Information Technology Reference
In-Depth Information
While hand measurements can provide dimensions and position over a few
meters, it is impractical to extend this to larger objects; and collecting many
measurements (for example 1,000 or more) would be a laborious and, therefore,
unattractive process.
For objects with too much detail e.g. the façade of a gothic cathedral that
has a large amount of small stone carving elements, it is impossible to do the
measurements manually.
Photogrammetry and laser scanning can be used to provide a greater number of
measurements for similar object sizes, and, therefore, are suitable for more complex
objects. Photogrammetry and laser scanning may also be deployed from the air so
as to provide survey data covering much larger areas.
While GPS might be used to survey similarly sized areas, the number of points
it might be used to collect is limited when compared to airborne or even spaceborne
techniques. This advice and guidance is focused closely on laser scanning (from
the ground or air), although the reader should always bear in mind that another
technique may be able to provide the information required. Laser scanning, from the
air or from the ground, is one of those technical developments that enables a large
quantity of three-dimensional measurements to be collected in a short space of time.
The term laser scanner applies to a range of instruments that operate on differing
principles, in different environments and with different levels of accuracy. A generic
definition of a laser scanner, taken from Böhler and Marbs is: “any device that col-
lects 3D co-ordinates of a given region of an object's surface automatically and in
a systematic pattern at a high rate (hundreds or thousands of points per second)
achieving the results (i.e. three-dimensional co-ordinates) in (near) real time.”
The scanning process might be undertaken from a static position or from a mov-
ing platform, such as an aircraft. Airborne laser scanning is frequently referred to as
LiDAR, although LiDAR is a term that applies to a particular principle of operation,
which includes laser scanners used from the ground. Laser scanning is the preferred
generic term to refer to ground based and airborne systems.
Laser scanning from any platform generates a point cloud: a collection of XYZ
co-ordinates in a common coordinate system that portrays to the viewer an under-
standing of the spatial distribution of a subject. It may also include additional
information, such as pulse amplitude or colour information (RED, GREEN BLUE
or RGB values). Generally, a point cloud contains a relatively large number of co-
ordinates in comparison with the volume the cloud occupies, rather than a few
widely distributed points. Laser scanning is usually combined with colour digital
images (RGB) that are then used over the laser structure to provide a virtual texture
to the object making that the object becomes a “virtual reality” object.
21.1.2.1 When to Use Laser Scanning
In order for a heritage expert to decide if the use of laser scanning is appropriate
depends on various factors about the “What does the heritage object look like?”
or “How big is it?” For example, a conservator might want to know how quickly
a feature is changing, while an archaeologist might be interested in understanding
