Quiz #2 SET 360

What are the attributes of a point cloud?

-Intensity -Scan identifier -Measurement's time

What is the reason why images are often not an optimal data structure for processing TLS data?

-The pixel size and pixels without laser points -The interpolated ranges for obstructed areas -Difficult recognition of multiple surfaces

Identify the steps in the Principal Component Analysis and list them in consecutive order.

1. Choose neighborhood 2. Compute covariance matrix 3. Decompose into eigenvalues and eigenvectors 4. Relation to orthogonal regression plane 5. Additional feature

Identify the steps in the free-form surface reconstruction process and list them in a consecutive order.

1. Determine patch layout and spline surfaces 2. Estimate parameters of spline surfaces 3. Ensure suitable continuity and patch boundaries 4. Volume and surface modeling by boundary representation

Triangulation is defined in _____ dimensional space.

2

How many registration parameters are needed for registering two scans each done with a different TLS?

3 translations, 3 rotations, 1 scale

During an airborne laser scanning operation, if the aircraft is moving at a cruising speed of 70 m/s and the on-board GNSS recording rate is set at 2 recordings per second, the position of the aircraft can be determined at ________ spacing.

35.0 m

Scanners max/min range distance is defined by the following laser scanning system elements:

A charged coupled device and laser

Inertial measurement unit is either fixed directly to the laser scanner or close to it on a stable survey platform. It typically records which of the following:

Acceleration Data Rotation rates

How is the signal to noise ration affecting the range-uncertainty value?

All of the above.

What are free-form surfaces?

Bivariate special-lines

What needs to be done before a terrestrial laser scanner is utilized for survey-grade data collection?

Calibration

Where is the location of the WGS84 origin?

Center of Earths Mass

Which Iterative Closest Point approach minimizes the sum of squares of Euclidean distances between slave and master point of a cloud surface?

Chen and Medioni approach

Complex objects require multiple scans of different sides and occlusions. Which of the following is achieved when we:

Combine triangulation of individual scans -> real 3D triangulation Fill holes -> Watertight models

To relate each scans intrinsic coord system into a common coord system, ___________ primitives in different scans are selected and used.

Coupled

What is a neighborhood?

Definition: A neighborhood is a set of points N

When creating a large free-form surfaces more than one patch is needed. However, each patch contains (Select all that applies):

Degree of basis function Knot vector

How does the scanning of a target differ from scanning of an object/building?

Density of the captured measurements

3D Hough transform method is used for detection of lines in a 2D image. What is the basis for the Hough transform?

Duality between points and lines

When is an Iterative Closest Point algorithm used for point clouds registration?

Exact point to point correspondence does not exist

If the object reflectivity is too high, the laser beam is fully deflected in the mirroring direction and will hit another surface or spread into the open. This creates a noise called a spackle noise. T or F?

False

The indirect georeferencing of two scans is usually performed for a free-station scanning method. For a reliable 3D solution, a minimum of two common targets with known 3D coordinates are needed. T or F?

False

The neighborhood provides information on the surface's points distribution. Neighborhood points can be identified as: (Select all that applies)

Fixed Distance Neighbors, K Nearest Neighbors, Neighborhood from topology

Independent of the specific type of airborne laser scanning, an airborne laser scanning project can be divided into three phases:

Flight planning, data processing, collection of ground reference data.

To accurately integrate/transform the on-board systems data, the spatial relationship between ___________, ____________, and __________ must be known (select all that apply).

GNSS on-board station Scanner assembly Inertial Measurement Unit

The Iterative Closest Point registration algorithm requires an input of:

Good orientation estimates

TLS intrinsic reference systems axes are coincidental with scanners:

Horizontal, Vertical, and Collimation Axes

Point Cloud is a set of points p in 3D space (x,y,z), possibly augmented by additional attributes a per point. Which of the following are points' typical attributes? (select all that applies)

Intensity Full waveform info Measurements time Measurements accuracy RGB Scan identifier

Point clouds from different terrestrial scanners' setups have different:

Intrinsic reference systems

Which of the following are preferred data structuring tools?

K-D tree, Octree, Delaunay triangulation

Single points position within NAD 1983 is represented by:

Lat and Long

Which of the following approaches are used for generating boundary representation(B-rep) models?

Manual approach Automated approach

What is the most important consequence of the laser beams divergency?

Mixed edge problem

Point cloud's record of the inherent strength of a reflected laser pulse can be used to create __________.

Monochromatic image

By setting the scanning resolution higher than the per-point accuracy, the scan will be _________. This will generate extra noise and the processing will _______.

Over-sampled; increase

Which of the listed primitives are parametric?

Planes, cylinders, spheres, tori, cones

When applying the manual approach for generating boundary representation models a digitalization of the point cloud is performed in the following three consecutive steps:

Point identification Edge = connect 2 points Face = enclosed by edges

The process of applying constructive solid geometry for point cloud modeling - surface recognition is called:

Primitive instancing

A Normal curvature is a curvature k(n) of a curve obtained by the intersection of the surface and a plane including the normal vector. How are the extreme values of the Normal curvature k(n) called?

Principle curvatures

TLS collects point data for the scanned object. Each point consists of:

Range measurement, return intensity, horizontal angle, and vertical angle

__________ is solving the problem of estimating the 3D rigid body transformation parameters of a point cloud between internally and externally defined coordinate systems?

Registration

The laser beam divergence has a strong influence on the cloud _______ and ________ uncertainty of a point.

Resolution, positional

What are the steps taken in surface growing point cloud segmentation process?

Seed detection & Growing

When extracting information from a point cloud, what is the first most commonly used method?

Segmentation

What are the consecutive steps which should be taken in reconstruction modeling with primitives?

Select points Select primitive type Primitive parameters determination

What is a Point Cloud?

Set of points 𝑝𝑝 in 3D space (𝑥 𝑦 𝑧), possibly augmented by additional attributes 𝑎 per point.

The random sample consensus (RANSAC) algorithm can be used for detection of:

Simple shapes

The number and the signal shape of return echoes depend on the type and orientation of the illuminated surface. If the surface is wet, water, or slate there wont be a return signal because of the _________ reflection.

Specular

Which point cloud processing method groups nearby points based on surface's planarity or smoothness?

Surface growing

Which of the following are NOT a result from a point cloud feature computation and segmentation?

Surfaces ARE: Points, attributes, features, segments

Multiple scans can be transformed into the spherical coordinate system of one master scan, and triangulation can be performed in the angular domain of this master scan. T or F?

T

Laser beams divergence is directly proportional with:

The absorption of the signal, the angle of incidence, and the reflectivity of the object

The laser beam in a panoramic type of scanner is reflected by rotating mirror. This type of scanner offers:

The fastest scan time and the largest FOV

TLS emits a short pulse of light and measure the time for the return signal to reach the detector and uses that to calc the distance of the object. If the intensity of the returned signal is higher than usual:

The measured distance will be shorter than the actual distance.

The panoramic-type scanners rotating mirror directing the laser beam towards an object is located at:

The origin of scanners Intrinsic coordinate system

What measurement is used to calculate the two-way travel time of the Phase-based laser signal:

The phase-shift between emitted and received signal

Which of the following elements are required to build a point cloud registration model?

The primitives, the similarity measure, the transformation function, an the matching strategy

What automated process was developed as a general approach to robust model fitting?

The random sample consensus

What are the two most common laser scanners systematic errors?

The temp drift and non-linearity of the time measurement unit.

The multiple scans registration process applies matching strategy for establishing mathematical relationship between different coord systems. Which of the following is included in this strategy?

The transformation function, the registration primitives, and the similarity measure.

Data compression coding of the octree is executed in a _______________ and ___________ order.

Top-down, Breadth-first

TLS's axes are not perfectly aligned due to a manufacturing limitation causing collimation and horizontal axis error. T or F?

True

The GNSS and IMU data of an airborne laser scanning project are used to reconstruct the flight trajectory. T or F?

True

The surface reconstruction process that is based on triangulation of the visible parts also is used for generation of a water tight model. T or F?

True

How are the coordinates of the center of a spherical target determined when it is used for a registration of a point cloud?

Via last squares

What method is commonly used to inspect a point cloud and obtain a first impression of the data quality?

Visualization

When a point cloud is considered a final model which of the following operations are applicable?

Visualization Interface checking Simple Measurements

Information on, or property of, one point from the point cloud is called a

feature

Which of the following conditions a line segment should satisfy when doing scan line segmentation? i. All pairs of neighboring points on two-line segments should be within specific distance. ii. All segment lines should have a minimum length. iii. The segments should overlap for some percentage. iv. The segments should have north/south orientation.

i. ii. and iii.

Point cloud registration can be performed in two modes. Which solution is applied when direct point correspondence does not exist?

indirect

A triangulation by parametrization over angles is recommended for triangulation of ______ scan/s.

one

Triangulation / meshing adds _______ to the point cloud.

surface