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Мобильное лазерное сканирование

Mobile LiDAR scanning

Mobile LiDAR Scanning

Spatial data acquisition for road monitoring and certification, design of traffic management and restoration projects.

1. Mobile LiDAR Scanning

A typical task in the operation of the road network is the development of traffic management projects and the creation of technical passports for motorways. To solve them, it is necessary to collect accurate spatial data. As a rule, the most effective way to obtain such information is aerial photography. However, there is an alternative approach - mobile laser scanning. This method can be used in conjunction with aerial photography, or independently, if aerial photography is not possible.

Mobile laser scanning (MLS) is performed on a vehicle with a laser scanner and satellite geodetic equipment installed. Thus, the resulting cloud of laser reflection points has a precise geographic reference. Examples of MLS and aerial photography results are shown in the illustration below. On the left is a fragment of an orthophotoplan, on the right is the same area in the form of a dense point cloud.

1. Mobile LiDAR Scanning

A typical task in the operation of the road network is the development of traffic management projects and the creation of technical passports for motorways. To solve them, it is necessary to collect accurate spatial data. As a rule, the most effective way to obtain such information is aerial photography. However, there is an alternative approach - mobile laser scanning. This method can be used in conjunction with aerial photography, or independently, if aerial photography is not possible.

Mobile laser scanning (MLS) is performed on a vehicle with a laser scanner and satellite geodetic equipment installed. Thus, the resulting cloud of laser reflection points has a precise geographic reference. Examples of MLS and aerial photography results are shown in the illustration below. On the left is a fragment of an orthophotoplan, on the right is the same area in the form of a dense point cloud.

afs_mls

Let's compare both methods and name their main features.

Aerial surveying

  • Performance of up to 150 km of road with one UAV per day
  • Orthomosaic with all road infrastructure elements captured in high-detail (<10 cm per pixel)
  • Spatial positioning of any object down to 10 cm

Mobile LiDAR scanning

  • Performance of 100-200 km with one car per day
  • Dense LiDAR point cloud, covering everything within a 150 m radius from the scanner
  • Spatial positioning of any object down to 5-7 cm

Among other differences, it is worth noting the following advantages of MLS. There is no need to obtain flight permits. It is possible to do scanning at night. You get accurate elevation of objects and slopes. However, this technology also has some limitations. The width of the capture strip is 80-150 meters and depends on the surrounding terrain. Data processing is more complex and time-consuming than with aerial surveying. As a result, one kilometer of MLS surveying costs approximately twice as much as aerial surveying.

Let's compare both methods and name their main features.

Aerial surveying

  • Performance of up to 150 km of road with one UAV per day
  • Orthomosaic with all road infrastructure elements captured in high-detail (<10 cm per pixel)
  • Spatial positioning of any object down to 10 cm

Mobile LiDAR scanning

  • Performance of 100-200 km with one car per day
  • Dense LiDAR point cloud, covering everything within a 150 m radius from the scanner
  • Spatial positioning of any object down to 5-7 cm

Among other differences, it is worth noting the following advantages of MLS. There is no need to obtain flight permits. It is possible to do scanning at night. You get accurate elevation of objects and slopes. However, this technology also has some limitations. The width of the capture strip is 80-150 meters and depends on the surrounding terrain. Data processing is more complex and time-consuming than with aerial surveying. As a result, one kilometer of MLS surveying costs approximately twice as much as aerial surveying.

2. Mobile LiDAR Scanning Results

The resulting point cloud allows you to determine:

 

 

  • roadway boundaries
  • road contours within the berm edges
  • fills and mounds edges and their heights
  • junction and side road positions
  • road sign locations
  • barrier placement
  • lighting positions
  • placement of traffic lights
  • public transport stops
  • sidewalks and footpaths
  • road service facilities
  • and other elements of the road infrastructure
point_cloud_MLS_square

Accurate maps of roads and surrounding areas can be generated based on mobile LiDAR scanning data.

road_map_MLS_square

It is possible to create technical road documentation too:

road_passport

2. Mobile LiDAR Scanning Results

The resulting point cloud allows you to determine:

 

  • roadway boundaries
  • road contours within the berm edges
  • fills and mounds edges and their heights
  • junction and side road positions
  • road sign locations
  • barrier placement
  • lighting positions
  • placement of traffic lights
  • public transport stops
  • sidewalks and footpaths
  • road service facilities
  • and other elements of the road infrastructure
point_cloud_MLS_square

Accurate maps of roads and surrounding areas can be generated based on mobile LiDAR scanning data.

road_map_MLS_square

It is possible to create technical road documentation too:

road_passport
Съёмка горного участка

Survey in the mountains

Survey in the mountains

The survey in a mountainous area with 500 meters of height difference was carried out literally in one day.

Based on the survey results, we created survey grade 3D model of the slope, XYZ point cloud and contour lines with the step of 1 meter.
Check out the result below!

Geoscan 401
Geoscan 401
Camera
metashape
3D model
3D model
Съемка с беспилотника в зимнее время и отметки высот

UAV surveying in winter and the elevation points

UAV surveying in winter

 and the elevation points

Why winter surveying is different.

We are often asked about the topographical drone surveys in winter. After all, the main essence of this technology is to obtain the surface model and extract the elevation points remotely. An aircraft or a quadcopter takes photographs of the area while simultaneously recording the geodetic coordinates of the image centers. After that, photogrammetric data processing is performed and an orthomosaic of the surveyed area is produced along with the DEM. The orthomosaic is used to draw a topographic plan, and the height map (DEM) is used to obtain elevation points. Since all data is based on the images, all derivative materials will only have the information that was captured on the images. Thus, two questions are always asked regarding the winter time: how do snow cover and forest affect the final materials?

When it comes to snow, the answer is clear - photogrammetric software builds the surface of the snow, meaning that we can only obtain elevation data from the snow's surface. If the distribution of snow is relatively uniform, we can simply subtract the thickness of the snow cover. However, when it comes to trees, the question is more complex and is related to the algorithms of the photogrammetric software. If you fly not too close to the ground, the program may not "see" small elements, such as branches, tree trunks, and wires, when constructing the digital elevation model. In fact, if a deciduous forest is photographed without leaves in winter, we will only get marks of snow cover on the site instead of trees.

Survey example, described by a survey engineer

The aerial surveying performed with a Phantom 4 Pro PPK drone from https://uav-design.com/ru/

phantom_PPK

Photogrammetric processing performed using the Agisoft Metashape software.

The site was a snow-covered field with longitudinal and transverse ditches, partially overgrown with willow 2 to 7 meters high. Compared to classical geodetic survey, when topographical survey of a site is carried out by a surveyor, unmanned aerial technologies provide better accuracy for this type of terrain. Why?

Firstly. The accuracy of the terrain is generally higher, since the flight materials allow the automatic reconstruction of a 3D model from a dense point cloud (ranging from 100 thousand to tens of millions of points, depending on processing requirements). On this particular site, the thickness of the snow cover was uniform. It was measured at several control points, and did not exceed the average statistical error of classical surveying. Also, with classical geodesy, the surveyor will take measurements at points with the density of 10-20 meters, or only measure the key points of the relief. Thus, the resulting surface model will be much less detailed, compared to the aerial survey outcome.

Here is an example of an orthomosaic and a DEM of the site:

ortho-dem-field

Secondly. The contours of the boundaries of ditches, cliffs, slopes and other key elements of the relief, visible on a detailed surface, allow one to determine the “fracture line” more accurately and draw it on the topographic plan. In field conditions, the surveyor does not always see a clear boundary of the "relief fracture".

ortho-dem-field2

Thirdly. The Agisoft Metashape omits the trees and bushes from the terrain reconstruction, when processing the materials of a winter survey with low forest density (overgrown ditches in this example). In other words, it does not see these elements when creating a height map.

The elevation points obtained using Metashape are almost identical to the control with field measurements in areas of tree vegetation.

elevation_points
ЛЭП, отметки изоляторов и провис проводов. Проверка точности.

Power lines, insulator marks and wire slack. Checking the accuracy.

Power line monitoring

Insulator heights and wire slack. Checking the accuracy.

1. Power line inspection with drones

The survey and inspection of power lines with unmanned aerial vehicles have become quite widespread. Survey data are used for cadastre, engineering and surveying. Quite often the question arises: is it possible to obtain elevation marks of power lines, sags, wire dimensions and other metric characteristics using a UAV? And with what precision?

The main sensor of the UAV is a camera. The 3D image is built from overlapping photographs and information about the exact centers of photography. But as always, there are nuances. After processing a set of images of power transmission lines in photogrammetric software, you will not see supports and wires in 3D. The most that you can get is a dense cloud. Unfortunately, only a few of the many supports will be reconstructed properly. See example below:

LEP_point_cloud1

Power lines are too thin to be reconstructed in 3D by photogrammetric software. The algorithm simply can't detect the long "thread" several pixels wide. In this case, computer vision comes to the rescue and there are multiple technologies for automated wire detection. Despite not having a large team of programmers, we have developed a technology for accurately detecting and reconstructing the position of power lines using standard tools. This allows us to obtain cable and ground height in any location with high precision, and to tackle related tasks such as determining wire dimensions, sag, turning angles etc.

2. Control measurement of wire heights

In order to check the accuracy of the wire height detection we surveyed a small section of a power line. First, we set up ground control points and obtained their coordinates using the EFT M3 GNSS in RTK mode. Using the AR600E high-voltage cable height meter, we measured the height of the bottom wire at each control point.

cable_height

This way we collected the control wire measurements using the instrumental method. Next, we conducted aerial survey of the site and determined the height of the wire above the control point using the photogrammetric method. Last step was to compare the instrumental measurements with the aerial survey based data. The aerial survey was performed with a Phantom 4 Pro v2 PPK survey grade quadcopter from uav-design.com

uav-d_copt

3. Data processing and accuracy assessment

Coordinates of image centers were processed in Magnet Tools software. Photogrammetric processing was carried out in Metashape. Images were captured from 100 meters altitude in two passes. Survey technology with a geodetic drone from uav-design allows to consistently obtain high accuracy results. For our flight, we got average camera alignment error within 2 cm. The smaller the error in the relative position of the cameras, the more accurate the wire position will be determined:

2019-08-12_11-33-52

The root mean square error at ground control points was also several centimeters:

LEP_err_opo

Using our own technology, the power line heights were determined at the control point locations. A total of 5 measurements were taken with the photogrammetric method at different sections of the wire.

LEP_points_wires3

The final table below shows the difference in the height of the wires measured by the instrumental and photogrammetric method. The maximum deviation was 12 cm, the average deviation was 6 cm.

2019-08-12_14-00-18

Conclusion: Photogrammetric survey of power transmission lines from drones using the technology developed by Aerostream allows calculating wire marks with an accuracy of about 5-12 cm, compared to instrumental survey.

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