Airborne Bathymetric LiDAR Survey

 

 

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Definition: Bathymetry is the study of under water (ocean, lake) topography where as bathymetric LiDAR is an airborne acquisition technology used to capture a continues topographic data of both land and seafloor at the same time. This is the most effective and cost efficient data capturing method applied in many important issues like:

  • Fundamental understanding of risk area of high vulnerability
  • To provide detailed 3D elevation models along cost lines
  • To capture on both sides of the cost line, over areas stretching more than 100km along the cost, has made it the ‘gold standard for coastal’ vulnerability and nearshore bathymetric habitat modeling.

 

 

Principle of Bathymetric LiDAR


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It works based on principle on differential timing of laser pulses reflected directly from the water surface and reflected under the water bottom (sea floor).

Bathymetric LiDAR is also slightly different from its correspondent airborne topographic LiDAR in that the airborne topographic LiDAR using a light with wavelength of 1,064nm where as bathymetric LiDAR uses a light with 532nm wavelength.

 

 

The simplified components of Bathymetric LiDAR


As its corresponding topographic LiDAR system do the bathymetric LiDAR system can be simplified to four components.

 

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  • Gps receiver to define the aircrafts position.
  • The IMU (inertial measurement unit) to define the roll, yaw and pitch
  • The leaser scanner to emit the signal in a particular pattern.
  • The sensor which reads the Return.

The position and orientation from the above components of bathymetric LiDAR enables the system to record accurate measurements. Some of the sensors can measure 100,000 points per second even more, implies 10 points/m2 in shallow water, and with a depth of up to 75m.

 

It’s Adverse Impacts: By comparison bathymetric surveys are more vulnerable to the adverse impacts of environmental effects, than their counterparts topographic surveys which can result in data gaps, reduced data coverage and measurement quality.

 

Factors which Should be focused: Understanding and managing of some special conditions in bathymetric LiDAR can mean the difference between success and failure in its whole process. For a successful bathymetric LiDAR the following factors need to be considered such as weather for flying, air traffic controls, turbidity, tides, sea state, vegetation condition and ground control accessibility.

 

The major hindrance of shallow water from bathymetric LiDAR sensors is water clarity, or lack thereof. Where as high turbidity, sea grasses and low-reflectance seafloors pose risks to the success of a survey with a LiDAR System.

 

 

Charactorstics of Bathymetric LiDAR Sensors In shallow and in deep water.


 

LiDar system for depth water (>10m) survey generally have more laser power/pulse but a lower measurement frequency (low resolution), a larger laser footprint and receiver field-of-view than LiDAR systems for shallow (<10m) water systems. These deep-water bathymetric LiDAR systems vary in depth penetration capability from between 2.0 to 3.0 times the Secchi depth (a disc to measure water transparency) measurement. Bathymetric survey operators nowadays utilizes shallow-water and deep-water sensors simultaneously in twin optical port survey aircraft to maximize detail and coverage.

 

The scan patterns for sensors are composed of the shape, tilt and method.

 

The scan shapes vary between:

 

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  • Rectilinear
  • Circular arc,
  • circular
  • Elliptical arc
  • Elliptical
  • Non-Oscillating Elliptical Arc

 

 

NB: The circular and elliptical scanners are able to look forwards and backwards, increasing the number of times an area is sampled, although this can result in oversampling along the edges of the scan where as the remaining shapes are usually tilted forward or backwards with respect to the aircraft.

 

The scan methods vary between:

  • Oscillating mirrors;
  • Rotating prisms;
  • Palmer scanners;
  • Rotating multi-facet mirrors and
  • Oscillating raster scanners

All of which result in slight differences in the scan pattern and can be seen in the subsequent point cloud.

 

Decision to Buy a Bathymetric LiDAR System: The best Bathimetric LiDAR survey sensors can be sensors which are manufactured by the considerations of all the great qualities that were tried to be incorporated above in our article. Experts recommend that  the individual characteristics of the sensor and their environmental impact should be examined as well. Obviously, the practical experience and knowledge  and expertise of the operator actuates the final decision at a certain level (i.e. Operators with best experiences in this survey should be involved in the decision).  From a LiDAR perspective, points per meter, avoiding shadows and swath width seem to be the most desired features. The components that should be reconsidered during buying such a system can much depend on the often intended purpose of the expected outputs of the survey, its area, environment.

For example:  usually, what type of environment is expected to be surveyed?, the water clarity situations that we have seen above.  These and all use and  usability analysis by reverse engineering should ofcourse be considered to reach the best decision in buying and planning to buy process.

 

NB: This note is taken from Gim International online magazine from the article “”Bathymetric LiDAR” posted 22/11/2016

LiDAR System and UAVs

In this section we’ll see the pros and cones for the two categories of drones (Rotary Wing Vs Fixed Wing UAVs) interns of being platformed for a LiDAR system.
It’s obvious that each of them possess set of advantages and limitations interns of their contents and the service that they can offer for this system.
Here the purpose of the data planned to be captured (for what purpose we’ll use the system matters) and size of the survey should be our starting grounds to start to prefer one over the other.

 

Advantages of Fixed Wing UAV: It’s known that a data acquisition on a bigger area can demand a UAS with longer operation time and support larger payloads better fuel economy. In this sense as we know most fixed winged UASs have generally special capabilities to longer flying time, stability, and speed, which makes them suitable for large areas or long-distance missions while we know that rotary-winged generally struggling with inertia in a more complex way.

 

Fixed Wing UAV Limitations: However fixed wing UAVS also have some limitations in that they need more space at turns which results in increasing the path length employing decreasing in efficiency. Their speed can also claiming for more faster, more advanced, high performance and expensive sensors to fulfill the needed point density.

 

Rotary Wing UASs Leverage : These UAVs generally enables us to use low-cost sensors, which makes UAS-based laser scanning accessible to a wider user community. Specially they are good for small but object rich areas and more complex terrain survey purpose. Rotary winged UASs exceeds the fixed winged because they have comparatively excellent maneuverability and enabling the operator for densified point scanning, they don’t need larger turning area, or to takeoff or landing for they are known for their (VTOL) capability. They allows for a low or even stationary speed controlling so that it can be possible to scan most important areas with a more densified point data collection.

 

In summary: Because they have an excellent speed, economical fuel conception, long flying time and higher system stability during flying fixed winged UASs preferred over rotary wings for a larger data acquisition in at a larger survey area with less topographic and detail complexities where as these restrictions make a fixed-wing platform less favorable for small-area surveys or with complex terrain/objects and rich in features areas of surveys.

 

Industrial sensors for a UAS leaser scanning


 

Some industrial scanners manufactured equipped with affordable sensors suitable for UAS laser scanning. These sensors are designed for industrial applications and robotics.
The main advantages of these sensors are their low price, small size and durability,all of which are advantages for UAS-based laser scanning. But performance of these types of sensors is relatively limited in some ways like Their:

  • speed of data collection,
  • ranging accuracy,
  • limit for maximum range and often lack of implemented or available signal processing techniques means that they are not always suitable for commercial heavy-user operations,
    but they suits for many other (research) applications.

 

Examples: The most known sensors in this group is UAS-compatible systems that have recently emerged multi-layer laser sensors originating from the automotive industry such as Velodyne LiDAR Puck LITE and Quanergy M8-1 LiDAR.

These systems offer high data rates and some desirable features at a reasonable price. And the most important features of these sensors are Their:

  • ability for Multi-layer data capture
  • 360-degree field of view

Both of which plays big role to improve the along-track sampling and enables more comprehensive data collection of complex scenes such as the urban environment.

 

Other sensors Suit UAV leaser caning


 

aeroscout_vux_laser_scout_groundThe sensors at riegl VQ-480-U or VUX-1 variants are examples of professional sensors which are specialized for laser scanning. Even though their specialty is due to their sophistication in their

design which results in a slightly higher price in the market. And this makes it not to be explored by mapping industries in as a larger scale as they are doing with the other UAV systems. Even though their capability to operate at higher altitudes usually(500 to 1,500 meters) is an advantage, the stricted regulations on UASs make the corresponding manned aircraft conveniently preferred over them.

 

Low Altitude Leaser Sensors which Can Suit UAS


 

Because of the strict regulations on UASs it is preferable using as low altitude as possible. Examples of UASs which are very convenient in this aspect are the above mentioned Hokuyo and Velodyne LiDAR Puck LITE. These sensors can be operated lower than 100m altitude.Their 20 minutes overall flight time makes flight path length as lower as 1.5-2km. To capture an 80m wide strip they should fly with 60 degree field of view and as high as 70m Altitude.

 

UASs LiDAR Application in Urban Area


 

The remote Sensing application in urban area is generally very diverse. The following are the likely usual mapping types in urban areas:

  • For planning or building purpose
  • Routes like Scale detection of power-line, or maintaining of roads.
  • For analyzing some social, economical, political issues.ef2f4695d6631a81cdc9c4a9b8ce1b11a1f46d24

Most of the time all applications generally needs for higher accuracy because a little error in this application results in higher risk in damaging property or even people. The figure at this side showing an example of this type of data.  An autonomous UAS system used to collect this data is:

  • A rotary-wing UAS equipped with a Velodyne LiDAR Puck
  • Sensor package used here also included a NovAtel IGM-S1 GNSS-IMU device for observing and recording the sensor flight path and orientation.
  • A display connected to the system via WLAN

 

Accuracy Depends On: Here one must not forget that the resulting point cloud accuracy depends not only the sensors performance but also directly depends on the flight-path solution accuracy. Therefore during post processing GNSS based station data and data from virtual network was considered for accurate (corrected) flight path computation. With this procedure one can expect 5-10cm accuracy. The point density can be thousands of point data per square-meter compared to the traditional ALS data-set (i.e usually 1-20 points per sqm). To take advantage of simultaneous capturing of the terrain, street infrastructure, building walls and roof structures using a wider FOV is preferable.

 

 

The Upcoming  LiDAR Technologies


 

Flash LiDAR technology and development of Solid state photon counting sensors are the upcoming trends if they fully developed and be operational, they will be expected to push the field of LiDARlidarmaxresdefault to a completely new era.

Flash LiDAR is really focal plane LiDAR, with an array of sensors to capture returns with the CCD sensors of today’s digital cameras.

 

Advantage and Limitations: Due to framed sensing, flash LiDAR allows for faster, simultaneous areal coverage and overlapping, this will really be even more advantageous in object detection
and recognition applications which follows generally multi-look principle. Their limitations will simply be their expensive price and because of their military background they are not be explored much in the civilian users.

 

Solid state photons counting sensors: these can be larger sensors suitable for manned aircraft or larger UASs, or can be extremely small in size based on single photon avalanche diodes lidarsspclast-ned-2
(SPAD). According to their current research lab results SPAD can perform same as the currently low-cost leaser scanning. Which indicates that there’ll come time with a great opportunity of having LiDAR sensors with highest performance with a reasonable cost.

 

 

NB: we have adopted this note from an article “The Current State of the Art in UAS-based Laser Scanning”, posted 22/06/2016, Gim International online Magazine, just with a little modifications to not to copy and pest directly. (Dr Antero Kukko, Dr Anttoni Jaakkola, Prof Juha Hyyppä) are referenced on the magazine as original authors of this article.

 

Lidar-

LiDAR is a very fast growing remote sensing technology which becoming quite important issue to be discussed in Governmental, industries, professionals, Students levels. It is a quickly updating technology which we are interested to discuss about it in this section.

Even though originally it was for usage of measurement of terrain,the interest in LiDAR has been grown through time in it’s last two decades of age in areas like:

  • DEM developing
  • Its application independent to cloudy/sunlight conditions unlike the other remote sensing technologies.
  • Its applications in urban, forestry and agriculture to record quality forest attributes and object heights to sub meter accuracy level.
  • Surface height estimation and the structures of forests especially assessed by this technology.

Due to these specialties it did not take time to be adopted by the geomatics, agriculture and forestry industries.

As you may know LiDAR stands for Light detection and ranging technique which uses leaser to measure ranges directly and heights and elevations indirectly.

Even though it has a kind of similarities with the other mapping technologies like radar and sonar, it is different because it uses light instead of radio or sound.

Eye Affection of Laser: Leaser is a light portion of a electromagnetic spectrum which lies b/n green and Near-Infrared of which type strongly reflected off of vegetation. Most of them are around 1064 nm wave length which impossible to see them with naked eyes. Pulses affects eyes but the Australian studies finds out some googles as solutions to secure the affection and became safe for all species and will not be a negative impact anymore on eyes.

LiDAR is an active type of remote sensing technology unlike of (areal photography, Landsat and spot which are all passives), due to the system emits light as a pulse and receives a reflection back as a Return. This will make the system more controllable using some sort of sensors in the platform. And with that sense we can say that this is an accurate technology.

There are three most common data collecting ways in LiDAR.

  • Terrestrial using some forms of scanners on a tripod or on a vehicle
  • In air on an air plane, UASs
  • Airborne LiDAR bathymetric technology generally use 532 nm frequency.

Air borne LiDAR is the most available way of the technology. Therefore airborne data is the most freely available in the observatory networks(National Ecological Observatory Networks) now a days.

lidar-scanThe components of LiDAR System are

  • LiDAR unit
  • GPS
  • IMU and
  • Computer.

 

How LiDAR Works: The part of the system enabling laser scanning from side to side, is the LiDAR unit, where as GPS is used to measure the x,y,z position of the system on the aircraft. Here in the leaser’s pulse and Return, the system can calculate the height of the system from the surface, that the Return is reflected. So together with the GPS height and leaser measurements those are already calculated, the basic elevation of the surface will be indirectly computed.

And finally the pitch, yaw and roll measurements from IMU, and the angles b/n the Nadir-line and each of the offset-ed Pulse (off Nadir pulses) , both will be considered by the computer of the system, to calculate the accurate heights of each and every surface points on which the Returns are reflected back from.lidarimages-3

Number of Returns: The leaser that reaches to a canopy will not be returned direct only from the top of the canopy, but it will continue to reach to the branches, then the possible available shrubs and finally to the ground before returned back to the LiDAR system. And that is this ability of the LiDAR system, of recording all the available information from the top of the canopy to the ground that makes it unique, and highly valuable to plant study science. Because all the reflected backs Returns tells all the available information not only on the top but also under the canopy all the way to the ground. The shape, density, the possible available shrubs and structures of the forest will be uncovered in this process.

Application

The application of LiDAR system includes:

  • Agriculture in Classification in plant species, feature extraction, Reflectance features, Geometrical Features
  • Archaeology
  • Autonomous vehicles
  • Biology and conservation
  • Geology and soil science
  • Atmospheric remote sensing and meteorology
  • Law enforcement
  • Military
  • Mining
  • Physics and astronomy
  • Rock mechanics
  • Robotics
  • Spaceflight
  • Surveying
  • Transport
  • Wind farm optimization many more

Sigma Spaces’s Single Photon LiDAR The Next Generation LiDAR Capabilities

The airspace’s optical instrumentation company Lincoln Laboratory has pioneered in the development of mid and long infrared (IR). In the development process, it was possible to hybridized an Infrared (IR) detector to a readout integrated circuit to form a focal plane array.

Single photon LiDAR (SPL) is a system developed by the company for NASSA. The technology is also essential very helpful and profitable for the other industries like Geo systems. SPL is the LiDAR capable of detecting single photons (single particle of light) which makes the efficiency of the overall system higher than all the systems ever known before. With this capability it was possible to divide the leaser beam spectrum in to multiple hundred beams. This intern enables to identify the terrain, wall or water and also some atmospheric instruments is made using this photon counting technology. With single photon LiDAR a range of measurement as much as 6 million times/sec could be provided, which is a mile beyond (about 6 times as effective as hexagon latest generation LiDAR sensor) and other commercial instruments capabilities.

Relations With Leica Geo-system: This capability attracts the industries like Leica Geo-system to consider this technology for their imagery programming by adding elevation component to it. This efficiency can make the future mega projects that the Leica Geo-system may have feasible, in addition to getting wider spread adoption of the technology for their commercial instrument productions.

Space Science Application: The application of this new technology will also be expected to be used in mapping asteroids and planets in the process of attempting the plan to land on the planets like Mars by mounting it on the space craft that will be employed for this project. Generally it can be said that it is the technology which is revolutionizing the whole LiDAR system in to it’s completely new dimension.

The above note is taken from Wikipedia, POB (Point of Begining), Geoinformatics

Geiger-Mode LiDAR (GML) and Single-Photon LiDAR (SPL)

The 17th international LiDAR mapping forum (ILMF) , which will be taken place in Denver, Colorado from 13-15, 2017, will include sessions to provide answers for questions concerning the LiDAR technologies GML/SPL, said the organizers of the event.

ilmf-200x150As the the Geoinformatics magazine stated, the events director Lisa Murray said,  the event will begin with answering the important questions What’s the Difference (GML/SPL) & Why Does SPL Matter? including many questions about the new LiDAR techinology and un understood matters about the next generation LiDAR technology among  geospatial professionals will be addressed and in addition it will Reporting out:
SPL Applied to Large-Scale Projects, including lessons learned from the nation’s first statewide program to leverage Geiger-mode technology (North Carolina).

This first session of the event will include also:

  • Sensor characteristics,
  • Data density,
  • Accuracy and stability of point clouds
  • And filtering techniques which will appear under this new tech. will all be addressed.

In the other sessions also big issues including Multi-Sensor & Data Fusion, UAVs & LiDAR, Updates from the USGS (The united States Geological Survey) concerning LiDAR technology and many others will be covered… Please read more about this at Geoinformatic Website.

 

UAVs’ Fit-For-Purpose Way as Cadastral Solution

The ongoing supporting of Cadastral process using conventional areal imagery and high resolution satellite imagery in some counties like Rwanda, Ethiopia and Lesotho e774fd59cfa9aa1db33f3ab8f80ebf4a3016de20which were based from the similar older activities of Thailand gives raise for unmanned areal system (UAVs) interest to be received in the field of land administration.

Some results of the ongoing exploratory work of this method presented revels the opportunities and challenges for embedding this technologies into the existing Cadastral survey process as a fit-for-purpose way.

The effort for developing cloud services using imagery, often captured using multi-copters, for 3D reconstruction of the built environment, makes the Linköping and Stockholm based Swedish company Spotscale unique in this area of exploration.

Read the full news here…