Review of Project Specifications

Planning is performed after careful review of the project specifications and answering a series of questions: Should maps be compiled to NAD83 (HARN) for the horizontal datum and NAVD88 for the vertical datum? Should elevation data (orthometric heights) be produced by converting from ellipsoid heights using the GEOID12A model? Should the coordinate reference system use the relevant State Plane Coordinate System or Universal Transverse Mercator (UTM) coordinates? (Note: State Plane coordinates are more accurate for typical requirements). Should the units be feet or meters? If feet, should U.S. Survey Feet or International Feet be used? What should be the nominal point density? What classifications should be included i.e., ground, water, buildings, vegetation, etc? Are planimetric features such as roads or buildings needed to be extracted from the LiDAR data? Are there limits on environmental factors such as shadows, clouds, topography, climate, snow cover, standing water, tidal and river levels? Will DEMs, DSM, Contours, or other derivative products be produced? What are the metadata requirements? How are accuracies to be reported in the metadata; will the accuracy be reported using the accuracy at the 95% confidence level for the NVA? Are waveform data needed? If yes, what is the data format?

(1) LiDAR Point Density. LiDAR data can be collected with a wide variety of point densities depending on the needs of the project. The selection of point density is a big driver of the overall cost of a LiDAR project and should be selected with consideration to the end uses for the LiDAR. Modern LiDAR sensors are capable of acquiring LiDAR data with a higher density than previously available and can do so at higher altitudes and with less overlap. A LiDAR product with 1 point per square meter (ppsm) is sufficient for many applications such as flood mapping in many areas. Higher point densities (4-8 ppsm) allow for greater utilization of the data for mapping planimetric features such as roads and structures as well as for vegetation analysis such as biomass and canopy studies. Additionally, specialized LiDAR at very high densities > 20 ppsm are often used for mapping infrastructure in greater detail such as power lines, pipelines, and for Department of Transportation (DOT) significant features such as mile posts and signs. The ground conditions should be considered when selecting a point density as well. If the area is covered with dense vegetations such as a coniferous forest a higher density and more overlap would be required to penetrate to the ground than an area where leaf-off conditions exist.

(2) Swath Overlap. Planning for swath overlap should also be included in the overall planning of the point density. A higher percentage of overlap may be beneficial in an area with dense vegetation as there will be more look angles from the sensor to the ground at any given points. The result would mean that there could be a lower point density requirement in an individual swath with the overlap accounting for an overall higher point density on the project. Depending on the scanning pattern, data from the extreme edges of the swath may be unusable due to geometric nature of the scan pattern. Typically, 10% overlap between swaths is the minimum requirement for an airborne LIDAR collect. However, most LiDAR flights are conducted with 30% overlap, and those that require higher pulse density in vegetated areas are often flown with 55% overlap.