05 Triangulation Concepts Data 2-1 t1

8/10/2019 05 Triangulation Concepts Data 2-1 t1

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Concepts of Triangulation

Sgt Imagery

Data 2-1


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AimTo introduce the concepts of triangulation,the preparation and computation of a block

of imagery.

(based on LPS)


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Why?Triangulation is the main technique by which

imagery is controlled to enable the accurate

extraction of geo spatial data.

Exterior orientation parameters may needcalculating or improving.


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References

Erdas Imagine LPS Users Guide,,Leica Geosystems.

SOCET Set Users Manual, 2002,BAe Systems.


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Scope

Concepts of triangulation.

Definition / Terminology / Points intriangulation.

Stages of digital triangulation:

Preparation

Automatic tie point generation

Interactively adding GCP / Check points.

The computation process.

Some practical tips and summary.

Review Aim


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Triangulation.

Triangulation involves many aspects, not onlycomputation as many texts tend to emphasise.

It also involves the stages of:

Planning & Preparation.

Data collection / Observation.

Adjustment, computation methods & stages. Analysis.


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Triangulation

Triangulation can be undertaken onvarious imagery sources, therefore thetitle varies accordingly:

Aerial triangulationbased on aerialimagery.

Block triangulationbased on satelliteimagery.

Terrestrial triangulationbased on

terrestrial imagery.

This lecture is based on aerialtriangulation.


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Why do it?

Stereo model preparation.

Feature collection.

Accurate/Precise point positioning. DEM extraction.

Orthorectification.

Generate 3D data.


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Some history

Triangulation was undertaken anddeveloped by analogue / analyticaltechniques that involve greatartisan skills.

Now it is mainly based on digitaltechniques involving automationand so the operator skills requiredhave changed.

Digital triangulation is very flexiblebut great care must be taken in themanagement of the data/results.


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GCP for a Single Model(traditional photogrammetry)

+ +b

b

Plan Height Check plan Check height


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The Strip

The strip consists of overlapping

images or connecting models.


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Old Concepts 1

For a strip of 10 aerial images,

there are 9 models and 20 controlpoints are necessary.

Note that some points are common

to 2 models and fall on 3 images


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The Blockmany images inmore than one strip.


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Old Concepts 2

For a block of 3 strips, a total of 30 images

and 27 models, 40 controls are required.


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New Concepts 3 (Strip)

For a strip of 10 aerial images,

there are 9 models and 14 controlpoints are necessary.

Note that some points are common

to 2 models and fall on 3 images.


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The Concepts 3 (Block)

For this task - 18 plan and height points

were used.


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The Concepts 4

Theimage is the basic unit and is2D.

Themodelconsists of 2 imagesforming a stereo pair.

Thestr ipconsists of models linkedtogether.

Theblock consists ofstrips ofimagery linked together.

Scope


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A DefinitionThere are many definitions, this is one mainly

based on the definition from the SOCET setusers manual :

The process of calculating (or improv ing) theexterior sensor parameters for one or more

images based on ground control, selected tiepoints and sensor information. Also known

as Exterior Orientation, Image registration orGeo positioning.


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Typesof points

GCP- points of known geodetic location to a certaindegree of accuracy that are clearly visible on theimagery; X,Y,Z or X,Y or Z only.

Tie points- Image points located in 2 or more images

used to connect the block to solve the sensorparameters. The X,Y,Z are solved duringtriangulation.

Check point- a type of GCP not used in the solutionof the triangulation but used to check the result.Sometimes known as a diagnostic point.

Image EO parameters (PCs)The exterior orientationparameters are either known accurately /approximately or are solved during triangulation.They must be considered along with the points.


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Common / Tie pointsalong a strip

Image 1 Image 2 Image 3

Tie points along a strip appear on 3 images


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Common / Tie pointsbetween strips

Tie points across strips can appear on up to

6 images for regular lyflown aerial imagery.


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Summary on first 2 parts

There is aerial, block and terrestrialtriangulation.

There are images, models, strips,and blocks.

There are tie points, GCP, Checkpoints and Perspective centres.


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Outline Stages of DigitalTriangulation.

Preparation.

Measurement of Tie Points, GCPsand Check points.

Computation / Adjustment.

Analysis.

Publish.


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Preparation for Digital AerialTriangulation

Create a Block file - define datum, projection,vertical datum etc.

Import images (scanning may be necessary).

Create a camera calibration file.

Import image exterior orientation of images, ifknown.

Interior orientation.

Import / create GCPs file.

Set standard deviations and status of data.

NOTEThis will vary with other sensors. Digitalsources of imagery will have interior orientationparameters defined and may also have the exteriororientation defined.


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LPSThe main window

Demonstrations and practical exercises will teach the details of this window


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Automatic Tie Point Collection(Measurement).

This can be done manually but isvery labour intensive.

Estimate the exterior orientation

parameters of the images frommaps and knowledge of imagescale.

If no map available, estimaterelative positions at ground scale.

Set status of PCs to Init ial.


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Point measurement tool


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Setting the parameters forAutomatic tie point generation


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The Block after AutomaticTie point generation

2 fold point3 fold point

6 fold

point


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Tie point generation inLPS

LPS does not select tie points in theclassical positions.

It attempts to collect a cloud of

points distributed as evenly aspossible over the image area (up to500 per image).

This does mean an there could be

areas with little or no points. Editingis done MANUALLY.

This may a problem with so manypoints!!


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Interactive PointMeasurementAdding GCP

Points added manually must beidentified and measured on every

image.

Measure GCP on imagery - Humanprocess.

Measure operator selected tie pointson imagery.


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GCP

GCP are either

X, Y, Z Full

X, Y - Horizontal Z only - Vertical

Check points are treated the sameas GCP.


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Summary of preparation

A defined project in Geo terms.

Tie points transferred between allimages.

Control points identified andmeasured.

Block ready for computation.


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The Bundle method

The simultaneous enforcement of theCollinearity condition for all observed

points in a block of imagery.

The unit of adjustment is the 2D image.(Based on 2D measurements).


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Basis of Bundle adjustment

Observations are 2D image co-ordinates.

No intermediate processes for adjustment -direct to block from 2D.

Intermediate processes only for checking

data, error elimination and estimating initialvalues.

Offers far greater flexibility in imageryexploitation - sensor models andrefinements.

GPS/INS technology compatible.


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Collinearity in a blockBundle method

X,Y,Z

XA,YA,ZA

X,Y,Z

X,Y,Z

X,Y,Z X,Y,Z

X,Y,Z


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The Triangulation window


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Summary of thecomputation process

Simultaneous enforcement of theCollinearity condition

Image is the unit of adjustment - 2D.

Adaptable for scanner sensors egSPOT.


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Summary of DigitalTriangulation

A progression to automation, preparationis not so manual.

Digital images.

Total solution. Multi image / sensor approach to point

transfer and measurement.

The imageis the basic unit.

Digital triangulation is now being adoptedas the standard method.


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Look Forward

Demonstration

Set-up Orthobase project

Interior Orientation

Exterior Orientation

Triangulation

Analysis


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Any Quest ions?

05 Triangulation Concepts Data 2-1 t1

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