DRAFT Lidar Base Specification: Deliverables DRAFT
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Bare-Earth Surface (Raster Digital Elevation Model)
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- Product metadata files shall comply with the Federal Geographic Data Committee (FGDC) "Content Standard for Digital Geospatial Metadata" (CSDGM) (FGDC, 1998).
- Metadata deliverables shall include the following:
- A survey report detailing the collection of all ground survey data including the following:
- Control points used to calibrate and process the lidar and derivative data.
- Check points used to validate the lidar point data or any derivative product.
- A collection report detailing mission planning and including detailed flight logs. Flight logs are expected to include:
- A unique ID for each lift.
- The take-off and landing times for each lift.
- The aircraft make, model, and tail number.
- The instrument manufacturer, model, and serial number.
- The date of the instrument’s most recent factory inspection/calibration.
- General weather conditions.
- General observed ground conditions.
- All inflight disturbances and notable head/tail/crosswinds.
- All inflight instrument anomalies and any inflight changes in settings.
- A processing report detailing:
- Calibration and instrument settings by lift and identified by the lift ID.
- Classification methods.
- Product generation procedures including methodology used for breakline collection and hydro-flattening (see the “Hydro-Flattening” section and appendix 2 for more information on hydro-flattening).
- A QA/QC report, detailing procedures for analysis, accuracy assessment, and validation of the project data, including the following [NOTE: The following four reports may be compiled as separate documents, or combined into a single document, at the discretion of the data producer]:
- The expected horizontal accuracy of the lidar data, as described in ASPRS (2014).
- The assessed relative vertical accuracy of the point data (smooth surface repeatability and overlap consistency). Relative vertical accuracy requirements are listed in table 2.
- The assessed NVA of the unclassified lidar data in accordance with the guidelines set forth in ASPRS (2014). Absolute vertical accuracy requirements for the unclassified point data using the ASPRS methodology are listed in table 4.
- The assessed NVA and VVA of the bare-earth surface in accordance with the guidelines set forth in ASPRS (2014). Absolute vertical accuracy requirements using the ASPRS methodology for the bare-earth DEM are listed in table 4.
- QA/QC analysis materials for the absolute vertical accuracy assessment.
- A georeferenced, polygonal representation of the detailed extents of each lidar swath collected, as a GIS layer. The goal is a set of polygons that define the area actually covered by the swaths, not merely the points collected in the swaths.
- The extents shall be those of the actual coverage of the collected swath, exclusive of peripheral TIN artifacts:
- Minimum bounding rectangles or simplified rectangles are not acceptable.
- The boundary will generally follow the overall shape of the swath.
- Each swath polygon shall be attributed with the following:
- The lift’s unique ID (string format).
- The unique Point Source ID of the swath (string format).
- The type of swath (string format):
- "Project"
- "Cross-tie”
- “Fill-in"
- “Calibration” or
- “Other”
- Start time in adjusted GPS seconds to the nearest integer.
- End time in adjusted GPS seconds to the nearest integer.
- Esri polygon shapefile or geodatabase or geopackage is required. Geopackage is preferred.
- The extents shall be those of the actual coverage of the collected swath, exclusive of peripheral TIN artifacts:
- A georeferenced, digital spatial representation of the detailed extents of each delivered dataset.
- The extents shall be those of the actual lidar source or derived product data, exclusive of peripheral TIN artifacts or raster NODATA areas.
- A union of tile boundaries or minimum bounding rectangles is not acceptable.
- For the point data, no line segment in the boundary will be longer than the four times the ANPS from the nearest lidar point.
- Esri polygon shapefile or geodatabase is required.
- Product metadata (FGDC compliant, XML format metadata).
- One XML file is required for each of the following deliverable product groups:
- Classified point data.
- Bare-earth DEMs.
- Breaklines.
- Any other datasets delivered (digital surface models [DSM], intensity images, height above ground surfaces, and others).
- Metadata files for individual data files within a deliverable product group are acceptable but are not required.
- FGDC-compliant metadata shall pass the USGS Metadata Parser (MP) without errors.
- One XML file is required for each of the following deliverable product groups:
- A survey report detailing the collection of all ground survey data including the following:
- A block of lidar-related metadata tags specified by the USGS shall be included in the CSDGM (FGDC, 1998) metadata files for all lidar data deliverables. All tags are required.
- Tags requiring a numeric value shall not contain text (that is, units) because the required reporting units are defined in the appendix 4. The descriptive template of this lidar metadata block is provided in appendix 4 and a completed example is provided in appendix 3.
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Description: Swath separation images (figure 6) use color-coding to illustrate differences in elevation (z-) values where swaths overlap. The color-coded images are semi-transparent and overlay the lidar intensity image. They are ancillary metadata used as visual aids to more easily identify regions within point cloud datasets that may have suspect interswath alignment or other geometric issues. These images may be produced by a variety of methods; however, their usefulness will be ensured by following the specification requirements and documenting the processes used to create the images in the lidar acquisition and processing report.
- Image creation:
- All 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 will 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 50 percent transparent RGB layer overlaying the lidar intensity image.
- 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):
- 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).
- color choice of GREEN, YELLOW, and RED is suggested but not required.
- no pixel shall remain uncolored (transparent) in the overlap areas.
- 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):
- Where swaths do not overlap, pixel values shall be intensity alone.
- Image file formats and version control:
- Swath difference image format may be delivered as GeoTIFF or JPEG (with world file) by tile or as a single compressed JPEG 2000 (JP2) image mosaic.
- The point cloud geometry and intensity data delivered shall be identical to the point cloud geometry and intensity data used to create the difference images. Changes in the point cloud geometry or intensity requires recreation of the difference images.
- Spatial extent and coordinate reference system:
- Spatial resolution (pixel dimension) of the images shall be between 2 and 4 times the Nominal Pulse Spacing (2-4 x NPS) in the project’s linear unit (meters or feet).
- The difference images must be representative of the associated data delivery.
- The images shall be in the same CRS as the point cloud data to ensure alignment with the point cloud.
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- Classified point data deliverables shall include or conform to the following procedures and specifications:
- All project swaths, returns, and collected points shall be fully calibrated, adjusted to ground, classified, and segmented into tiles. Project swaths exclude calibration swaths, cross-ties, and other swaths not used, and not intended to be used, for product generation.
- LAS Specification version 1.4, PDRF 6, 7, 8, 9, or 10.
- Overage (Overlap) and Withheld flags set as appropriate.
- If collected, waveform data in external auxiliary files with the extension .wdp. See LAS specification version 1.4–R15 (ASPRS, 2011) for additional information.
- Correct and properly formatted georeference information as WKT (OGC, 2001) included in all LAS file headers.
- GPS times recorded as Adjusted GPS Time at a precision sufficient to allow unique timestamps for each pulse.
- Intensity values, normalized to 16-bit. See LAS specification version 1.4–R15 (ASPRS, 2011) for additional information.
- Tiled delivery, without overlap, using the project tiling scheme.
- Classification, as defined in table 5, at a minimum.
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Bare-Earth Surface (Raster Digital Elevation Model)
- Bare-earth surface deliverables shall include or conform to the following procedures and specifications:
- Bare-earth DEM, generated to the limits of the DPA.
- DEM resolution as shown in the table 6.
- 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.
- DEM data shall be in the same CRS as the lidar data.
- Georeference information in or accompanying each raster file, as appropriate for the file format. This information shall include both horizontal and vertical systems; the vertical system name shall include the geoid model used to convert from ellipsoid heights to orthometric heights.
- Tiled delivery without overlap.
- DEM tiles with no edge artifacts or mismatch. A quilted appearance in the overall DEM surface will be cause for rejection of the entire DEM deliverable, whether the variations are caused by differences in processing quality or character among tiles, swaths, lifts, or other artificial divisions.
- Void areas coded using a NODATA value of ‘-999999’ and shall be defined in GDAL_NODATA tag #42113.
- Hydro-flattening as outlined in the “Hydro-Flattening” section. Depressions (sinks), whether natural or man-made, are not to be filled (as in hydro-conditioning). The methodology used for hydro-flattening is at the discretion of the data producer (refer to appendix 2 for more information on hydro-flattening).
- Bridges removed from the surface (refer to the “Glossary” section for the definition of “bridge”).
- Road or other travel ways over culverts remain intact in the surface (refer to the “Glossary” section for the definition of a culvert).
- A report on the assessed absolute vertical accuracy of the bare-earth surface in accordance with the guidelines set forth in ASPRS (2014). Absolute vertical accuracy requirements using the ASPRS methodology for the bare-earth DEM are listed in table 4.
- QA/QC analysis materials used in the assessment of absolute accuracy.
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- Delivery of all breaklines collected on or used in support of the project is required for USGS–NGP lidar projects. This includes breaklines used for bridge and saddle treatments and any additional breaklines required by project cooperators.
- Breaklines representing all hydro-flattened features in a project, regardless of the method used for hydro-flattening, are required for USGS–NGP lidar projects. Specific research projects may be exempt from these requirements with prior approval of the USGS–NGP.
- Breakline deliverables shall include or conform to the following procedures and specifications:
- Breaklines developed to the limit of the DPA.
- Breaklines delivered in shapefile or file geodatabase formats, as PolylineZ and PolygonZ feature classes, as appropriate to the type of feature represented and the methodology used by the data producer.
- Breakline data shall be in the same CRS as the lidar data.
- Each breakline feature class shall have properly formatted, accurate, and complete georeferenced information stored in the format’s standard file system location. Each shapefile shall include a correct and properly formatted .prj file. All CRS information for 3-dimensional (3D) data shall include the vertical reference and identify the geoid model used to convert from the ellipsoid to orthometric heights.
- Breakline delivery may be in a single layer or in tiles, at the discretion of the data producer. In the case of tiled deliveries, all features shall edge-match exactly across tile boundaries in both the horizontal (x, y) and vertical (z) spatial dimensions. Delivered data shall be sufficient for the USGS to effectively recreate the delivered DEMs using the lidar points and breaklines without substantial editing.