What is lidar?
Lidar Specifications for the State of Idaho
The lidar data should meet the following specifications:
- Pulse (point) density of >12 pulses/m2; the pulse density should be determined based on the needs of the project; this minimum is suggested in order to make these data useful for a wide range of applications. Lower pulse density is unlikely to result in useful data for most quantitative analyses.
- Capability of 4 returns per pulse (minimum)
- Flight lines with 50% side-lap (or at least 150 ft. side-lap)
- Scan angle of <30o (+/- 15o)
- Vertical RMSE commensurate to objectives; over flat/open surfaces <5 cm;
- Horizontal RMSE commensurate to objectives; over flat/open surfaces <30 cm (based on flight altitude). Horizontal errors should demonstrate randomness relative to direction (N-S versus E-W), and horizontal RMSE should be evaluated with control points.
- Spatially distributed real-time kinematic control points collected across each landcover type (e.g. dense overstory with grass/shrub understory, riparian, grass/shrub, urban, agriculture, and bedrock versus regolith) and slope category (0-5, 5-10, 15+o) in the study area;
- Control points should be spatially distributed across each flightline and include a minimum of 3 permanent recoverable control points (monuments or benchmarks) tied to the most current national geodetic datum (e.g., as of this writing, NAD83(2011) should be used).
- Calibrated intensity data (e.g. with Automatic Gain Control (AGC) correction, if needed, given the given the sensor type, but without normalization)
- All-return unclassified point cloud delivered in LAS format (v 1.4 or other agreed upon format)
- All-return classified point cloud delivered in LAS format (v 1.4 or other agreed upon format); classification in ASPRS LAS standards
- Complete metadata in XML following FGDC standards detailing data quality information (accuracy assessment) and processing steps including software used to achieve the delivered point cloud data
- Flight path trajectory information (SBETs) (at least X, Y, Z, time, roll, pitch, heading velocity)
- Project and data collection reports including equipment used, equipment accuracy (assumed and manufacturer’s stated accuracy), details of mission efforts including a QA/QC assessment (summary statistics broken down by land cover and slope type, histograms, etc), survey extent, positional accuracy and accuracy assessment, and classification of points.
- Survey report with locations and accuracy of all control and reference points including permanent monitoring locations, equipment used, and equipment accuracy (assumed and manufacturer’s stated accuracy)
The following information need to be contained in the LAS file for each return (note: this follows the Point Data Record (Format 3) of the ASPRS LAS specification)
- X, Y, and Z coordinates
- Return Number
- Number of Returns for given pulse
- Scan Direction Flag
- Edge of Flight Line
- Scan Angle Rank
- User Data
- Point source ID
- GPS Time (GPS Week Time and Absolute GPS Time, POSIX)
- Red, Green Blue (if digital imagery are collected)
The LAS file header information should include the following at a minimum (note this follows the Public Header Block of the ASPRS LAS specifications).
- Global Encoding Information
- System Identifier
- Generating Software
- Georeferencing information (in GeoTiff Specification)
The vendor should also provide the following:
- All derived products should be in the requested file formats (e.g. TIFF or GeoTIFF) with coordinate system embedded and with complete metadata including software and processing steps used to create the derived products. Coordinate systems and projections: Horizontal: GCS NAD 1983(2011) Vertical: NAVD 1988
- Bare Earth Model
- Surface Model
- Hydro-enforced DEM with break lines
- Hydro-flattened DEM
- Contour lines (2 foot, 1 meter, or other as appropriate)
- Ground point density image
- Intensity return image
- Clearly delineate the project goals with the vendor and ask to be involved in the flight planning process
- The best practice is to have an independent licensed surveyor provide the above ground survey (instead of the lidar vendor); the data requester should also perform an independent survey. While the independent survey may add to the cost, it ensures an unbiased data accuracy assessment.
- Consider timing of the acquisition (e.g. leaf on/off; high/low river flows; snow on/off)
- Optical data (e.g. digital imagery) should be co-acquired with lidar due to minimal additional cost and the opportunity to check lidar for accuracy, as well as a wide range of additional other uses of the aerial imagery.
- At minimum two flightlines should be perpendicular to all other flightlines, one at each end of the study area
- Consider the scan angle and flight line orientation in reference to the landforms in the study area
- Consider locations where higher and/or lower point densities may be needed in the study area
- Define the coordinate system to be used (including Geoid). NAD83(2011) is recommended.
- The vendor should coordinate with land-owners for site access.
Other helpful resource links:
Oregon Lidar Consortium and Lidar Specification
Minimum Lidar Data Density Considerations for the Pacific Northwest (PDF)
USGS NGP Lidar Base Specification
ASPRS Guidelines LAS
Vertical Accuracy Reporting of Lidar Data V1.0, (ASPRS, 2004)