds for absolute and relative accuracy may be increased by the USGS–NGP when any of the following conditions are met:
    • A demonstrable, substantial, and prohibitive increase in cost is needed to obtain this accuracy, which is often the case in heavily vegetated project areas.
    • An alternate specification is needed to conform to previously contracted phases of a single larger overall collection effort such as for multiyear statewide collections.
    • The USGS–NGP agrees that the use of an alternate specification is reasonable and in the best interest of all stakeholders.
Approved change: (items that have changes are bolded)
Absolute Vertical Accuracy
  • Absolute vertical accuracy of the lidar data and the derived DEM will be assessed and reported in accordance with ASPRS (2014).
  • Vegetated and nonvegetated land cover types shall be assessed for absolute vertical accuracy.
  • Federal Emergency Management Agency (2003) identifies seven land cover types; National Digital Elevation Program (2004) and ASPRS (2004) reiterate the first five of those types. The way in which each of the seven classes was reported under the previous standards and how they are reported under the new ASPRS standards and by this specification are shown in table 3.
  • Four absolute accuracy values shall be assessed and reported:
    1. NVA for the point data
    2. VVA for the point data
    3. NVA for the DEM
    4. VVA for the DEM
  • The minimum NVA and VVA requirements for all data, using the ASPRS methodology, are listed in table 4. Both the NVA and VVA required values shall be met.
  • NVA for the point data shall be assessed by comparing check points surveyed for NVA assessment (see Check Points) to a triangulated irregular network (TIN) constructed from ground-classified lidar points in those areas.
  • VVA for the point data shall be assessed by comparing check points surveyed for VVA assessment (see Check Points) to a triangulated irregular network (TIN) constructed from ground-classified lidar points in those areas.
  • NVA and VVA for the DEM are assessed by comparing check points to the final bare-earth surface.
  • The minimum required thresholds for absolute and relative accuracy may be increased by the USGS–NGP when any of the following conditions are met:
    • A demonstrable, substantial, and prohibitive increase in cost is needed to obtain this accuracy, which is often the case in heavily vegetated project areas.
    • An alternate specification is needed to conform to previously contracted phases of a single larger overall collection effort such as for multiyear statewide collections.
    • The USGS–NGP agrees that the use of an alternate specification is reasonable and in the best interest of all stakeholders.