LiDAR, which stands for Light Detection and Ranging, is a remote sensing technology that uses laser pulses to measure distances and create 3D models of surfaces. LiDAR data can be used for a variety of applications, including urban planning, forest management, and topographic mapping. One of the most important steps in processing LiDAR data is classification, which involves identifying and labeling different types of objects and terrain features in the LiDAR point cloud.
There are several software packages available for LiDAR classification, each with its own unique features and benefits.
1. LAStools:
LAStools is a popular software package for LiDAR processing and classification. It includes a suite of tools for filtering, quality control, and classification of LiDAR data. LAStools can classify LiDAR data into ground, non-ground, buildings, vegetation, and other features. It also includes tools for point cloud thinning, which can significantly reduce processing times.
2. TerraScan:
TerraScan is another powerful software package for LiDAR classification. It includes a range of tools for point cloud management, filtering, and classification. TerraScan can classify LiDAR data into ground, buildings, vegetation, and other features, and can also perform advanced classification tasks such as building footprint extraction and power line detection. TerraScan is widely used in the forestry, transportation, and utility industries.
3. ArcGIS:
ArcGIS is a popular GIS software package that includes tools for LiDAR data processing and classification. It can classify LiDAR data into ground, vegetation, buildings, and other features, and can also perform advanced classification tasks such as tree species identification and canopy height modeling. This software is widely used in the urban planning, forestry, and environmental management industries.
4.CloudCompare:
CloudCompare is an open-source software package for LiDAR data processing and classification. It includes a range of tools for point cloud filtering, registration, and classification. CloudCompare can classify LiDAR data into ground, non-ground, buildings, vegetation, and other features. It is widely used in the surveying, archaeology, and geology industries.
5.TopoDOT:
TopoDOT is a software package for LiDAR processing and classification that is specifically designed for the transportation industry. It includes tools for point cloud management, filtering, and classification, as well as advanced features such as automated roadway extraction and sign detection. TopoDOT is widely used in the transportation industry for highway and rail planning, design, and maintenance.
6.GlobalMapper:
One of the key features of Global Mapper’s lidar classification tool is the ability to distinguish between ground and non-ground points. This is an essential step in many lidar applications, as it enables accurate terrain modeling and surface analysis. Global Mapper offers several algorithms for ground point classification, including the progressive morphological filter (PMF) and the triangulated irregular network (TIN) method.
In conclusion, LiDAR classification is an essential step in processing LiDAR data for a variety of applications. There are several software packages available for LiDAR classification, each with its own unique features and benefits. LAStools, TerraScan, ArcGIS, CloudCompare, and TopoDOT are some of the most popular software packages for LiDAR classification, widely used in different industries for different purposes. It is important to select the appropriate software package based on the specific requirements of the project and the industry.
What is Total Station? Principles and fundamentals – Advantages and Disadvantages
Total station is used for computing slant distances, horizontal and vertical angles, elevations in topographic and geodetic works, tacheometric surveys, etc. The total station is a pre-eminent contribution to modern surveying and hence the equipment is designed for speed, range, and accuracy. They are a combination of Theodolite and Electronic Distance measurement (EDM). This enables computing the vertical, horizontal as well as slope measurements.
They acts as a substitute for theodolite, EDM, Data collector, and a Microprocessor. Moreover, they are lightweight and compact machines and perform like transit stadia and plane table alidades.
The integration of microprocessors helps in the data collection and measurement computation process. Further to that, the inbuilt software helps to generate the maps instantly.
Applications
Apart from taking the measurements, the total station helps in computing, interpreting, and documenting the data. Here is a list of activities that are computed, interpreted, and analyzed.
Horizontal angle
Vertical angle
Slope distance
Coordinate of point
Missing line measurement
Area calculation
Contour level
Principle of Total station
The total station consists of a built-in emitter, capable of emitting microwaves and infrared signals. The wavelength of these emitted waves helps in calculating the distance between the points. Distance = velocity * time
Here the distance is calculated by multiplying the time taken to cover a certain distance by the velocity. However, Triangulation and trigonometry methods are adopted for computing the angles and determining the coordinates.
Total Station Components
The Equipment is composed of different parts. Below listed are the major components.
Components of total station
Handle
Handle securing screw
Data input/output terminal (Remove handle to view)
Instrument height mark
Battery cover
Operation panel
Tribrach clamp
Baseplate
Levelling foot screw
Circular level adjusting screws
Circular level
Display
Prism and prism pole
Objective lens
Tubular compass slot
The optical plummet focusing ring
Optical plummet reticle cover
Optical plummet eyepiece
Horizontal clamp
A horizontal fine motion screw
Data input/output connector
External power source connector
Plate level
Plate level adjusting screw
Vertical clamp
A vertical fine motion screw
Telescope eyepiece
Telescope focusing ring
Peep sight
Instrument centre mark
A tripod enables to affix the equipment onto the ground. A handle is available on the top of the equipment for holding it. There are a Data input and output terminal below the handle enabling the data transfer to the computer.
The total station comes with inbuilt software, an operation panel, keyboard, and a screen. The prism and prism poles helps in measuring distances.
Advantages of Total station
There are a lot of advantages of total station as follows.
High precision and accuracy.
Requires limited manpower
Perform quick field work
Reduction in manual errors.
Correction for temperature, pressure etc and digitally rectified.
Storage and interpretation of data is easy
Time consumed is less.
Inbuilt GIS software helps in instant map creation
Assists in local languages.
Disadvantages of Total station
The cost of the instrument is high.
Need high skilled surveyor with AutoCAD knowledge and professional training.
Checking errors during the operation is impossible.
All you should Know about Surveying and Its Classification
Here is everything you need to know about surveying and its different classifications.
Surveying is a fundamental element of civil engineering since it is the initial stage in initiating a new civil engineering project.
A student must master the fundamentals of surveying in order to fully understand the procedures.
To begin learning surveying, one must first understand what surveying is and why it is important.
So first of all :
What is surveying ?
Surveying is simply the process of using direct or indirect measurements to determine the relative positions of various features on, above, or beneath the earth’s surface, and then putting them on a sheet of paper known as a plan or map.
Surveying skills are helpful in various of engineering processes. Any engineering project necessitates the use of surveying.
Below are some of the most important aspects of surveying.
You’re just about to know why surveying is that important , keep reading :
Why is it important ?
The first phase in surveying is to draw out a plan and a portion of the region that will be surveyed.
Based on the nature of the project, the best potential alignment, quantity of earthwork, and other relevant information can be computed using these prepared maps and sections.
Surveying measurements are used in the planning and design of all Civil Engineering projects, including railways, highways, tunneling, irrigation, dams, reservoirs, waterworks, sewage works, airfields, ports, enormous structures, and so on.
So, to conclude, any project of any size is built along the lines and points specified by surveying during its implementation as an initiation to its success and full achievement in the best possible ways.
Surveying most common classifications:
In order to get more in depth with surveying, you need to know that its most common classifications are:
Plane surveying
Geodetic surveying.
Let’s start with the first one :
Plane surveying :
Plane surveying refers to surveying in which the earth’s mean surface is treated as a plane and the spheroidal shape is ignored.
Plane triangles include all triangles created by survey lines. All plumb lines are parallel and the level line is deemed straight.
We are only concerned with a small portion of the earth’s surface in everyday life, and the above assumptions appear reasonable in light of the fact that the length of an arc 12 kilometers long lying on the earth’s surface is only 1cm greater than the subtended chord, and that the difference between the sum of the angles in a plane triangle and the sum of those in a spherical triangle is only one degree.
And that was all you need to know about plane surveying for now.
Now, let’s jump into the second type of surveying which is « the geodetic surveying ».
Geodetic surveying :
Geodetic surveying the second method of surveying that takes into consideration the shape
of the earth.
The lines on the surface are all curved, and the triangles are all spherical.
As a result, spherical trigonometry is required to be able to master the different phases of
this kind of surveying.
All geodetic surveys entail labor at a bigger scale and with a high level of precision.
We can say that the goal of a geodetic survey is to identify the precise location on the earth’s surface of a set of widely separated sites that serve as control stations for less precise surveys.
To conclude, there are two types of surveying : the plane surveying and the geodetic one.
As a matter a fact, it’s up to the civil engineer to choose the best type based on each project he’s asked to accomplish.
The question now is :
Are there other ways to classify surveying ?
The answer is a massive YES and you’re about to discover some other ways of classifying surveying.
Classification based on nature of field:
There are three categories of surveying that are classified depending on the nature of the field:
Land surveying : which is divided into three categories: topographical survey, cadastral survey, and city survey.
It is concerned with natural and man-made characteristics on land such as rivers, streams, lakes, wood, hills, highways, trains, canals, towns, water supply systems, buildings and properties, and so on.
Marine surveying : This classification of surveying is also known under the name of hydrographic surveying, is simply the species and elements related to the water for the purposes of navigation, water supply, harbor construction, and mean sea level determination.
Measurement of stream discharge, topographic survey of coasts and banks, taking and locating soundings to establish water depth, and recording ocean tidal fluctuations are actually all part of the job we’re talking about.
Astronomical Surveying: This type of surveying allows a surveyor to determine the absolute location of any point on the earth’s surface, as well as the absolute location and direction of any line.
This entails making observations of celestial bodies like the sun or any fixed star. (this one is quite interesting).
Classification based on instruments used :
Surveying can be split into six groups based on the different types of instruments employed :
Surveying in a chain
Surveying using a compass
Surveying on a plane table
Surveying using a theodolite
Tacheometric surveying is a method of measuring the distance between two points.
Surveying using photographs
Methods used for classification: Or in other words, classification based on the method used .
Surveying can be classified into the following categories based on the methodologies used:
Surveying via triangulation
Surveying in a straight line
And … the last surveying classification , and my favorite one is :
Object-based classification:
There are four different forms of surveying based on the object:
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.
Compass surveying is the branch of surveying in which directions of survey lines are determined with compass and the lengths of the lines are measured with a tape or a chain. In compass surveying the direction of survey line called the bearing of line is defined as the angle made by the line with the magnetic meridian. In practice compass is generally employed to run a traverse. Traverse consists of series of straight lines connected together to form an open or closed traverse.
COMPASS
The commonly used instrument for compass surveying is Compass. A compass is small instrument which consists essentially of magnetic needle, a graduated circle and a line of sight. When the line of sight is directed towards a line, the magnetic needle points towards magnetic meridian and the angle which the line makes with the magnetic meridian is read at the graduated circle.
Compass consists of cylindrical metal box of about 8-12 cm diameter in the center of which is a pivot carrying a magnetic needle which is already attached to the graduated aluminum ring. The ring is graduated and is read by reflecting prism. Diametrically opposite to the prism is the object vane hinged to the box side carrying a horse hair with which the object in the field is bisected.
TYPES OF COMPASS
There are two forms of compass as under
Prismatic compass
Surveyors compass
The two compass are almost same except few differences so far as their construction is concerned. Prismatic compass uses WCB (0⁰-360⁰) circular ring while as the surveyors compass uses Quadrantal Bearing (0⁰-90⁰) circular ring system. Besides bearing system, the former has graduated ring attached to magnetic needle as the result of which when compass box and sight vane is rotated the needle remains stationary on the other hand in surveyors compass graduated ring being attached to compass box moves with it as the box is rotated
ADJUSTMENT OF COMPASS (WORKING)
Working of compass involves three steps:
CENTERING
LEVELLING
OBSERVING THE BEARING
Centering involves to align the compass is such a way the Centre is placed vertically over the station point. It is done with the help of tripod stand and is checked by dropping a small pebble below the Centre of compass.
Levelling is done so that the graduated ring swings quite freely. It is done with the help of ball and socket arrangement and can be checked by rolling a round-pencil on the compass box
Observing the bearing once centering and levelling has been done, raise or lower the prism until the graduations on the ring are clearly visible when looked through the prism. Afterwards turn compass-box until the ranging rod at the station is bisected by horse-hair of objective vane. At this position note down the reading.
COMPASS TRAVERSING
Whenever in traversing compass is used for making angular measurements, it is known as compass traversing or compass surveying. In compass traversing, the compass is used to determine the direction of survey lines of the framework of the traverse for measuring the angles which these lines make with the magnetic meridian.
The process of chaining and offsetting is the same as in chain surveying and running the check lines is not necessary. Compass traverse may be closed or open. Close traverse starts from one traverse station and closes either on same or on another traverse station whose location is already known. On the other hand an open traverse starts from one station and closes at other station whose location is neither known nor established.
EQUIPMENT USED IN COMPASS SURVEYING
PRISMATIC COMPASS
MEASURING TAPE
RANGING RODS
PLUMB BOB
CHAIN
CROSS STAFF
MEASUREMENT OF INCLUDED ANGLE
In a compass (prismatic) which uses Whole Circle Bearing (0⁰ to 360⁰) system the included angle is given by
Included angle = F.B of next line — B.B of previous line at same station
PRECAUTIONS
Compass surveying is used in case of rough surveys where speed and not accuracy is main consideration. One of the biggest disadvantages of compass is that magnetic needle which gives bearing of lines is disturbed from its normal position in presence of materials such as iron-pipes , current carrying wires , proximity of steel structures , transmission lines etc. called sources of local attraction.
The value of LiDAR lies in the fact that it virtually places you on your target site without having to leave the office. Combining imagery with LiDAR point cloud data is the next best thing to being there. Plus, it’s fast, accurate, and affordable.
2. Can I Collect Other Information While I’m Gathering LiDAR Data?
You name it — cameras, video imaging systems, multi-spectral and hyper-spectral imaging systems can all be mounted on and operated with most current LiDAR systems. Because you can operate these multiple systems using the same components, you’ll save time and money, a big benefit to corridor mapping projects such as transportation, pipeline, and transmission mapping.
Mobile mapping systems typically include two or more cameras within the system. One drawback: These passive systems require a light source, which means your collection times are lim-ited. Unlike LiDAR, they can’t see in the dark.
3. Do I Need Breaklines If I’m Using LiDAR?
Not necessarily. It all depends on the requirements of the product you’re generating. Typically, LiDAR is very good at defining the surface, as long as the sample spacing is ade-quate and there isn’t too much vegetation. Most features and terrain are very well defined in LiDAR data.
The rule of thumb relative to breakline usage comes down to edge recognition needs in the surface. If you have key elements, such as a back of curb line or a lip of gutter line, you will want to collect breaklines. If you’re only producing large-scale contour maps, you may not need the accuracy that breaklines provide.
4. Where Can I Find an End-to-End Solution for LiDAR Data?
Try Autodesk. Specifically, you’ll find an end-to-end solution in AutoCAD 2011, AutoCAD Labs, AutoCAD Civil 3D, Map 3D, and Navisworks products. Another robust data extraction solution for feeding all these applications with classified LAS and featurized GIS is the Virtual Geomatics solution.
5. Classical Photogrammetric Data Collection Works for Me — How Does LiDAR Compare?
LiDAR is about 40 percent less expensive than classical pho-togrammetric collection. And it takes less time to collect, process, and extract the needed information from LiDAR com-pared with traditional methods.
6. Is LiDAR Data Accurate Enough to Use on Road Overlay Projects?
You bet, as long as you use the appropriate collection method with the sufficient survey control. Just be careful when you pick your method and plan the control.
7. What Is Corn-Rowing?
You can’t eat it. The term corn-rowing refers to an artifact of LiDAR sampling that typically occurs at the edges of scans and overlapping data areas. It’s caused when LiDAR points are sampled close together and the difference in the sampled points is greater than the relative accuracy. With proper col-lection and filter processes that remove the points causing the trouble, you can minimize corn-rowing.
8. What Type of LiDAR Data Do I Really Need?
The best way to determine what LiDAR products you need is to really understand the application for the data. Call a qualified LiDAR collection agency for a recommendation on the appropriate collection methods based on your specific requirements. Here’s the info you’ll need to pass along:
✓ Accuracy requirements for data.
✓ Extraction requirements — do you just need points, or linear features, too?
✓ End products you’ll require. Do you need a triangulated surface, a classified LAS, and/or GIS features?
9. What Is an Intensity Image?
An intensity image is a monochromatic (shades of gray) image of the illumination (energy) returns from the LiDAR system. These images can be used for generating planemetric and breaklines by using LiDARgrammetry. The intensity image is typically a Geotiff, and the accuracy of the image is a function of the horizontal accuracy of the LiDAR, along with the inter-polation of the point to a raster image.
10. You Just Said LiDARgrammetry—What’s That?
LiDARgrammetry is the process of using intensity images to generate synthetic stereo pairs, much like the stereo pairs used in photogrammetry. The data generated from LiDARgrammetry tends to be only as accurate as the LiDAR from which it’s generated. There are varying opinions regard-ing the usefulness of this information and how accurate it is. Still, it’s a good byproduct of LiDAR, and whether it’s useful to you just depends on the scope of your project.
If you want to feel a lot more fluent in the language of LiDAR, you must know theses terms:
Repetition rate: This is the rate at which the laser is pulsing, and it’ll be measured in kilohertz (KHz). Fortunately, you don’t have to count it yourself, because these are extremely quick pulses. If a vendor sells you a sensor operating at 200 KHz, this means the LiDAR will pulse at 200,000 times per second. Not only does the laser transceiver put out 200,000 pulses, the receiver is speedy enough to receive information from these 200,000 pulses.
Scan frequency: While the laser is pulsing, the scanner is oscillating, or moving back and forth. The scan frequency tells you how fast the scanner is oscillating. A mobile system has a scanner that rotates continuously in a 360 degree fashion, but most airborne scanners move back and forth.
Scan angle: This is measured in degrees and is the dis-tance that the scanner moves from one end to the other. You’ll adjust the angle depending on the application and the accuracy of the desired data product.
Flying attitude: It’s no surprise that the farther the plat-form is from the target, the lower the accuracy of the data and the less dense the points will be that define the target area. That’s why for airborne systems, the flying attitude is so important.
Flight line spacing: This is another important measure for airborne systems, and it depends on the application, vegetation, and terrain of the area of interest.
Nominal point spacing (NPS): The rule is simple enough — the more points that are hit in your collection, the better you’ll define the targets. The point sample spac-ing varies depending on the application. Keep in mind that LiDAR systems are random sampling systems. Although you can’t determine exactly where the points are going to hit on the target area, you can decide how many times the target areas are going to be hit, so you can choose a higher frequency of points to better define the targets.
Cross track resolution: This is the spacing of the pulses from the LiDAR system in the scanning direction, or per-pendicular to the direction that the platform is moving, in the case of airborne and mobile systems.
Along track resolution: This, on the other hand, is the spacing of the pulses that are in the flight direction or driving direction of the platform.
Swath: This is the actual distance of the area of coverage for the LiDAR system. It can vary depending on the scan angle and flying height. If you’re flying higher, you’ll have a larger swath distance, and you’ll also get a larger swath distance if you increase the scan angle. Mobile LiDAR has a swath, too, but it is usually fixed and depends on the particular sensor. For these systems, though, you might not hear the word “swath;” it may instead be referred to as the “area of coverage,” and will vary depending on the repetition rate of the sensor.
Overlap: Just like it sounds. It’s the amount of redundant area that is covered between flight lines or swaths within an area of interest. Overlap isn’t a wasted effort, though — sometimes it provides more accuracy.
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.
Learn What Is Photogrammetry And Its Various Applications
In this article, we are presenting a brief introduction to what is photogrammetry and its various applications for those who are new to this technology.
What is Photogrammetry?
In a straightforward language, Photogrammetry is a technology that combines photography and geometry. It has a significant impact on the current architectural works.
As the name implies, Photogrammetry is a 3-Dimensional coordinate measuring method that makes use of photographs as the primary medium for measurement. The classical definition of the Photogrammetry is the simple process of deriving metric information about an object through measurements made on the photograph of that object.
Furthermore, photogrammetry is the science or the art of making measurements from the photographs. It means the measuring of features on photographs.
Photogrammetry uses the fundamental principles of triangulation called as Aerial Triangulation. In this method, a photograph gets snap from at least two different locations called “Line of sight,” and it can develop from each camera to points on the object.
The mathematical intersection of these lines can generate the 3D coordinates of the points of interest.
History of Photogrammetry
The Photogrammetry method was initially in use by the Prussian Architect in 1867 who designed some of the earliest topographic maps and some elevation drawings. The photogrammetry service in the topographic mapping is well-established but in the current scenario, the application of photogrammetry is common in the fields of architecture, engineering, forensic, underwater, medicine and much more for the production of accurate 3D data.
The term photogrammetry describes from the three simple words:
‘Photo’ – Light
‘gram’ – Drawing
‘metry’ – Measurement
“Photogrammetry means Light Drawing Measurement”
The output of this method is typically a map, drawings, measurement or a 3D model of some real-world objects. Many of the maps we are using are generated with the help of this technique, while the photographs are taken from the aircrafts.
Application of Photogrammetry
The categorization of the photogrammetry is based on the camera location during the actual photography. On these terms, we have Aerial Photogrammetry, Terrestrial Photogrammetry and Space Photogrammetry.
Now let’s understand each application of Photogrammetry in detail.
Aerial Photogrammetry
In this type of photogrammetry, the cameras are launch on a machine that flies aircraft and therefore takes pictures. These pictures are useful in generating the measurements. In this case, for the statistical comparisons, at least two photos of the same object or surface are clicked. This type of photography uses special design planes.
The aircrafts fly over a preset piece of land, pointed with a particular landmark. The camera speed is controlling accordingly to the speed of the plane. Also, the height of the plane from the land is initially defined. The stereo-plotter (an instrument that allows an operator to view two photos at once in a stereo view) processes the photographs. The photographs are also useful in automation processing for Digital Elevation Model (DEM) creation.
Terrestrial Photogrammetry
It is this kind of Photogrammetry technique in which the camera is usable in a stationary position, and hence photographs need to capture from a fixed, known position on or near the ground. The camera tilt and other specifications are in command. Photo Theodolite is a unique instrument that utilizes in exploring the photographs.
Space Photogrammetry
The space photogrammetry adapts all the aspects of extraterrestrial photography as well as measurements wherein the camera is non-moving on the earth or place on artificial satellite or in the space.
The term Photo Interpretation is applicable to that branch of photogrammetry wherein aerial or terrestrial photographs utilize to calculate, analyze, classify and interpret images of objects that are visible on the photographs. As a result, Photogrammetry is a combination of measurement and interpretation of a particular object.
Advantages of Photogrammetry
Photogrammetry has numerous advantages that are beneficial in modern construction as well as various other sectors like:
Covers large areas quickly.
The photogrammetry technique is cost-efficient.
The method is the easiest way to obtain or access information from the air.
The photographic images illustrate great details.
Application of Photogrammetry
To quickly verify the spatial positions of the ground objects.
To prepare topographical maps (surveying/mapping).
Helpful in Military/Artificial Intelligence.
For the interpretation of Geology/Archaeology.
Analysis of crop damages due to floods or other natural disasters.
To prepare a composite picture of ground objects.
To relocate existing boundaries of properties.
Helpful in the field of medicine.
The photogrammetry can generate a data set that will help many organizations or the stakeholders, therefore, helping to create most efficient and effective plan for any construction project.
