Theodolite. It is mainly used to measure vertical and horizontal angles. Distance meter

33. How many instruments do you know that belong to numerical acquisition systems? Can you describe their main characteristics?
-Theodolite. It is mainly used to measure vertical and horizontal angles.
-Distance meter. Fundamentally, it works with electromagnetic energy. In the process there are instrument sends (Emitter) the pulse to object (Reflector) and part of the energy is absorbed by instrument (receiver) again to measure the distance. 2D=c*dt>>D=c*dt/2
-TOTAL STATIONS: it is combination of electronic theodolite, electronic distance meter and firmware. Total stations have the ability to record measurements and angles to data recorders or cards which can be processed using computer software upon return from the survey. For optimal accuracy, they are used with a prism which reflects light waves back to the measurement device to determine a measurement
-GPS – Global Positioning System. It measures 3D locations on earth surface with the aid of satellites. GPS consist of orbit satellites, earth control stations, receivers. GPS components are space component, control component, user component. Generally, at least two GPS stations should be used, one on a point of known coordinates (reference receiver or "master station") and the other ("rover station") on the points whose 3D coordinates are to be obtained, in same system of the known point. The two stations must "see" the same 6 satellites at least at the same time, in order to obtain the differential correction
-Laser scanner. A 3D scanner can be defined as an instrument capable of recording three-dimensional coordinates of distributed points on a portion of the surface of an object, automatically and with high density. It is an optical-mechanical device capable of emitting an electro-magnetic pulse, the … laser (Light Amplification by the Stimulated Emission of Radiation) and of receiving the reflected signal, measuring the elapsed time interval and, therefore, the distance between the instrument and the detected point.The laser is directed by a mechanism of rotating and oscillating mirrors which, with the variation of the azimuth and zenith angle, illuminate the object to besurveyed in some points, according to a predefined grid (resolution). This system works by measuring even thousands of points per second forming point clouds models. For each measurement, the system provides, in addition to the position of the point in space (x, y, z coordinates), also the reflectance value (the reflective power of a body subjected to radiation, therefore the quantity of light returning to the instrument after hitting the object). Some laser scanners, equipped with internal or external cameras, can also associate the corresponding RGB color value to the detected point.
34. What’s the main difference between how a laser scanner works and all the other instruments work?
There are 3 main differences:
-With Laser scanner we can obtain huge number of points
-In Laser scanner we cannot choose one specific point, in automatic way we choose many points
-In addition to the position and space, in one singe Laser scan instrument can obtain reflectance value and color value (We can take photos)

35. Why do we talk about “massive acquisition” of data when we use a laser scanner? What can we control and what not?

Because, thanks to laser scanner we can obtain up to million points, with their X, Y, Z relative coordinates, reflectance value and RGB color value.
In laser scanner what we can control:
-number of points with regulating dimension of horizontal and vertical angular steps;
-the position of Laser scanner
-final resolution, number of points
What we cannot control is that surveying specific point on an object.

36. Speaking of how laser scanning works, what’s the “shadow cone” and what’s a “shadow area”?

Instrument does not cover a vertical angle of 360 degrees since the base of the instrument usually rests on the ground or on a tripod, so an absence of light pulse in the angular sector below the instrument itself is "shadow cone". Absence of points in the object is called “shadow area”. Final system of result is spherical grid.
37. Speaking of how laser scanning works, what are “internal form factors” and “external interference factors” and what do they generate?
The choice of the scanner position has the purpose of "touching" as much surface as possible of the object and therefore reducing the "shadow areas", avoiding gaps in the point cloud.
“Internal form factors” and “external interference factors” generate the “shadow area” on to be surveyed object.
Internal form factors - relating to the formation of the object to be surveyed, therefore depend on the geometry of the object and the positioning of the instrument in reference to these geometries.
External interference factors -Related to the presence of natural or artificial obstacles that are between the instrument and the object to be surveyed.
38. Speaking of no-contact survey with laser scanner, what can be done to fill in the shadow areas in a single scan?
In order to fill the point gap on the object we need to choose several optimal positions for Laser scanner.
39. Speaking of no-contact survey with laser scanner, what’s the instrumental resolution?
instrumental resolution is an aspect that can be controlled by the operator, who sets either the values of the horizontal and vertical angles on a sphere with a given radius, or the number of points he wishes to obtain. However, these settings do not give in a homogeneous result, because the distances of the points on the horizontal planes are inhomogeneous.
40. Speaking of no-contact survey with laser scanner, why is the spherical grid not an homogeneous reference system?
Because in Spherical grid we have equal meridians but different parallels. Any resolution set on the sphere involves an intensity of equal points on the meridians but different intensity on the parallels with a thickening of points the closer one gets to the pole.
41. Speaking of no-contact survey with laser scanner, what’s the real resolution? What does the real resolution depend on?
Real resolution is number of points on an object, while instrumental resolution is number of intersected points on spherical grid.
Real resolution depends on:
-the distance between the station (the scanner) and the object itself
- the angle with which the laser beam hits the surface
- the geometric conformation of the surfaces.
42. Speaking of no-contact survey with laser scanner, what’s the difference between the instrumental resolution and the real resolution?
When we talk about instrumental resolution it is the numbers we set on an instrument and number of intersections on spherical grid, while real resolution is about number of points on an object.
Example: if we set following setting to an instrument:
spherical grid with 10 m radius; points on every cm on horizontal and vertical resolutions.
Which gives us 2πr = 2 * 3,14 * 10m = 62, 8m nHp= 62, 8 / 0,01m = 6280 points on Horizontal
In vertical resolution, length of half meridian, the angle is 152.5 (shadow cone is taken into consideration)
number of Vertical points = [nVp(2π) * 152,5] / 360 = [6280points * 152,5] / 360 = 2660points
in total 6280*2660= 16 704 800 points
Then we scan the building with elevation of 25 m and 10 m width, in our point cloud we might have around 10 000 000 points.
Real resolution is usually less then instrumental resolution.