"LiDar" SURVEYS

Light Detection And Ranging

We utilise the latest technology in the form of Lidar surveying methods were we offer the following unique capabilities that will assist our clients to maximize the benefits offered by LiDar scanning. Our clients can have the confidence that we will provide innovative technology with high accuracy and fast data capture, a proven survey process that ensures accuracy, completeness and consistency. In addition we provide unmatched experience for fast, safe and complete results, utilising the latest software to achieve maximum return on investment pertaining to our professional service.

How does it work?

Lidar Simultaneously....

· Measure trajectory of aircraft in three dimensions
· Measure orientation of laser scanner about three rotations axes
· Transmit and receive laser pulses, measure the time-of-flight (i.e. range to reflecting surface)

Consequently LIDAR

· Know the coordinates of thousands of points surveyed per second
· Map the surface of the Earth in high density

Click here to see a video of this service

Deliverables

Point Cloud

Comprised of millions of points collected from the Laser onboard the aircraft, the Point Cloud is combined with IMU and GPS data to ensure that every point in the cloud is within sub 10cm accuracy

Imagery

Hi resolution ortho-rectified photos are overlaid on the data to create extremely high detailed maps. Using a 39 Mega Pixel Medium format camera to ensure the highest resolution and quality.

Contours

A contour line joins points of equal elevation above a given level, such as mean sea level. A contour map is a map illustrated with contour lines, for example a topographic map, which thus shows valleys and hills, and the steepness of slopes. The contour interval of a contour map is the difference in elevation between successive contour lines.

DTM

Digital Terrain Model (DTM) is created by digitally removing all of the cultural features inherent to a DTM by exposing the underlying terrain. The quality of a DTM is a measure of how accurate elevation is at each pixel (absolute accuracy) and how accurately is the morphology presented (relative accuracy). Several factors play an important role for quality of DTM-derived products:

• Terrain roughness

• Sampling density (elevation data collection method)

• Grid resolution or pixel size

• Interpolation algorithm

• Vertical resolution

• terrain analysis algorithm

Uses for LiDAR Scanning

• 3D City Models
• DTM of Large areas
• Calculation of accurate Volumes over large areas, being much more cost effective than conventional surveys
• Updating of Statutory Mining plans
• Roads / Rail and Power line routes for Planning and Design
• Measuring of Carbon Credit
• Bulk property valuations

Please contact us should you want any more detail of the above

Technical Background of LiDAR

Technical Introduction 

Light Detection and Ranging (LiDAR) is a surveying technique which enables highly accurate, rapid and high-density three-dimensional mapping of large areas.

The Street and Air Mapper LiDAR system as used by ACS is superior to any other LiDAR system on the African continent, due to its very high scan rate and the low flying altitude, which yield point densities of up to five times that of older generation LiDAR systems. Our Riegl VQ450 scanner has and effective 360 Degree measurement rate of up to 550,000 points per second. A point cloud density of up to +-100 point per Square meter can be expected by flying at very low altitudes (+-30m AGL).

In addition, high resolution camera flown in conjunction with the LiDAR system yields full-colour, geo-referenced orthophotos at the highest possible resolution of up to +-1cm per pixel by flying at very low altitudes (+-30m AGL).

Surveying an area of interest using LiDAR and high-resolution photography would save the client time, money and in some cases the logistic requirement of providing surveyors with access to the area.

Mission Planning and Preparation 

Appropriate mission planning will ensure the highest possible accuracies of surveyed data.  The following factors are taken into account when deciding on the best time to conduct the survey:

• The GPS satellite constellation, which has a 23 hours and 56 minutes repeat cycle, needs to be taken into account to maximise the number of satellites available when conducting the survey.  The Position Dilution of Precision (PDOP) is a quantification of how accurately one would be able to measure positions at a given time.  ACS will conduct the survey during a time window, determined for the area of interest, during which PDOP is sufficiently low (typically below 3.5).
•  For safety purposes and to ensure data quality, the weather conditions are taken into account.  LiDAR surveys are not recommended in high winds (over 30 knots) or when visibility is poor.  Because of the lower flying altitude employed ACS (typically below 500 m), surveys can even be conducted during cloudy periods; however, very dense low-level fog will force surveying to be postponed until the weather improves.
• A GPS base station has to be set up during the aerial survey within 25 km of the survey area.  ACS uses 2 state-of-the-art Trimble R8 GPS receivers as base stations, which are operated on the ground during the aerial survey.  Collected data can then be used during post-processing to find accurate differential positions of the helicopter along its trajectory.
• The sun angle is taken into account when selecting an appropriate survey time window.  In order to obtain good orthophotos, surveys are typically conducted between 09:00 and 15:00 local time.

The accuracy obtained for measurements of the LiDAR system’s position along the aircraft trajectory should be within 2.5 cm horizontally and 3.5 cm vertically.  The obtained accuracies will be reported with the results.

Data Processing Workflow 

The process of deriving mapping products from LiDAR survey data can be summarised as follows:

• Trajectories are determined for the LiDAR system along the helicopter’s flight path.  Firstly, differential GPS data (relative to the GPS base stations) are processed using the NovAtel Waypoint software.  Thereafter, the GPS-derived trajectory is refined by incorporating inertial measurement unit (IMU) data in Leica’s IPAS Pro software.

• The Streetmapper software is used to combine the measured laser ranges with the positions (from differential GPS) and attitude (from IMU measurements) to produce a so-called laser point-cloud.  This point-cloud contains all the spatial information from the survey.

• All surveyed data will be in UTM or WGS84, and projected the projection specified beforehand by the client.

• TerraScan is used to classify all the point-cloud data, based on observed intensities and three-dimensional coordinates, to different classes, including ground, vegetation, buildings. etc.

• Contouring and DTM/DEM generation is achieved through the use of established processing techniques in the TerraModeler software.

• Orthophotos are merged and rectified using the TerraPhoto software.

• Quality checks are conducted by comparing the aerial survey data with ground control. Orthometric heights will, unless otherwise specified in this document, be determined by applying the EGM96 geoid model to the survey data and fitting it to local trigonometric beacons, if available and required.

The above-mentioned process will ensure the timely delivery of the client’s requested products to the highest possible degree of accuracy.

Personnel & Equipment 

The following staffs are typically made available on most of our projects:

• 1 Project Manager
• 2 Pilots
• 1 System Operator
• 6 Data Processors
• 1 Base Station Operators
• 1 Surveyor for Ground Control

Helicopter

Most of our International LiDAR projects are flown with a cost effective Robinson R44. The R44 is a single-engined helicopter with a semi-rigid two-bladed main rotor and a two-bladed tail rotor and a skid landing gear. It has an enclosed cabin with two rows of side-by-side seating for a pilot and three passengers. Tail rotor direction of rotation on the R44 is reversed. On the R44 the advancing blade is on the bottom.

Designed during the 1980s by Frank Robinson and his staff of engineers, the R44 first flew on March 31, 1990. The R44 Astro was awarded an FAA Type Certificate in December 1992, with the first deliveries taking place in January 1993. In January 2000, Robinson introduced the Raven with hydraulically-assisted controls and adjustable pedals. In July 2002, Robinson introduced the Raven II featuring a more powerful, fuel-injected engine and wider blades, allowing a higher gross weight and improved altitude performance

Gyrocopter

 It is the ideal easy-to-fly and stable gyroplane for those pilots who enjoy long flights in absolute safety. The Voyager has been developed starting from such needs and the result is a gyroplane unique in its category. Large and comfortable, this model maintains a simple and stylish design and features about 150 liters of available storage space for our equipment.

The characteristic baggage strakes on each side are well harmonized with the fuselage design and are accessible through three compartment doors with double locks. In-flight comfort is provided through a large cockpit that ensures good protection for both the pilot and the passenger, the seat padding and all the accurate details make flying an enjoyable and safe experience. The Voyager has a longer maximum range; the new 80 liters fuel tank can easily ensure up to 4 flying hours, with cruising speed between 120 and 150 km/h. The M22 Voyager is powered by 115HP turbocharged Rotax 914 and features Magni’s latest system innovations as the newest cooling and lubrication systems with heat exchanger and thermal expansion valve.

Our corridor and Local (SA, All Neighbouring Countries and as far as Zambia and Malawi) LiDAR projects are flown with a this very cost effective Magni M22 Gyrocopter. The reason for this is that the aircraft transported on a trailer, hereby reducing double mobilisation cost as our surveyors anyway have to drive to site to survey the ground control. Another advantage is that we do not have to wait for weather to clear before we mobilise or return to our home base, thus increasing delivery time.

Fixed-wing aircraft

While a helicopter-based survey is ideal for corridor mapping, ACS also employs fixed-wing aircraft to reduce costs to the client for large-scale topographic mapping and to make work in very remote parts of Africa possible.

Vehicle Mounted

The StreetMapper system comprises state of the art DGPS and IMU components combined in the TERRAcontrol system, a roof mounted laser scanner platform, and a rack-mount PC/instrumentation cabinet. A custom-designed power supply and operator workstation is provided for each system depending on the survey vehicle. One 12 megapixel geo-referenced camera is also included for documentation purposes. The system is extremely versatile and can be deployed on various vehicles:

• Typical van (usually with a high roof)
• 4x4 vehicle
• Quad bike (larger models)
• Boat
• Train
• Gyro Plane

StreetMapper uses 360 degree laser scanners, with a range of 800m. The scanner performs up to 550,000 measurements per second with a scanning rate up to 200 scans lines per second.  

Accuracy

The StreetMapper survey vehicle uses well-proven laser scanning technology to capture the position of up to 600,000 3D points per second whilst in motion (By driving the rout twice). The typical positional accuracy is 2-3cm (for good GPS conditions) and the point-to-point accuracy within the data is 1cm. We however use our surveyed control to adjust the Scanned point cloud to accuracies of less than 1cm.

LiDar

ACS uses two of the latest LiDar systems to be produced by the German Riegl and IGI manufacturers, which is also the newest LiDar equipment in South Africa.

LMS-Q560

The RIEGL LMS-Q560 is a revolutionary 2D laser scanner applying the latest state-of-the-art digital signal processing technique which meets the most challenging requirements in airborne laser scanning.
The RIEGL LMS-Q560 gives access to detailed target parameters by digitizing the echo signal online during data acquisition, and subsequent off-line waveform analysis. This method is especially valuable when dealing with difficult tasks, such as canopy height investigation or target classification.
The operational parameters of the RIEGL LMS-Q560 can be configured to cover a wide field of applications. Comprehensive interface features support smooth integration of the instrument into complete airborne scanning systems. The instrument makes use of the time-of-flight distance measurement principle of nanosecond infrared pulses. Fast opto-mechanical beam scanning provides absolutely linear, unidirectional and parallel scan lines. The instrument is extremely rugged, therefore ideally suited for the installation on aircraft. Also, it is compact and lightweight enough to be installed in small twin- or single-engine planes, helicopters or UAVs. The instrument needs only one power supply and GPS timing signals to provide online monitoring data while logging the precisely time-stamped and digitized echo signal data to the rugged accompanying digital data recorders RIEGL DR560 or DR560-RD. These high performance data storage devices are capable of handling the continuous high speed data stream. The Data Recorder DR560-RD, using two removable disks for smooth operation, supports RAID 1 to achieve higher data integrity and RAID 0 for increased data throughput. Additionally an online data integrity check is performed prior to transferring the full waveform data to the hard disks.

RIEGL VQ-450

The RIEGL VQ-450 is a very high speed, non-contact profile measuring system using a narrow infrared laser beam and a fast line scanning mechanism, enabling full 360 degree beam deflection without any gaps. It is characterised by a Laser Pulse Repetition Rate (PRR) of up to 550 kHz and a scanning rate of up to 200 lines per second. Multitarget capability based on echo digitization and online waveform analysis offers superior measurement capabilities even under adverse atmospheric conditions

Inertial Measurement Unit (IMU)

The CCNS4 (Computer Controlled Navigation System, 4th generation) is a guidance, positioning and management system for aerial survey flight missions.

The basic system consists of the Central Computer Unit (CCU), the 5'' TFT Command and Display Unit (CDU), a state-of-the-art GPS receiver with antenna, necessary cabling and a shock-absorbing mounting plate.The system is universally usable and can operate and integrate all common digital and analog aerial camera systems. Together with IGIplan, it provides a complete and comprehensive solution for mission planning, aircraft guidance and sensor management.The CCNS4 controls the camera and other sensors, including crab/drift setting(s), forward overlap, V/H computation and provides data for data annotation on film; the coordinates may be WGS 84 or the country's X/Y - coordinates. The CCNS4 has the benefit of a fully automated flight control system for aerial surveying and reconnaissance. A pilot's Control & Display Unit (primary) and an operator's Control & Display Unit (secondary) - both 5 inch TFT - are available. All operations are activated easily via one control dial and five buttons. The EFIS type display, which is operated like an aircraft instrument, is divided into guidance and system/sensor management information (right side of the TFT). The pilot merely has to "follow the line". CCNS4 features outputs with selectable sensitivity for HSI and CDI instruments.

The CCNS4 requires position and velocity information from a GPS receiver and optional directional information from the aircraft's directional gyro (DG). The CCNS4 can be operated by a variety of external GPS receivers that already may be installed in the aircraft by using the receiver specific data format or the NMEA 0183 data format. The integrated GPS receiver (DGPS) operates according to the RTCM-104 format and can receive real-time differential corrections from WAAS and EGNOS satellites. Directional gyro information is used by a Kalman filter process for stable position information and drift/crab calculation. Corrections for local variations and aircraft deviations can be used. AEROcontrol is IGI's GPS/IMU system for the precise determination of position and attitude of an airborne sensor. All operations and the management of the AEROcontrol system is controlled by the CCNS4. All raw data of the IMU are stored on the AEROcontrol system. The software uses a forward/backward Kalman filter algorithm to achieve optimal results. The CCNS4 is able to control up to two sensor systems. The actual flight data - including the aircraft's position in WGS 84 coordinate or local grid system - are computed and can be provided for data annotation on film. Waypoint/photo data, flight information and GPS positions are stored and transferred to the CCNS4 Mission Card for post processing, analysis and plotting of the flight index or the complete mission. The system has the advantage of no mechanical (moving) parts, no hard or floppy disk to crash or wear out from dust, humidity, acceleration or vibration. More than 300 installations - worldwide - show that the CCNS4 is a very reliable system. Using the CCNS4, no specialized photo pilot or photo navigator is required.

GPS Base Stations 

The Trimble R8 GPS System is a multi-channel, multi-frequency GPS receiver, antenna, and data-link radio combined in one compact unit, with the following features

• Advanced Trimble Maxwell™ Custom Survey GNSS Chip;
• High precision multiple correlator for GNSS pseudorange measurements;
• Unfiltered, unsmoothed pseudorange measurements data for low noise, low multipath error, low time domain correlation and high dynamic response;
• Very low noise GNSS carrier phase measurements with <1 mm precision in a 1 Hz bandwidth;
• Signal-to-Noise ratios reported in dB-Hz;
• Proven Trimble low-elevation tracking technology; and
• 72 Channels: GPS L1 C/A Code, L1/L2 Full Cycle Carrier.

Quality 

The following steps are taken to ensure that the client’s products meet the set requirements:

• All internal offsets within the LiDAR system are to be calibrated twice a year, as per the Street/Air Mapper system specifications.  ACS’s last system calibration was done in October 2011 during which vertical accuracies, compared to surveyed ground control points, of < 2.5 cm were obtained.  Furthermore, the system’s lever-arm measurement (offset between the GPS antenna and the system’s internal reference point) was surveyed to sub-centimetre level and certified by an independent registered professional surveyor in October 2011.
• To ensure the highest possible relative accuracies, a flight line perpendicular to the lines covering the area of interest, is to be flown at the end of the survey.  Overlapping data should correspond to each other at levels within the product specification.
• A further quality check is incorporated in the data processing algorithm, by manually inspecting data in the TerraScan software, to check for any un-calibrated offsets between data from different flight lines.
• The obtained point cloud could be compared to a sufficient number of ground control points (GCPs) which are surveyed onsite before or after the actual LiDAR survey, thereby ensuring the accuracy and quality of the data.

Although all steps possible will be taken to ensure data accuracies, ACS will not be liable for any inaccuracies due to inaccuracies in the trigonometric network used.

Aircraft safety

All legislation regarding aviation will be strictly adhered to during the survey, to ensure the safety of the survey team, as well as lives and property around the survey site.

Eye safety

Our Street and Air Mapper system is eye safe as an FDA certified Class 1 laser

Safety Plan

A comprehensive safety file can be made available once the proposal has been accepted and all the project details have been transferred to us.

Example Data from Previous Surveys 

Low altitude LiDAR scan of substation

Low altitude LiDAR scan of earth wire

Zoom of Low altitude LiDAR scan of earth wire

Orthophoto – 10 cm resolution

Orthophoto ZOOM of above image – 10 cm resolution