03 TLS-Training Data Registration v1.0!30!01-2012

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Module 3 Data Registration www.riegl.com

RIEGL Laser Measurement Systems GmbH

3580 Horn, AUSTRIA

Training Material for RIEGL VZ-XX PRELIMINARY VERSION

Module 3

Data Registration

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Module 3 Data Registration

Coordinate Systems and Transformations

Registration Methodes

Registration by Use of Reflectors (Control Points)

Coarse Registration

Backsighting Orientation

Multistation Adjustment (MSA)

Practical Examples

POSE Estimation (GPS, Inclination sensors and Integrated Compass)

Combination Coarse Registration & MSA

Combination Reflectors & MSA (Chain, Ring)

Table of Contents

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COORDINATE SYSTEMS

AND TRANSFORMATIONS

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SOCS (Scanners Own Coordinate System)

PRCS (Project Coordinate System)

GLCS (Global Coordinate System)

Coordinate Systems

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XPRCS YPRCS

ZPRCS

YGLCS XGLCS

ZGLCS GLCS

GLobal Coordinate System

SOCS

Scanners Own Coordinate System

PRCS

Project Coordinate System

MSOP

... Scanners orientation and position in PRCS

MPOP

... Project coordinate systems orientation and position in GLCS

Coordinate Systems

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REGISTRATION METHODES

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Registration by Use of Reflectors (Control Points)

Coarse Registration


Multistation Adjustment (MSA)

Registration Methods

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REGISTRATION BY USE OF REFLECTORS (CONTROL POINTS)

Registration method

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The standard registration process in RiSCAN PRO is based on corresponding tiepoints (finescanned reflectors).

Reflectors Overview

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Registration by Use of Reflectors

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Find Corresponding Points

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MODE:

by Link

the SOP will be calculated without changing the corresponding points (links)

by Name

retrieve the corresponding points by comparing their names (the SOP will be recalculated)

Minimize Error (Default) - Recommended

the point-pairs will be detected automatically (the SOP will be recalculated)

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PARAMETERS:

Tolerance

defines the search radius (the maximum distance between two corresponding points in order to recognize them as corresponding).

Default settings: Tolerance = 0.1m

Minimum N

defines the minimum number of point-pairs.

Please note: Minimum 3 corresponding points are needed for a unambiguous solution

(If you set this value too high, you might get bad results because points might be linked together that are not related.)

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OPTIONS:

Close gaps in chained or ringed scan positions

Activate this option if your scan positions are organized as a ring or a chain.

Rename tiepoints

Selecting this option will rename the tiepoints with the corresponding name of the linked tiepoint.

Use existing link

If this option is activated, RiSCAN PRO will use existing links to tiepoints/controlpoints to speed up the process. You may also use this option if RiSCAN PRO doesn't find the correct solution automatically due to many reflectors.

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Please note: Minimum 3

corresponding points.

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COARSE REGISTRATION

Registration methode

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Coarse Registration Overview

Like the registration based on corresponding tiepoints you can also do a coarse registration by temporarily defining some (at least 4) corresponding points. (2D-/3D mode)

Corresponding points can be:

well known features like corners,

edges,

points with high reflectivity

between a dataset (a point cloud or a mesh) of a registered scanposition (or the PRCS) and a dataset of the scanposition to be registered.

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Coarse Registration

View A: reference data (already registered)

View B: scanposition to be registered

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Define corresponding points in view A and view B. To do so, hold down the SHIFT key and left-click on a point in view A. Proceed the same way with the corres-ponding point in view B.

A small sphere with a label showing the point number represents the clicked point.

When the point pair is well defined confirm the settings by clicking the "+" button.

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Coarse Registration

If at least 4 point pairs are defined you can click the button "Register". Now the proper SOP matrix is calculated and written to the scanposition. The object(s) of view B are automatically added to view A.

The field "Standard deviation" shows the quality of the registration

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Coarse Registration

Calculate origin of scan position only:

If the orientation is already defined by the internal inclination-sensors and the internal compass, only the origion of the scan position is calculated, when activating this option. In this case only one point-pair is necessary.

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Registration Coarse Registration (Result)

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BACKSIGHTING ORIENTATION


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You can use this tool to register the scan position using the well known coordinates of a certain point (precisely surveyed point on the ground, or exact coordinate from mounted external GPS) and the coordinates of a remote object.

The scanner must be either leveled manually or with the optionally built-in inclination sensors.

Backsighting Orientation Overview

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6-Degrees of Freedom:

3 Degrees of Freedom Translation (e.g. GNSS)

2 Degrees of Freedom Inclination Sensors

1 Degree of Freedom Backsighting / Heading

Overview 6-Degrees of Freedom

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Start wizard "Backsighting orientation: Open the wizard "Backsighting orientation" by right-clicking the SOP matrix of the scan position and selecting "Backsighting orientation...".

A wizard pops up requiring the input of the scanners position coordinates

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Own GPS position: On the first page enter the scanner's own position in global coordinates. The scanner's own position can be either: global (e.g.: WGS84) or local (e.g.: Mining System, with Projection)

ones.

"Instrument height": Insert the vertical offset between the well known ground point and the center of SOCS, indicated at the scanner head.

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Own GPS position:

Read from file: If you are using a GPS mounted on the scanner you can import the coordinates by using a *.uda file (Format: "Name,X,Y,Z"). Open the *.uda file and select one entry (=position) from the list.

Read from scan : Read out the GPS position which has been acquired with the internal GPS receiver (L1).

Use GLCS tiepoint: Known coordinates, measured with total station or DGPS, of each position can be used which have already been imported by user before running Backsighting orientation.

Click on Next, when finished.

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Import local grid coordinates (measured with external device, e.g. Totalstation, DGPS), which are normally common with the targets that have been fine-scanned.

Your grid coordinates can be in various text-file formats, with or without header.

The format is normally: Point_ID, E, N, H or Point_ID, X, Y, Z.

Importing ASCII Grid Coordinates

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Skip lines: This causes the import function to ignore the first n lines of the file (e.g. header)

Column separator: is used to the correct character in order to recognize the data columns from the file.

Column association: Just drag the column from the list-box showing all columns and drop it on the corresponding column of the preview.

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Alignment via remote object

Enter the coordinates of a remote object in global coordinates (the GPS import via *.uda-file is also available)

against north The scanner should be aligned towards north (switch to following page).

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Alignment - Angle

Use northing angle [deg]: Based on measurements of the magnetic field and detailed information of the internal GPS receiver, the scanner calculates accurate northing angle.

Use tiepoint (finescanned): By using a finescanned reflector target, the northing angle is automatically calculated from the reflector position gained by the scanner.

Use inclination sensors OFFLINE Activate, if you want to read the inclination angles from an already acquired scan file.


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Summary

Summary You can see a summary of the given data and the used methods. In this step you can still go back to one of the previous pages in order to correct wrong settings.

Matrix

On the fourth page you can see a summary of the given data and the calculated matrix. In this step you can still go back to one of the previous pages in order to correct wrong settings.

Click on Set SOP, when finished.

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MULTI STATION ADJUSTMENT

(MSA)


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Multistation Adjustment (MSA) is a RiSCAN PRO tool, designed to improve other registration methods such as Backsighting Orientation or Coarse Registration.

Flat surface patches are detected within the pointcloud. These patches are represented as a point, indicating the center of gravity of the plane patch, and a normal vector on this point, representing the orientation of the plane patch. The following alignment is based on a modified ICP algorithm (iterative closest point algorithm).

To make use of this procedure, data must be pre-adjusted (using a prior orientation method).

MSA Overview

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To extract the plane patches from the scan the option Prepare data is used.

Registration > MSA > Prepare data...".

This opens the filter dialog.

MSA Prepare Data

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MSA Prepare Data (Plane patch filter)

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MSA Prepare Data (Plane patch filter)

Plane patch filter: Maximum plane error [m]:

This parameter is used to decide whether a couple of points represent, a plane patch or not. It defines the threshold for the standard deviation of residuals. 0.003 0.01 m for architectural applications 0.02 0.03 m for mining applications

Minimum number of poitns per plane: A plane is calculated from at least this number of

points. Minimum search cube size [m]: Defines the threshold of the calculated

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MSA Prepare Data (View)

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Activate the point normals to view the normal vector.

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MSA Prepare Data (View)

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MSA Start Adjustment (Shortcut F4)

To start the Multi Station Adjustment select the menu item

Registration > MSA > Start adjustment..."

The MSA adjustment tool will appear.

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MSA Start Adjustment The adjustment iteratively modifies the position and orientation of each scan position until the error is below a user defined threshold.

If no control points are available you should lock the position and orientation of at least one scan position (e.g. the scan position that defines your PRCS).

You can also lock any of the 6 parameters separately with the checkboxes in front of the parameters.

Identify and fix reference scan position; other positions will be oriented to this position.

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MSA Start Adjustment

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There are several options to align scan positions: Use tiepoints:

uses common tiepoints from the tiepoint list (tiepoints must be linked between the scan positions)

Use tieobjects: manually created planes, stored within the tie object list (TOL) of each scan position and linked between the scan positions

Use polydata objects: uses common planar surfaces created during the data preparation (Plane patch filter)

Use measured scan positions: can be used to tie the position to measured coordinates (e.g. measured by GPS or totalstation). Limits adjustment of position by use of measurement uncertainty (see sop-matrix)

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MSA Parameters

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Nearest point search:

Mode: All nearest points (recommended) is a set of points containing the nearest points of each remaining dataset. One nearest point (fast) searches for the nearest point in all datasets.

Search radius [m]: defines the distance within the algorithm is searching for corresponding plane patches

Max. tilt angle [deg]: Max. tilt angle is used to remove incorrect point-pairs. Each point represents a plane whereas each plane has a surface normal. If the angle between the surface normals of two planes is smaller than the maximum tilt angle then the two planes are considered to correspond.

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MSA Parameters

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Adjustment: Min. change of error 1 [m]:

If the improvement of the alignment between two following iterations is less than the given value the algorithm is stopping and searching for new corresponding planes. Then the alignment starts again.

Min. change of error 2 [m]:

The iterative alignment is running till the improvement between two following iterations is less than Min. change of error 2. In that case the final alignment is reached.

Outlier threshold:

When Min. change of error 2 has been reached, optionally outliers can be removed and a final iteration for the alignment is calculated. The error histogram should show a symmetric distribution (bell curve) around zero. An outlier threshold of one, defines that all corresponding planes outside the one sigma value of the error histogram are removed.

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MSA Parameters

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Adjustment: Calculation mode:

Least square fitting (recommended): Using the square distance (of the point-pairs) is taken, which means that outliers will have a greater influence to the overall result. Robust fitting: Using the absolute distance, because this mode is more stable. It may take more time because of smaller steps between the iterations. Once the data is aligned, there is not difference between calculation modes.

Update display: You can watch the progress of the updates in a 3D view. Available steps: never, seldom and often. (Recommended: never)

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After defining all parameters, click on Analyse.

The Analyse function will show a histogram and a polar plot of the corresponding planes.

Histogram: shows the error distribution of the corresponding planes

Polar plot: Each plane is shown as a point, the positon of a point is set by the normal vector of a plane which gives you the orientation of all the used planes.

The wider the distribution of planes within the polar plot the more robust the final alignment will be (minimum should be 3 clusters of plane-orientation to cover the 6 parameters of freedom).

MSA Analyse

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Plane surface filter

Max. plane error [m]: This parameter is used to decide whether a couple of points represent a plane surface patch or not. The points need a lower standard deviation from the estimated plane to define a valid plane.

Max. edge length [m]: For better visualization and edge detection the remaining points are triangulated. Use this parameter to remove too large triangles.

Min. rane [m]: Points closer than this value will be ignored.

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Error (StDev) is around 1m, with around 500 planes.

MSA Analyse

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Recommended workflow:

Double the value of the Error (StdDev) and set it as Search radius.

e.g.: 1m 2m

Min change of error 1: approx. 10cm

Min change of error 2: approx. 1cm

MSA Calculation

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Start the calculation! Take a look at the Error (StdDev) Polydata Objects If the values of the planes increase, it means that the alignment become better and better.

MSA Calculation

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1st run has finished:

The MSA found much more planes: 15.000 polydata.

The distribution of points is even throughout the polar plot and the histogram displays a steep bell curve.

Overall Error: 0.0066 m

MSA Calculation

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For 2nd run:

Repeat the same steps as descirbed before.

Now using a much smaller search radius, we also have to decrease the values of the two errors.

Calculate

MSA Calculation

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The 2nd run has finished:

The MSA found even more planes: 22.000 polydata.

The distribution of points is even throughout the polar plot and the histogram displays a steep bell curve.

Overall Error: 0.0023 m

MSA Calculation

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MSA Final Result (View)

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Mark points in the point-cloud and click on the information button.

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PRACTICAL EXAMPLES

POSE Estimation


Combination Reflectors & MSA (Chain, Ring)

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POSE ESTIMATION BY USE OF GPS, INCLINATION SENSORS AND INTEGRATED COMPASS

Practical Example

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This practical example shows the use of:

GPS

Internal measured GPS position. (quality of L1 receiver)

Inclination sensors

Levelling by use of inclination values

Integrated compass

Alignment of Scanners Own Coordinate System to true north

POSE Estimation Practical Example

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POSE Estimation (Preparation)

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POSE Estimation (Indoor Measurement)

Special case:

For indoor measurements, the GPS position has to be fixed either with an

manual input or

GPS position from

previous position measured outdoor

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Scans are automatically registered

Read out the Information for Roll / Pitch / Yaw

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POSE Estimation (Situation before)

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Results from POSE Estimation: Gaps between the scans are caused by inaccuracies of internal sensors (mainly GPS and compass)

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After importing the scans

directly use MSA

POSE Estimation

1st step: Prepare data

2nd step: Start adjustment

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POSE Estimation MSA 1st step

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POSE Estimation MSA 2nd / 3rd step

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POSE Estimation (Results after Running MSA)

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Please note: Accuracy of compass: typically 1 deg

(one sigma value for vertical scanner setup position)

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POSE Estimation MSA Calculation

MSA calculation after 2nd run: Error (StdDev): 0.0023

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POSE Estimation SOP

SOP ScanPos01:

SOP ScanPos02:

SOP ScanPos03:

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Analyse your MSA-calculation by checking the results with the information tool.

POSE Estimation Checking Results - show plane info

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Please note: StdDev. of

residuals to estimated plane

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COMBINATION

COARSE REGISTRATION & MSA

Practical Example

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After using registration mode: Coarse registration

directly use MSA

1. Prepare data

2. Run MSA

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Situation before:

By analysing a large number of points (~5.500), we get to a result of

StdDev: 0.078 m

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Analyse:

After preperation of the dataset, run the Analyse-tool in the MSA.

The MSA found around

43.000 polydata objects.

Error (StdDev): 0.0130 m


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Final result:

One run is enough to fit all pointclouds together.

The MSA found around

17.000 polydata objects.



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Situation after:

By analysing a large number of points (~5.500), we get to a result of

StdDev: 0.003 m

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COMBINATION

REFLECTORS & MSA

Practical Examples

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1st step: Register SP1 and SP6 by using controlpoints.

2nd step: Register SP2 onto SP1 & SP5 onto SP6

Register SP3 onto SP2 & SP4 onto SP5

3rd step: Link SP3 and SP4 together.

The deviations between SP3 & SP4 will be higher than within the other scan positions.

To do so run "Find corresponding points" for SP4 again, select only SP3 and activate "Close gaps in chained or ringed scan positions".

Chain of Scan Positions

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Please note: Only links between the tiepoints

of SP4 and SP3 are created. The SOP-matrix is not affected!

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Ring of Scan Positions

1st step: Controlpoints are available at scan position SP1.

2nd step: Register SP2 onto SP1 & SP3 onto SP2

Register SP4 onto SP3 & SP5 onto SP4

There will by a higher deviation between SP5 and SP1.

3rd step: Run "Find corresponding points" for SP5 again, select SP1 and activate "Close gaps in chained or ringed scan positions".

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Please note: Only links between the tiepoints

of SP4 and SP3 are created. The SOP-matrix is not affected!

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After using registration mode:

Find corresponding points by use of Close gaps in chained or ringed scan position

Follow the steps described on slide 86. Thereafter directly run MSA and use only teipoints as input data.

Chain Practical Example

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Adjust ScanPos:

- Lock ScanPos001

- Lock ScanPos009

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Analyse:

After adjusting the scanpositions, run the analyse-tool in the MSA.


by using 42 tiepoints


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Calculation:

In this case MSA is doing a bundle adjustment on the whole chain or ring of scan positions.


by using 38 tiepoints


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RIEGL Laser Measurement Systems GmbH

3580 Horn, AUSTRIA

Training Material for RIEGL VZ-XX PRELIMINARY VERSION

End of Module 3

Data Registration

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Documents

03 TLS-Training Data Registration v1.0!30!01-2012