Geo-Referencing

of Scans

Oliver BürklerTechnical Product Manager

May 2010

Geo-Referencing of Scans Laser Scanner Training – Europe –

Training Objectives:

• Difference between local registration and geo-referenced registration

• Typical coordinate systems for geo-referencing

• How to do get a geo-referenced registration in Scene

• What does “Traversing” mean?

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Positioning the Scanner

The scanner will record only what it can „See“

This means that the references and the target object must be in the line of sight of the unit.

For registration each scan requiresa minimum of 3 visible references

These references should be evenly spaced around the scanner. For better control it is better to use 4 references per scan.

Automatic Target Identification

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Reference Targets

The FARO references are normally

• Spheres and Paper targets

References should be

• Easily visible • Spaced 2 to 15 from the scanner

(depending on size and resolution)• Secure

Starter Kit

SuperSpheres

Checker Board

Circular Flat

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Local Registration

• One scan serves as origin of thecoordinate system

• This “Reference Scan” isdefined

• Randomly by Scene (default)or

• Manually by the user

Reference Scan

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referenced Registration

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referenced Registration

• A surveyor is providing the coordinates of the reference targets based on a geodetic coordinate system

• Normally, the coordinates are determined with

Total Stations – or – GPS systems

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

• Land surveying is a very old technology• Traditionally, mathematical calculation capabilities have been very limited

Until today, surveyors are working in 2-dimensional coordinate systems• The surface of the earth is assumed to flat• Coordinates are often given as “Easting and Nording” or “Right and Up” (German:

“Rechtswert und Hochwert”)• Then, after defining the 2-dimensional position on the earth surface, a height

information is added• The height is determined independently from the position and is typically less

accurate

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

Unfortunately…

…the earth is not flat!

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

…therefore geodetic coordinate systems are trying to provide detours to avoid 3D and a curved surface.

Many common coordinate systems are based on the UTM system

UTM means “Universal Transverse Mercator” which is a geometrical transformation projecting each point on the global ellipsoid onto a flat plane

To make this work, the globe is cut into “zones” of 6° width

As mathematical model of the earth´s shape the so-called WGS84 or GRS80 ellipsoid is used

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

Principle of UTM transformations

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

Global “Zones”

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

UTM zones for Europe

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geodetic Coordinate Systems

But this is not everything:

• Military is using a UTM system as well, but with a different grid subdivision (100km x 100km grid size). This is called Military Grid Reference System (MGRS)

• Germany, for example, is using a system called “Gauss-Krüger” named according the mathematician Carl Friedrich Gauss and Johann Heinrich Louis KrügerThis uses the same mathematical transformations, but a Bessel or Krassowski ellipsoid and 3° wide zones

• GPS systems are using longitudinal and latitudinal coordinates based on the WGS84 or GRS80 ellipsoid (note: also the height is missing here and needs to be specified separately)

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referencing in Scene

Independent from the used coordinate system, what you need is a list of coordinates from the surveyor:

• ASCII file format (file extension *.txt or *.cor)• One coordinate per line• Point name -comma- X coordinate -comma- Y coordinate -comma- Z coordinate• “Point” as decimal separator

• Example:A,146.445675591,-6.8416812176,0.12194667699B,142.674728212,-2.9329090693,-0.14458280531C,139.682633752,4.3632461477,0.0004202706739D,153.66088266,0.47645329791,-0.021972010218E,148.270912355,5.4490686746,-0.13309600625F,134.643476,-14.297264,0.089182

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referencing in Scene

Things to be aware of:

• Keep the coordinates within “kilometer range”• Scene will have difficulties in 3D view if the coordinates become too large• Typical surveyor coordinates are too large!

I.e. the coordinate of the city of Dresden, Germany in UTM (WGS84) is:Zone 33-NorthEasting 411.777,6 mNording 5.655.984,3 m

Tell the surveyor, to cut away all leading digits which do not change(if necessary they can be added as a global transformation to the Workspace again)

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referencing in Scene

Things to be aware of:

• Make sure the surveyor gives the coordinates in the correct, mathematical sequence

• Sometimes surveyors use tell the up value first and then the right value• If you import this into Scene, the scans will be rotated by 90°

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Geo-Referencing of Scans Laser Scanner Training – Europe –

Geo-Referencing in Scene – Workflow –

1. Create your workspace as usual (import scans, filter etc.)2. Import the coordinate file from the surveyor

a. Drag & Drop the file into Sceneb. Use “File – Import” to import the coordinate file Scene should automatically create the Folder “References”.

For each coordinate from the file, there should be a point object.Each point will have to coordinates given by the file

3. Run the normal scan placement processa. Detect targets if not done yetb. “Place scans”

4. Scene will automatically identify the suitable survey coordinates from the geometrical pattern of targets.

5. Target names will be derived from point numbers given by the coordinate file

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Geo-Referencing of Scans Laser Scanner Training – Europe –

“Traversing” in Scene – Workflow –

“Traversing” is a common surveyor technique where the instrument (total station or scanner) is placed on top of a known coordinate.

This technique helps to minimize the amount of targets. The scanner position is known, the inclination sensor forces the scan to be leveled.

Only one additional target is needed

This target is typically placed on another tripod of which the position is also known

Depending on whether this target is in front or behind you (seen from the scanning workflow), it is called front-sight or back-sight in surveyor-slang

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Geo-Referencing of Scans Laser Scanner Training – Europe –

“Traversing” in Scene – Workflow –

Scene is capable of doing this (it´s just a special case of geo-referencing):

1. Each scan needs to have a point object with the coordinate 0,0,0 (= scanner origin) which has the same name as the scan itself.

2. Coordinate import and scan placement works as usual.You just need to make sure that the coordinate file consists of coordinates for the scanner position (naming!)

3. Scene will make sure that scan gets exactly the given position

4. If there are tensions in the calculation, they will appear at the reference targets