Multi Scale Image Based Methods

Traditional Photogrammetry

Image-based methods can be considered as the passive version of SL. In principle, image-based methods involve stereo calibration, feature extraction, feature correspondence analysis and depth computation based on corresponding points. It is a simple and low cost (in terms of equipment) approach, but it involves the challenging task of correctly identifying common points between images. Photogrammetry is the primary image-based method that is used to determine the 2D and 3D geometric properties of the objects that are visible in an image set.

The determination of the attitude, the position and the intrinsic geometric characteristics of the camera is known as the fundamental photogrammetric problem. It can be described as the determination of camera interior and exterior orientation parameters, as well as the determination of the 3D coordinates of points on the images. Photogrammetry can be divided into two categories. These are the aerial and the terrestrial photogrammetry.

In aerial photogrammetry, images are acquired via overhead shots from an aircraft or an UAV, whilst in terrestrial photogrammetry images are captured from locations near or on the surface of the earth. Additionally, when the object size and the distance between the camera and object are less than 100m then terrestrial photogrammetry is also defined as close range photogrammetry. The accuracy of photogrammetric measurements is largely a function of the camera’s optics quality and sensor resolution. Current commercial and open photogrammetric software solutions are able to quickly perform tasks such as camera calibration, epipolar geometry computations and textured map 3D mesh generation. Common digital images can be used and under suitable conditions high accuracy measurements can be obtained. The method can be considered objective and reliable. Using modern software solutions it can be relatively simple to apply and has a low cost. When combined with accurate measurements derived from a total station for example it can produce models of high accuracy for scales of 1:100 and even higher.

3-NEW Overlapping area of images captured at A and B are resolved within the 3D model space to enable the precise and accurate measurement of the model.

Semi Automated Image Based Methods

In recent times, the increase in the computation power has allowed the introduction of semi automated image-based methods. Such an example is the combination of Structure-From-Motion (SFM) and Dense Multi-View 3D Reconstruction (DMVR) methods. They can be considered as the current extension of image-based methods. Over the last few years a number of software solutions implementing the SFM-DMVR algorithms from unordered image collections have been made available to the broad public. More specifically SFM is considered an extension of stereo vision, where instead of image pairs the method attempts to reconstruct depth from a number of unordered images that depict a static scene or an object from arbitrary viewpoints.

Apart from the feature extraction phase, the trajectories of corresponding features over the image collection are also computed. The method mainly uses the corresponding features, which are shared between different images that depict overlapping areas, to calculate the intrinsic and extrinsic parameters of the camera. These parameters are related to the focal length, the image format, the principal point, the lens distortion coefficients, the location of the projection centre and the image orientation in 3D space. Many systems involve the bundle adjustment method in order to improve the accuracy of calculating the camera trajectory within the image collection, minimise the projection error and prevent the error-built up of the camera position tracking.

SMF1-WEB Diagram illustrating the principles of structure from motion (SFM) measurement from multiple overlapping images.

Long Mid Range Techniques

Time of Flight (TOF)

Time-Of-Flight (TOF) - also known as terrestrial laser scanning - is an active method commonly used for the 3D digitisation of architectural heritage (e.g. an urban area of cultural importance, a monument, an excavation, etc). The method relies on a laser rangefinder which is used to detect the distance of a surface by timing the round-trip time of a light pulse. By rotating the laser and sensor (usually via a mirror), the scanner can scan up to a full 360 degrees around itself.

The accuracy of such systems is related to the precision of its timer. For longer distances (modern systems allow the measurement of ranges up to 6km), TOF systems provide excellent results. An alternative approach to TOF scanning is Phase-Shift (PS), also an active acquisition method, used in closer range distance measurements systems. Again they are based on the round trip of the laser pulse but instead of timing the trip they measure the wavelength phase difference between the outgoing and return laser pulse to provide a more precise measurement.

Diagram illustrating the principles of time of flight (TOF) measurement devices.

Airborne Laser Scanning

In addition to ground based systems Airborne lidar (light detection and ranging), also known as Airborne Laser Scanning (ALS) has also been used as a data capture technique.

A lidar system is based on sending out thousands of laser pulses per second. The laser pulses are reflected by vegetation or buildings or soil. By measuring the time that passes while the light travels from the airplane to the ground and back, the distance between airplane and ground can be measured. The current position of the airplane and the direction into which each laser pulse was sent are measured using GPS and inertial navigation units. By combining millions of such measurements, a very detailed three-dimensional model of the earth surface can be computed.

However the true power of lidar mapping systems is that some laser light is reflected from the vegetation canopy, but some reaches the ground – the scanner onboard the airplane receives multiple reflected signals. To visualise the ground surface beneath the canopy the data needs to be filtered. For every pulse that was sent out, only the last reflected signal - the one that has travelled the longest way - is chosen. Some laser pulses do not reach the ground at all. They can be identified and discarded by comparing neighbouring points. The result is a three-dimensional model of the ground surface from which the bushes, hedges and trees have disappeared. Barrows, hollow ways, mining pits or former field structures become visible in this virtually de-forested landscape and can be mapped by archaeologists.

Schematic principles of data acquisition in Airborne Laser Scanning.

Short Range Techniques

Laser Triangulation (LT)

One of the most widely used active acquisition methods is Laser Triangulation (LT). The method is based on an instrument that carries a laser source and an optical detector. The laser source emits light in the form of a spot, a line or a pattern on the surface of the object while the optical detector captures the deformations of the light pattern due to the surface’s morphology. The depth is computed by using the triangulation principle. Laser scanners are known for their high accuracy in geometry measurements (<50µm) and dense sampling (<100µm). Current LT systems are able to offer perfect match between distance measurements and colour information. The method being used proposes the combination of three laser beams (each with a wavelength close to one of the three primary colours) into an optical fibre. The acquisition system is able to capture both geometry and colour using the same composite laser beam while being unaffected by ambient lighting and shadows.

















Diagram illustrating the principles of laser triangulation (LT) based range devices.

Structured Light (SL)

Structured Light (SL) - also known as fringe projection systems - is another popular active acquisition method that is based on projecting a sequence of different alternated dark and bright stripes onto the surface of an object and extracting the 3D geometry by monitoring the deformations of each pattern. By examining the edges of each line in the pattern, the distance from the scanner to the object’s surface is calculated by trigonometric triangulation. Significant research has been carried out on the projection of fringe patterns that are suitable for maximizing the measurement resolution. Current research is focused on developing SL systems that are able to capture 3D surfaces in real-time. This is achieved by increasing the speed of projection patterns and capturing algorithms.


Diagram illustrating the principles of structured light (SL) measurement devices.


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