What is site survey

What is Lidar & Working with Photogrammetry

LiDAR technology competes with photogrammetry, but also complements it - this is how it works.

Contrary to popular perception, lasers are not just devastating rays. Lasers are one of the most precise ways to measure distance. This also includes LiDAR, which is becoming more and more accessible in consumer markets. The Pix4D technology is about measuring with the help of images, but LiDAR can also be integrated into the workflow. Here you will learn how LiDAR differs from photogrammetry, but also how both can work hand in hand.

What is LiDAR?

LiDAR is an acronym for "Light Detection and Ranging", also known as 3D laser scanning. LiDAR was first introduced in the 1960s1 It has been used for years and is used to measure distances on the basis of laser light. It works by illuminating a target object or a room with a laser. From the light travel time back to the sensor, distances can be measured with high accuracy.

The detailed point cloud of a LiDAR (picture by Daniel L. Lu)

3D laser scanning is already used in a variety of industries, with applications ranging from mobile to terrestrial and airborne data acquisition projects. Since laser scanning measures distances precisely, it is very suitable for creating digital 3D reproductions of rooms, objects and landscapes. In this aspect, it can rival photogrammetry as a tool for digitizing reality. LiDAR methods generate a georeferenced and uncolored 3D point cloud. This dense point cloud is very detailed and often captures small objects such as cables or wires that photogrammetry cannot easily detect. However, LiDAR can be expensive to perform, and it is often necessary to use a specialized laser scanning company to collect the data.

What is photogrammetry?

Photogrammetry is the process of measuring using images. The recorded, overlapping images are processed in order to generate precise models of the environment. Its main purpose is to digitize reality for surveying and mapping. Early photogrammetry relied on high-altitude viewpoints or hot air balloons to collect aerial imagery. Nowadays, in addition to image capturing close to the ground, more and more drones are used to capture survey data. Photogrammetry is used in a variety of industries with a tremendous range of applications, from forensic crime scene mapping to use in precision agriculture.

What is the difference between LiDAR and photogrammetry?

Firstly, LiDAR has a big advantage over photogrammetry: it generates its own light. This means that it is not affected by weather conditions such as cloud cover and changing lighting conditions, which can severely disrupt the data acquisition from the air for photogrammetry. Both terrestrial and 3D laser scanning from the air benefit from this, while photogrammetry with drones or handheld cameras is impaired in poor lighting conditions.

Second, LiDAR can penetrate the spaces between the trees and capture small details. The laser pulse looks through the leaves and provides a measurement directly to the tree trunk or the ground under the tree, while photogrammetry relies on photos and only reconstructs what is visible on the surface.

However, LiDAR is often expensive. With photogrammetry, a wide range of levels of precision can be achieved through the use of a large number of cameras. Depending on the requirements, also with consumer cameras, and not exclusively with highly specialized professional devices.

In addition, photogrammetry benefits from a range of output formats, including colored point clouds, textured meshes, and orthomosaics, while LiDAR only generates a point cloud.

How can photogrammetry and LiDAR work together?

LiDAR can be used as an alternative to photogrammetry to perform similar tasks, e.g. B. to reinforce their results. Using both techniques in combination can capture details that aerial photogrammetry might have missed.

Pix4Dsurvey can import and process billions of points from different sources in order to realize hybrid projects with LiDAR and photogrammetry.

One example of this is Pix4Dsurvey, the new vectorization software that closes the gap between photogrammetry and CAD. The software enables users to merge point cloud data from different sources, including LiDAR and photogrammetry. The combined reference to the original images and the point cloud makes the vectorization and extraction of points, polylines, polygons and overhead lines more precise and faster than ever before. For complicated projects, e.g. With, for example, narrow objects like power lines, LiDAR can help fill in the gaps that photogrammetric data acquisition methods may overlook. Pix4Dsurvey can process photogrammetry data in combination with terrestrial laser scanning.

A point cloud in Pix4Dsurvey that combines LiDAR and photogrammetry data

During vectorization, an area is broken down into polylines and polygons in order to define points of the geometry. As a result, the final vector files derived from large point cloud data from photogrammetry and LiDAR are engineering grade.

Photogrammetry and LiDAR in action

LiDAR and photogrammetry point clouds are often massive files made up of billions of points. The manipulation and smooth handling of this data is a challenge that Pix4Dsurvey easily masters.

In the following example, a drone was used to collect aerial photos of a railway line in Switzerland. The point cloud generated with the help of photogrammetry captured many details of the scene, but not the specifics of the cables on the track or the metal posts that support them.

The LiDAR data (overlaid in green) capture the details that were overlooked during photogrammetry

The LiDAR data recorded these details and filled in the areas that the drone images hadn't captured. As a result, a more complete model of the situation was created, revealing more information.

What's next?

Devices equipped with LiDAR are becoming more widespread, making the technology available to all by transforming popular consumer products into devices capable of precise 3D modeling. This is particularly the case with the iPad Pro 2020 and the iPhone 12 Pro, for which the Pix4Dcatch app has been optimized.

The workflow for terrestrial 3D scanning with Pix4Dcatch is simple: hold up the iOS mobile device and walk around the area of ‚Äč‚Äčinterest. Scanning is done with real-time 3D meshing and gives live feedback on the recording of the images and the completion of the scene. Pix4Dcatch records the GPS and IMU data and makes it possible to create scaled and georeferenced 3D outputs with Pix4Dmapper or Pix4Dcloud.

Pix4Dcatch enables the creation of precise 3D models with a handheld device

The arrival of LiDAR in the consumer market is an exciting development for 3D modeling. The precise resolution of 3D laser scanning in combination with the flexibility and diverse applications of photogrammetry enables an ongoing revolution in the professional surveying and mapping industry and ensures easier access to important information - and with greater accuracy than ever before.