Origin of a Point Cloud

Wikipedia has defined a “Point Cloud” as “a set of data points in some coordinate system”. In a three-

dimensional coordinate system each point is commonly defined by an X, Y, and Z value, and is often employed to represent the surface of an object. Point cloud geometry is often referred to, rather mistakenly, as LiDAR. LiDAR is a technology that can be used to make point clouds; so measurements produced with LiDAR technology is more aptly known as LiDAR data. Most commonly, point clouds are the result of data being output by 3D scanners. Aerial and Terrestrial LiDAR sensors are two such examples of 3D scanners that perform a large number of measurements on an object's surface to output a point cloud, but as you’ll read, point clouds may originate from various other sources and technologies.

Photogrammetric Detection and Ranging (PhoDAR) is an alternative technology to conventional photogrammetry and LiDAR to generate point clouds. Though, PhoDAR and LiDAR are both capable of producing point clouds, the primary difference being PhoDAR uses photographic data (i.e. pictures) collected with a camera to generate a point cloud, whereas LiDAR involves the collection of data measured directly from the reflection of an object using a laser.

Another technology from which topographical point clouds are regularly created is IFSAR (Interferometric Synthetic Aperture Radar). . IFSAR sensors are typically flown in fixed-wing aircraft or mounted to satellites, and primarily uses X-band data of the electromagnetic spectrum to perform measurements. IFSAR data collection involves placing two radar (radio detection and ranging) antennas on the vehicle such that one antenna transmits a radar beam, and then both antennae receive the radar beam reflected from the target (usually the earth’s surface). Captured signals are combined with aircraft telemetry and positional data are processed to generate an interferogram, which is employed to generate elevation products.

Sound Navigation and Ranging (SONAR) technology uses the propagation of sound in an underwater environment to target objects. It is most commonly used to in the surveying profession to determine water depth (bathymetry). Sonar is applied to water-based activities because sound waves attenuate (taper off) less in water as they travel than do radar and light waves.

In future posts I’ll provide detailed analysis of the pros/cons, uses, and accuracies of the various technologies, but for now, the following table provides a brief overview of each.

Technology	Coverage	Accuracy	Use of Technology
Terrestrial LiDAR (Mobile or Static)	Small-to-Medium area projects	1cm	Buildings, Corridor mapping, Asset Inventory, Engineering Design
Aerial LiDAR	Large area projects, in varied terrain conditions	10 cm	Flood plain mapping, Disaster Management, Transportation and Engineering design, Impervious surface mapping, vegetation mapping
PhoDAR	Medium-area projects	15+ cm (dependent on GSD)	Same as above
IFSAR	Large area projects in varied terrain conditions	30 cm	Same as above
SONAR	Small-to-Medium area projects	5-10cm	Underwater studies, dredging, navigation

Cheers!
Srini