Thursday, June 9, 2016

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

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What is Mobile LiDAR?

Mobile LiDAR (Light Detection and Ranging) systems are comprised of vehicle-mounted lasers, cameras and GPS/INS navigation systems to capture highly-detailed and accurate three-dimensional (3D) topographic information for surveying and engineering applications. Michael Baker International became an early adopter of Mobile LiDAR technology by acquiring our first system in 2009, with further expansions in 2014 and 2015 that increased our fleet to four (4) Optech Lynx SG1 Mobile LiDAR systems. Over that period our systems have completed more than 300 projects throughout 29 different U.S. States (and multiple countries), and encompassing hundreds of thousands of miles. Our project portfolio includes applications in roadway design, 3D modeling, railroad corridors, signaled intersections, utility infrastructure, asset management, pavement condition assessment and airport infrastructure.