LIDAR or Light Detection And Ranging uses lasers to measure the elevation of things like the ground forests and even buildings. It is lot like sonar which uses sound waves to map things, or radar which uses radio waves to map things, but a LIDAR system uses light sent out from a laser.
For the record, there are different ways to collect LIDAR data: from the ground, from an airplane or even from space.
Airborne LIDAR data are the most commonly available LIDAR data and airborne LIDAR data will also be freely available through the National Ecological Observatory Network or NEON. Many other sources are becoming free for many countries.
The four parts of LIDAR Sytem:
To understand how lasers are used to calculate height in airborne LIDAR, we need to focus on the four parts in the system.
1. LIDAR Unit – Scans the ground:
First, the airplane contains the LIDAR unit itself which uses a laser to scan the earth from side to side as the plane flies. The laser system uses either green or near infrared light because these wavelengths or types of light reflect strongly off of vegetation.
2. Global Positioning System – Tracks planes x,y,z position:
The next component of a LIDAR system is a GPS receiver that tracks the altitude and X,Y location of the airplane.
The GPS allows us to figure out where LIDAR reflections are on the ground.
3. Inertial Measurement Unit (IMU) – Tracks Plate Position:
The third component of the LIDAR system is what’s called an inertial measurement unit or IMU.
The IMU tracks the tilt of the plane in the sky it flies which is important for accurate elevation calculations.
4. Computer – Records Data:
Finally, the LIDAR system includes a computer which records all that important height information that the LIDAR collects as it scans the earth’s surface.
How these four parts of the system work together to get fantastically useful later dataset?
The laser in the LIDAR system scans the earth actively emitting light energy towards the ground. Now before we go any farther, let us get two key LIDAR terms associated with this emitted light energy out of the way.
First, let’s define the word “pulse”. A pulse simply refers to a burst of light energy that is admitted by the LIDAR system.
And second, lets define the word “return”. Return the first reflected light energy that has been recorded by the LIDAR sensor.
Pulses of light energy travel to the ground and return back to the LIDAR sensor.
To get height the LIDAR system records the time that it takes for the light energy to travel to the ground and back. The system then uses the speed of light to calculate the distance between the top of that object and the plane.
To figure ground elevation, the plane’s altitude is calculated using the GPS receiver and then we subtract the distance that the light travel to the ground.
There are two more things in a LIDAR system to consider when calculating height. First, the plane rocks a bit in the sky as it flies due to turbulence in the air. These movements are recorded by the inertial measurement unit or IMU so that they can be accounted for when height values are calculated for each LIDAR return.
An airborne system scans the earth from side to side to cover a larger area on the ground when flying. So while some light pulses travel vertically from the plane to the ground or directly at nadir, most pulses leave the plane angle or off nadir. The system needs to account for pulse angle when it calculates elevation.
How a LIDAR system works?
The LIDAR system emits pulses of light energy towards the ground using a laser, it then records the time it takes for the pulse to travel to the ground and return back to the sensor. It converts this time to distance using the speed of light.
The system then uses the plan’s altitude, tilt, and the angle of the pulse to calculate elevation. It also uses a GPS receiver to calculate the object’s location on the ground.
All this information is recorded on that handy dandy computer also mounted on the airplane.
GPS is a product of the Cold War. Developed by the US military during the President Reagan years, this system consists of a series of 24 satellites in geosynchronous orbit. That is, these satellites remain in the same fixed location in the sky. Out of 360 degrees of longitude, each satellite covers a 15-degree sweep of the globe. They orbit the earth twice each day, broadcasting a timing signal. These signals can be intercepted by ground antennae mounted on ships, tanks, planes—or the buckets and blades of heavy earthmoving equipment.
Though, like all broadcast signals travelling at the speed of light, there is a small-but-measurable time lag between the signals from adjacent satellites. The difference, measured in milliseconds, allows for the triangulation between the ground antennae and the satellites emitting the signal. This triangulation measurement allows for the measurement of the precise location (measured in latitude, longitude, and elevation above sea level) on the surface of the earth where the receiving antennae is currently located. As is true so often in the history of technology, a technical advancement meant for use in war has been modified and adopted for peaceful civilian applications. A system intended to track the movements of men, weapons, ships, and war planes is now used to follow the movements of commercial shipments, find lost hikers, and guide construction equipment.
GPS guides equipment operations through their Automated Positioning Report System (APRS). Using triangulation between the several of the system’s broadcasting satellites allows for positional measurements with accuracies up to 30 centimeters (1 foot). The use of APRS increases this accuracy to 1 centimeter when used in conjunction with GPS. What APRS does is integrate GPS with the equipment’s controls. An APRS replaces the manually operated hydraulic drive cylinders traditionally used to control the movements of an excavator’s arm or dozer’s blade with electronically controlled servo-type valves. These servos send an electric current that creates magnetic field that rotate suspended armatures, that are further connected to fixed flapper arms. These flapper arms provide the linkage to the rotary spools that increase and decrease hydraulic pressure to the hydraulic systems. These are part of the closed-loop hydraulic system that controls the direction, flow rate, and applied pressure of the hydraulic fluid.
Going from individual pieces to equipment to an entire fleet of equipment or trucks is also easily managed by GPS applications. This allows a fleet owner to coordinate and choreograph the movements and activities of an entire fleet of earthmoving equipment and also track the locations of trucks delivering material to the site or hauling dirt away for disposal. Like pieces on a chessboard (or, more accurately, objects in a video game) these activities are tied to an onsite digital terrain model (DTM) of the project site.
This is a three-dimensional (3D) model created by an AutoCAD program, which utilizes a patchwork of connected triangles. The corners of each triangle is defined mathematically by three special coordinates (northing, easting, and elevation). While not a perfect match (no model ever is) to the actual terrain, this geometric surface comes the closes to matching actual surfaces. This is especially true for post construction or excavation surfaces, which tend to be regular and smooth.
Leica excavator machine control solution
The software interacts with the model and the equipment hardware via sensors attached to the business end of the machine (the edge of the dozer blade, the teeth of an excavator bucket, etc.). These sensors continuously record and update the movements of the equipment using the same 3D location system as the DTM. The sensors relay their current location back through the system to the operating controls, which in turn direct the movement of the equipment in accordance with the programmed DTM for the proposed construction surface or excavation grades.
Coordinating all of the mobile GPS sensors on each piece of equipment is a stationary GPS sensor combined with an antenna with a receiver called the base station that is set up adjacent to the operating area. The base station is permanently located over a pre-surveyed reference point, such as a third order benchmark that has been established by ground survey. If necessary, some relative location (manhole rims, street curbs, and building corners, etc.) whose elevation is not exactly known but can be treated as a local datum for the project area can also be utilized for ground antennae setup.
Grader GPS Control
The hardware for these control signals consists of a control box connected to the servo valves via electrical cable, which, in turn, connects to the hydraulic control system that physically moves the equipment. It is the incoming satellite data from the GPS system that tells the equipment where it is. The DTM design files are stored in a compact-flash memory card, memory stick, or accessed externally from data broadcast by the site’s controlled area network (CAN). The database, CAN, and GPS operate in real time to place a blade or bucket exactly where it needs to go and move it so that it accomplishes its task with the need for rework or wasted effort. All three elements, mobile sensors, base sensors, and equipment operator are in continuous communication with each other. The blades and buckets simultaneously move back and forth, up, and down in combination to achieve the desired movement.
Where and When Is GPS Best Used?
The advantages of using GPS guidance systems are legion. Using advanced systems, an operator can increase productivity by greater than 50% compared to purely manual operations. The increase in productivity comes indirectly by the avoidance of having to perform rework at the site. Guided by GPS, an equipment operator can get his cuts and placements right the first time. Furthermore, material wastage is minimized. GPS doesn’t necessarily increase the number of productive hours per day so much as it makes every hour a machine is active much more productive.
This has all sorts of secondary benefits and cost savings. Getting the most out his earthmoving equipment allows a contractor to get the most out of his workforce as well. This reduces labor costs, another costs savings, while reducing the impact of local labor shortages—always an issue in a booming economy with a considerable amount of construction activity. Furthermore, grades and elevations can be checked in real time as the work is being performed. The operator can check his elevation from within his cab as he is working. There is no more need to stop work at regular intervals and have a manual survey performed of the work zone to check its accuracy. In the past, the accuracy-checking task has traditionally relied on a crewmember inside the trenches to manually check depths and slopes. Now with these systems, the need for that task is greatly reduced—thereby cutting job site costs and improving crew safety. This traditional cause of equipment downtime is mostly avoided with the use of GPS.
Giving operators the tools to perform accuracy checks also gives greater responsibility, initiative, and job satisfaction. The additional training that the operators receive has the effect of empowering them, increasing senses of purpose and self-worth. This alone has a marked, if indirect, impact on productivity. By eliminating rework and improving employee morale, any money spent training operators on GPS is a wise investment that pays for itself very quickly. Human factors also show improvement in the area of site safety. The precision guidance and ability to choreograph equipment movement across a busy site improves safety by maintaining safe work zones, avoiding known utility locations, preserving the foundations of existing structures, and maintaining a safe flow of traffic.
GPS systems are often augmented by laser guidance for precise finish work. Lasers can compensate for some of GPS limitations. GPS works best with an open sky and no significant overhead blockage. (For example, GPS is not used for tunneling operations.) Laser guidance stations and targets mounted on equipment blades can work in any outdoor situation, with or without overhead blockage from trees and tall buildings. GPS, however has a much greater effective operating range, limited only by the availability of open sky. Lasers systems are usually limited to about 1,500 feet. GPS can allow for the construction of complicated surface models as well as flat, sloping surfaces. Lasers, being line of site instruments, are usually limited to operations on long, flat, or sloping surfaces. (Though they, too, can build surfaces as directed by a 3D design model represented by a surface AutoCAD file.)
Grade Control System
Productivity can be measured in many ways: time savings, labor costs, material costs, fuel costs, quality bonuses, and finish bonuses. It begins as early as the job layout.
With Machine Control and GPS, you don’t have to wait for someone to stakeout the project or weather that permits someone to stakeout. This allows you to get started sooner. Any design change will also benefit as productivity increase due to not having to wait for restaking.
As the operator starts moving material you will see great value in being able to move the correct amount of material, to the correct location, the first time. This, along with only using the exact amount of material, will translate into a productivity savings.
You can also use GPS by having job layouts of the site so the machines or supervisors’ tablets can have the precise storage locations of materials, job trailers, or site boundaries. This can help having the right things in the correct location or, better yet, not in the way, therefore reducing excessive handling of material.
Satellite positioning (GPS), advantages and disadvantages for site engineers
A 1960s surveying text book consulted in 1980 would reveal little change in twenty years. That is not true today, with modern technology, systems and software are being continually updated. Nowhere is this more obvious than with satellite positioning.
It has exploded onto the construction market changing some operations beyond all recognition. As technology improves, accuracy increases and costs come down it becomes more economical to employ it on smaller and smaller jobs.
What is it?
Satellite positioning is the determination of the position of a point using a satellite receiver. Satellite positioning is generally known as GPS or global positioning system after the American military system, which was fi rst available for public use.
Unlike most surveying and setting out tasks, the skill required of the operator is minimal. The skill with GPS is with the management of the system’s input and out put data. The satellite receiver does all the work in gathering the data and outputting or storing it as required. With setting out it can provide the operator with predetermined setting out coordinates.
How accurate is it?
Accuracy depends on the methods employed and the equipment used. For construction setting out centimetre level accuracy is achievable. This makes it suitable for many setting out tasks. Unlike traditional survey methods, each point is independent of the points around it, and therefore each point is of a similar accuracy.
Degradation of accuracy (due to creep) with distance from the main station is no longer a prob lem. If used in unsuitable conditions, accuracy may be compromised.
An error in one point is not passed on to adjacent points.
What are the advantages?
When used for setting out, a single engineer with a setting out pole equipped with a satellite receiver can set out points almost as fast as he can mark them. With a road centre-line for example, the operator can walk the route and mark centreline points at whatever frequency is required. The setting out information can be taken straight from the design on disk without the need to input a mass of figures.
Work is unaf fected by weather or daylight or a lack of it. Visibility between points is not required, so local obstructions (shrubbery, mechanical plant, low buildings, walls etc.) do not hinder the process. Productivity increases are considerable. As well as giving plan coordinates (Eastings and Northings), it will automatically provide heights as a mat ter of course.
Satellite systems can also be integrated into computer-controlled plant, in which, for example, a grader has the road design in its memory. The grader blade is automatically adjusted to give the correct earthwork
profile. This eliminates the need for a complete setting-out team along with their instruments, forest of timber-work, chainmen and their transport.
What are the disadvantages?
Cost is always an issue, but this has to be balanced against productivity. GPS is not suited to all locations. Due to the fact the position of the receiver is derived from observing a number of satellites, a clear view of the sky is necessary. This may make GPS unsuitable for city centre sites shielded by adjacent tall buildings.
A received signal may give inaccurate results if defl ected off the side of a building. GPS is not suitable for tunnelling work. However GPS can be used very effi ciently to establish a control either side of an obstruction under which tunnelling is required.
GPS does not work well in tree-covered areas, again due to the need for a clear line of sight to the sky.
The height element of the output is of a lower order of accuracy than the plan coordinates. Additionally, heights given are not above mean sea level (as with traditional levelling), but above the mathematical model of the Earth, WGS84 (World Geodetic System 1984).
Unfortunately, for Europe this does not run parallel to mean sea level. However the GPS output can be confi gured to give correct information. GPS is not suffi ciently accurate enough to obtain the 1 mm precision that can be
achieved with a theodolite.
