NGP Standards and Specifications

Lidar Base Specification v. 2.1: Revision History

As lidar technology, techniques, applications and products continue to evolve rapidly, periodic updates to the LBS are also required to maintain its relevance and utility to the USGS and the public. A summary of the most notable changes in each revision follows.

Return to LBS v. 2.1 Table of Contents

Effective dates for each version are given in parentheses. During the transition from print publication to online publication, versions 1.3 and 2.0 were in effect concurrently. This ended with the publication of Version 2.1. 

Revisions for Version 2.1 (OCT 2019 - present)

Revisions for Version 2.0 (JUL 2018 [Spec-X version]/JUL 2019 [online version] - SEP 2019)

Revisions for Version 1.3 (FEB 2018 - SEP 2019)

Revisions for Version 1.2 (OCT 2014 - JAN 2018)

Revisions for Version 1.1 (AUG 2012 - SEP 2014)

Revisions for Version 2.1
1 Eliminated Buffered Project Area (BPA) language. 
2 Updated point classification requirements. The section on Point Classification is replaced with:
  • The minimum, required classification scheme for lidar data is found in table 5.
  • All points that fall within the minimum classification scheme (table 5) and not flagged as withheld shall be properly classified.
  • Additional classes may be used on specific projects.
  • Accuracy of point classification into classes beyond the minimum scheme (table 5) will not be assessed by the USGS, as documented in metadata.
  • Assessing and verifying accuracy of point classification into classes beyond the minimum scheme will be the responsibility of the partner requesting the additional classes.
  • No points in the classified LAS deliverable may remain assigned to Class 0.
  • Points classified as water will only be checked when associated with a breakline.
  • If it is necessary to identify overage points in overlap areas, the overage points shall be identified using the overlap bit flag as defined in LAS Specification Version 1.4-R13 (ASPRS, 2011).
  • No classification code may be used to identify points as overage points.
  • Model key points, if calculated, shall be identified using the key point bit flag as defined in LAS specification version 1.4–R13 (ASPRS, 2011). Model key points may, in addition, be identified using class 8 at the discretion of the data producer. 
3 Changed DEM delivery format from preferring ERDAS Imagine (.img) to requiring GeoTIFF with the following requirements:
  • 32-bit floating-point GeoTIFF raster format.
  • The NODATA value of '-999999' shall be defined in GDAL_NODATA tag #42113
  • GDAL version 2.4.0, or as otherwise agreed to in advance and specified in the Task Order, shall be used to populate GeoTIFF keys and tags.
  • Additional requirements for GeoTIFF tiling, compression, and internal overviews may be referenced in Task Orders.
4 Added a requirement for the use of ‘-999999’ as a single, consistent NODATA value:
  • Void areas coded using a NODATA value of ‘-999999’ and shall be defined in GDAL_NODATA tag #42113.
Revisions for Version 2.0 
1 Changed review and approval process. The National Geospatial Program's Elevation Specification Review Board (ESRB) and the 3D Elevation Program Working Group (3DEP WG) must approve all changes. 
2 Changed the delivery method--the Lidar Base Specification will be published online through the Specifications Explorer (Spec-X) database and web application as well as on the 3DEP Standards and Specifications website.
3 Added requirements for bridge treatment:
  • All instructions and requirements regarding the use of breaklines also applies to non-hydrographic terrain generation below bridges.
  • Any breaklines used to enforce a logical terrain surface below a bridge shall be considered a required deliverable.
  • The bare-earth surface below the bridge shall be a continuous, logical interpolation of the apparent terrain lateral to the bridge deck.
  • Where abutments are clearly visible, the bare-earth interpolation shall begin at the junction of the bridge deck and approach structure. Where this junction is not clear, the contractor shall use their best judgement to delineate the separation of below-bridge terrain from elevated bridge surface.
  • Streams, rivers, and water bodies meeting the criteria for hydro-flattening shall be monotonically continuous where bridge decks have been removed.
  • Bridges, as defined in the glossary, shall be removed from the bare-earth surface.
Revisions for Version 1.3
1 The requirement for delivery of raw, unclassified swath data has been removed.
2 The requirement for XML metadata files for the overall project and for individual lifts has been removed.
3 A requirement to use Geoid12b to convert from ellipsoid heights to orthometric heights has superseded Geoid12a.
4 A requirement that specific coordinate reference system (CRS) information for all projects be agreed upon prior to collection has been added.
5 A requirement for vertical CRS information has been added.
6 A requirement to include the geoid model as part of the vertical CRS name has been added.
7 A requirement to represent horizontal and vertical CRS information as a compound CRS has been added.
8 A requirement that delivered raster elevation files must contain complete and correct georeference information for horizontal and vertical systems, including geoid model used, has been added.
9 A requirement for horizontal accuracy reporting has been added.
10 A requirement for delivery of ancillary products used to support processing of the lidar dataset has been added.
11 A requirement for an attributed polygon feature class representing individual swath boundaries has been added.
12 A clarification on the well-known text (WKT) represen- tation of CRS has been added.
13 A clarification on intensity normalization has been added.
14 A clarification on handling of multiple CRS records in LAS files has been added.
15 A clarification on file source identifier (ID) for tiled LAS files has been added.
16 A clarification of the difference between overlap and overage has been added.
17 A clarification on the identification of overage (overlap) points has been added.
18 A clarification on requirements for the use of overlap and withheld point flags has been added.
19 A clarification on how model key points shall be identi- fied using the LAS key point bit flag has been added.
20 The recommended process for assessing intraswath rela- tive accuracy (repeatability, precision) has been refined to normalize for the natural slope.
21 The recommended process for assessing interswath rela- tive accuracy has been limited to areas with less than (<) 10-degree slope.
22 The maximum limits for interswath differences have been removed.
23 A prohibition on duplication of points within a project has been added.
24 The classification code for “Ignored Ground” (typically used for breakline proximity) has been changed from 10 to 20 to correct the conflict with the ASPRS defined code for “Rail.”
25 A classification code for “Snow” (21) has been added.
26 A classification code for “Temporal Exclusion” (22) has been added.
27 Definitions of swath types have been added to the “Glossary” section.
28 Guidelines for breakline collection, compliant with a newly added EleHydro data dictionary have been added.
29 All references to the National Elevation Dataset (NED) have been changed to “the standard national DEM available through The National Map.” The names “National Elevation Dataset” and “NED” are no longer used for data collected and processed for The National Map or 3DEP.
Revisions for Version 1.2
1 For clarification, the publication was modified to omit versioning from the main title. No changes were made to the content of the specification.
Revisions for Version 1.1
1 For clarification, numerous sections of the specification were editorially revised and there was minor reorganization of the document.
2 Concurrently with USGS development of the LBS version 1.1, the American Society of Photogrammetry and Remote Sensing (ASPRS) developed the “Positional Accuracy Standards for Digital Geospatial Data” (American Society of Photogrammetry and Remote Sensing [ASPRS], 2014). Glossary definitions in the LBS were updated to align with those in ASPRS (2014) and other industry publications, and several new definitions were added. Notable among these are the following:
  • aggregate nominal pulse density (and spacing)
  • bridge and culvert
  • vegetated vertical accuracy (VVA) and nonvegetated vertical accuracy (NVA)
  • percentile
3 With regard to elevation data, the new standards redefine how elevation accuracy is described and reported, and although any accuracy could be its own accuracy class, a number of specific common classes are explicitly defined. These new ASPRS standard classes are slightly different from those defined by the previous ASPRS standards. Earlier accuracy classes were the basis for the NEEA QL definitions; therefore, the QL accuracy definitions were adjusted to match the new ASPRS classes and to eliminate confusion about accuracy requirements as 3DEP moves forward. Another QL, QL0, was added as a placeholder for the higher-quality data anticipated with future advances in lidar technology. The requirements stated for QL0 are somewhat arbitrary and are subject to change in future revisions of this specification. The changes relevant to lidar data QLs in this revision of the specification were as follows:
  • QL0 was added with accuracy of 5.0-centimeter (cm) vertical linear root mean square error in the z direction (RMSEz) and density of at least 8 pulses per square meter (pls/m2). This aligns with the ASPRS 5-cm vertical accuracy class.
  • QL1 accuracy was changed from 9.25-cm RMSEz to 10.0-cm RMSEz. This does not correspond directly to any ASPRS accuracy class; it is a hybrid of QL2 accuracy and QL0 pulse density.
  • QL2 accuracy was changed from 9.25-cm RMSEz to 10.0-cm RMSEz. This aligns with the ASPRS 10-cm vertical accuracy class. QL2 pulse density remains unchanged at 2 pls/m2.
  • QL3 accuracy was changed from 18.5-cm RMSEz to 20.0-RMSEz and density was changed from 0.7 pls/m2 to 0.5 pls/m2. This aligns with the ASPRS 20-cm vertical accuracy class.
4 In addition, to align with the new ASPRS accuracy standards, accuracy reporting requirements were defined as based on NVA and VVA. These two classes replaced the previously used fundamental, supplemental, and consolidated vertical accuracy (FVA, SVA, and CVA, respectively) classes.
5 The new ASPRS standards include recommendations tying the quantity of vertical accuracy check points required for a project to the areal extent of the project. This revision of the specification required adherence to these recommendations.
6 QL2 was established as the minimum required QL for new USGS–NGP lidar data collections.
7 Relative accuracy requirements for lidar data, within swath (intraswath) and between overlapping swaths (interswath), were refined and established for each QL. A more detailed methodology for assessing and reporting these metrics was provided.
8 Lidar data delivery is now required in LAS specification version 1.4–R13 (ASPRS, 2011), point data record format (PDRF) 6, 7, 8, 9, or 10. Proper use of the overlap and withheld bit flags is required.
9 The block of lidar specific metadata tags recommended in the previous version of this specification was modified to reflect the other updates to the specification. The inclusion of this block is now required in all lidar data eXtensible Markup Language (XML) metadata files.
10 The 2-gigabyte (GB) limit on swath file size was removed, although the method for splitting large swath files remains in the specification for use in situations where a data producer needs to produce smaller files.
11 The test area for assessing classification accuracy was corrected from 1 kilometer square to 1 square kilometer.
12 Two additional point classification type requirements were defined:
  • Class 17, Bridges
  • Class 18, High Noise
13 Anticipating that projects will more frequently use multiple coverage collection (for example, overlap greater than 50 percent) to achieve the higher pulse density required, terminology and requirements for this data organization were added.
14 Requirements for datum and coordinate reference systems were refined and clarified.
15 Development and delivery of breaklines were required for all hydro-flattened waterbodies, regardless of the methodology used by the data producer for hydro-flattening.
16 Requirements and guidelines for flightline overlap and scan angle limits were removed. Data producers were cautioned that more rigorous attention will be paid to gaps in and the relative accuracy of the point data.