Lidar Base Specification 2022 rev. A
Lidar collection parameters are dependent on the environment of the project area and numerous additional factors. Although these variations must be accepted, this section defines a number of collection requirements that must be met to achieve the consistent national lidar collection at the heart of 3DEP.
- The Defined Project Area (DPA) shall be the Area of Interest (AOI) plus a 100-meter buffer.
- Data collection is required for the full extent of the DPA.
- All products shall be produced to 3DEP and Task Order requirements up the edge of the DPA.
- All data and products shall be delivered to the customer for the full extent of the DPA
- All products, including checkpoints, shall be located within or otherwise clipped to DPA extents.
- The minimum acceptable Quality Level (QL) for 3DEP collections is QL2 as defined in this specification. See tables 1-6 for detailed QL requirements.
- Deriving and delivering multiple discrete returns are required in all conventional lidar data collection efforts.
- Data collection shall be capable of at least three returns per pulse.
- Intensity values are required for each multiple discrete return.
- The intensity values recorded in the LAS files shall be normalized to 16 bit, as required by the LAS specification version 1.4–R15 (ASPRS, 2011).
- Intensity normalization shall be strictly linear.
- Common image stretches (minimum-maximum, standard deviations, percent clip, histogram, and so forth) are expressly forbidden.
For further explanation of ANPD and ANPS, see Lidar Base Specification v. 1.3.
- The required ANPS and ANPD by QL are listed in table 1.
- Aggregate Nominal Pulse Density (ANPD) and Aggregate Nominal Pulse Spacing (ANPS) shall meet the requirements of the Quality Level (QL) of the project with a minimum of QL2 for 3DEP collections.
- Aggregate Nominal Pulse Density (ANPD) shall be no less than 2 points per square meter (QL2); assessment to be made against single swath, first return data located within the geometrically usable center portion (typically ~95%) of each swath.
- Aggregate Nominal Pulse Spacing (ANPS) shall be no greater than 0.70 meters (QL2); assessment to be made against single swath, first return data located within the geometrically usable center portion (typically ~95%) of each swath.
- Dependent on the local terrain and land cover conditions in a project, a greater pulse density may be required on specific projects.
- A data void is considered to be any area greater than or equal to (4 × ANPS)2, which is measured using first returns only.
- Data voids within a single swath are not acceptable, except in the following circumstances:
- where caused by waterbodies;
- where caused by areas of low near infrared reflectivity, such as asphalt or composition roofing;
- where caused by lidar shadowing from buildings or other features; or
- where appropriately filled in by another swath.
- For projects designed to achieve the required ANPS through multiple coverage, the entire DPA shall be covered with the designed number of swaths. Areas meeting the size threshold defined above for single coverage that are not covered by the designed number of swaths are data voids (see figures 1-3).
The process described in this section relates only to regular and uniform point distribution. The process does not relate to, nor can it be used for, the assessment of NPS, ANPS, or data voids.
- The spatial distribution of geometrically usable points will be uniform and regular.
- Collections will be planned and executed to produce an aggregate first return point data that approaches a uniform, regular lattice of points.
- The regularity of the point pattern and density throughout the dataset is important and will be assessed by using the following method:
- Assess only nonwithheld, first return points of a single File Source ID.
- Exclude acceptable data voids previously identified in this specification.
- Generate a density raster from the data with a cell size equal to twice the design ANPS.
- Populate the raster using a count of points within each cell.
- Ensure that at least 90 percent of the cells in the grid contain at least one lidar point.
- The USGS–NGP may allow lower passing thresholds for this requirement in areas of substantial relief where maintaining a regular and uniform point distribution is impractical.
- Atmospheric conditions shall be cloud and fog free between the aircraft and ground during all collection operations.
- Ground conditions will be snow free. Very light, undrifted snow may be acceptable with prior approval.
- Ground conditions shall be free of extensive flooding or any other type of inundation.
- Leaf-off vegetation conditions are preferred.
- Penetration to the ground shall be adequate to produce an accurate and reliable bare-earth surface for the prescribed QL.
- Collections planned for leaf-on collections shall be approved by the USGS–NGP/3DEP prior to issuance of a task order or contract.