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CSI: Rombalds Moor

Photogrammetry Photography

CSI: Rombalds Moor

Photogrammetry Training 26th March

10:00 Welcome• Presentation – image capture

• Practice 12:30 – 13:15 Lunch• More practice

16:00 (ish) Finish or earlier

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What is photogrammetry'photo' - light

'gram' - drawing'metry' - measurement

“The art and science of obtaining reliable measurement by means of images”

(Remote Sensing and Photogrammetry Society, 2004)

Photogrammetry is underpinned by the principle of triangulation. If imagery of the same subject is taken from two different locations, lines of sight, or “rays” can be developed from each camera to points visible on both images. A 3 dimensional x,y,z

coordinate can then be calculated from the intersection.

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Photogrammetry Today Photogrammetry is an extremely flexible tool and is used in many different disciplines / fields such as:• Quantity surveying• Planning / surveying• Topographic mapping• Architecture • Engineering• CSI (ballistics and surveying road accidents) • Geological measurement• Heritage

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CSI Photogrammetry Equipment• A digital camera• Scales• Software• Tripod• Parasol • Control points / targets• 3D monitor / Glasses

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Metric Camera Photogrammetric photography is typically acquired using cameras with fixed focus lens – often referred to as ‘metric cameras’ – for which the precise focal length of the lens and its distortion characteristics are known. This ensures that the same focal length, and hence image geometry, is used for each image acquired resulting in high accuracy processing.

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Digital Camera Lower cost digital cameras (compact and

DSLR) can also be used for photogrammetryproviding their image geometry is known –the process of determining the geometry is

referred to as ‘calibration’ • Focal Length e.g. 24mm (CSI lens)• Lens Distortion• Principle Point – either of two points on the principal axis of a lens where the object and image have the same size and are not inverted in relation to each other • Resolution – sensor / pixel size

CSI using 2x Nikon D5000•12.3 Megapixels (4288 x 2848)•2.7" vari-angle LCD Monitor•Live View•720p HD Movie•4fps Continuous Shooting•19 Scene Modes

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Calibration

Both Nikons have been calibrated to a focus of 1.1m and their 24mm lenses have been ‘fixed’.

This fixed focus will allow us to capture sharp and usable imagery between

0.9m – 1.3m

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CSI Camera and Lens IDs To maximise accuracy within the photogrammetric models, ensure imagery is taken using lens 1 on

camera 1 and lens 2 on camera 2 - all have been labelled

If the imagery becomes slightly out of focus, the focus ring may

have moved – let us know. Whilst there shouldn’t be any need to

recalibrate, the focus will need re-fixing to 1.1m

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Image Coverage

Using the project cameras, imagery taken at a distance of 1.10m from the

object will cover an area approximately 1.0m x 0.7m

If you take imagery at 0.9m and 1.3m the area covered will differ

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Camera Settings - Image Quality and Size Image Quality: To ensure as much data as possible is collected, image quality should be set to NEF (RAW) + JPEG

The NEF/RAW format stores roughly 4 times as much info / data as the highest quality JPEG. The JPEG is captured for

easy access to the image bank and doesn’t require any specialist software or conversion. This may be revised (JPEG) capture is storage becomes an issue

Image Size: set to ‘Large’ 4288 x 2848. NEF/RAW

automatically defaults to large

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Camera Settings – Sensitivity

ISO Setting: Nikon is set to ISO 200, this should be more than

adequate for the lighting conditions to be encountered. Try not to go

above ISO 400

Digital cameras have an ISO rating which can adjust the sensors

sensitivity to light. Higher ISOsamplify the sensors sensitivity, so less light is needed. However

‘noise’ is also increased, which is visible as graining and can impact

on processing

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Camera Settings – Shooting Mode The preferred shooting

mode is Aperture Priority (A) this will allow greater control over the depth of field. Ideally the aperture should be set to f22, but a range from f11 – f22 will be

ok

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Shooting at f11-f22 will slow the camera shutter speeds down and introduce the possibility of camera

shake. To limit this, capture imagery using the tripod and the

remote shutter

Camera Settings – Tripod and remote shutter

The remote shutter is activated via the Release Mode menu on

the D5000. Note: you need to be stood in

front of the camera or at the side for it to fire

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• The 24mm lens provided with the camera only works in Manual Focus, it cannot be switched to Autofocus

• Turn off noise reduction, anti shake etc can impact on data stored and calibration

Camera Settings – other info

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Imagery to be captured – vertical• The whole of the panels

surface to be captured, not just the carved surface. This will be under constant review as the impact on storage and recording time may be significant

• Due to the shape of some panels, there’ll be a mixture of different types of stereo photography, vertical, convergent and possibly oblique

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The English Heritage specifications for photogrammetric survey require that:

• “Overlap between adjacent stereo images of at least 60% of the object area”

Overlap area of at least 60%

Imagery to be captured – vertical

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• In addition to vertical coverage, oblique stereo-imagery should also be obtained of each carved stone• This should ensure good stereo coverage is obtained of all identified rock art features, including any deeper, recessed hollows• If time allows, a convergent, oblique stereo-pair should be taken from all four sides of the stone NB – This type of

coverage will also allow calibration of camera information –lens distortion, focal length etc – to be later derived. Often referred to as ‘self-calibration’ this information is essential for accurate photogrammetric processing.

Imagery to be captured - oblique

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What needs to be in the image - scales• Scales are required for accurate measurement to be applied to a model. Set the scales up at right angles to each other• On large panels requiring multiple stereo pairs there’s no need to have the scales on each pair. Include a scale where you can • Ensure vegetation doesn’t encroach over the sections on the scale• The scales (or control points) should not be moved between the left and right image of the stereo pair. The slightest movement will cause the processing to fail

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What needs to be in the image – other• Colour Chart to ensure a consistent colour is captured (or later matched), the IFRAO colour scale should be placed within each stereo-pair (off the area to be modelled)• Additional targets or control points can be included in stereo pairs to aid the stitching process• A north arrow isn’t essential but may aid model orientation once processed

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• A Minimum of 2 images referred to as a ‘Stereo Pair’ are required to produce models

• An object / surface needs to be in view in both images for an accurate model to be recreated. If a surface can only be seen on one image the software will create a false surface between the areas it can see

• Using the Nikon D5000 we can expect accuracies of 1-2mm + depending on the number of images used and distance the imagery is shot at. 0.9m will produce greater accuracies than 1.3m, but more imagery is required

• Avoid shadows in motifs by using the parasol / umbrella. The software struggles to create surfaces in areas of dark or uniform colour without texture

General Information

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• Ensure no vegetation encroaches into image, cut blades of grass back. Overhanging vegetation can cause spikes on the processed models

• Carefully brush off droppings / detritus in a downwind direction

• Soak up any water in the motifs with sponges and allow to dry

• Wet or muddy stones can cause issues during processing. You’re best coming back later

Preparing the panel

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Stereo Imagery – GeometryStereo imagery is

comprised of a left and right image

The distance moved between the left and right image is known as the

‘camera base’. The camera base is

determined by the object distance

English Heritage suggest a

Camera base to object distance ratio

of 1:4 – 1:6

Others use 1:3 – 1:5

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Stereo Imagery – GeometryProject cameras and 24mm lenses

have been calibrated and

‘fixed’ to a focus of 1.10m. Applying the guidance to

1.1m this equates to a base movement

of0.18m – 0.36m

1.1m

Move camera 0.18m – 0.36m L to R

1.1m

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Stereo Imagery – GeometryThere is a degree of flexibility within the 1.1m focus,

allowing imagery to be captured between 0.9m –1.3m from the

object0.9m


0.9m

A distance of 0.90m from the object

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Stereo Imagery – GeometryThere is a degree of flexibility within the 1.1m focus,

allowing imagery to be captured between 0.9m –1.3m from the

object1.3m


1.3m

A distance of 1.30m from the object

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Types of stereo imagery – Geometry The English Heritage specifications for

photogrammetric survey require that:

“A variation in the scale between adjacent stereo-images

of no more than 5%.”XAt the focus distance the project is working at (around 1.1m) the English Heritage specification equates to a difference 5 - 7cm

between the left and right image

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The English Heritage specifications for photogrammetric survey require that:• “Alignment of each image plane with the principal plane of the object area to within +3 degrees of parallelism”

Object plane

Image plane

Stereo Imagery – Geometry

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Plan View

�X

Keep as parallel to the stone as possible when taking the imagery

Stereo Imagery – Geometry

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• “Alignment of each image plane with the principal plane of the object area to within +3 degrees of parallelism”

In other words try to maintain the same camera tilt for each oblique

stereo-pair

Image Geometry – Oblique Tilt

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• “Minimised vertical tilt of the camera in either an upwards or downwards direction, up to a maximum of +15 degrees”

In other words try to limit the

downwards (or upwards) tilt of the camera for

each oblique pair

Image Geometry – Oblique Tilt

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Types of stereo imagery – verticalMaintain the same

distance between the camera and object /stone when moving left to right

on the stereo pair

Remember the camera and 24mm lens

calibrated to a focus distance of

1.1m

�

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Types of stereo imagery – divergent Images taken with camera angled away from object

A model can be produced with this imagery, but the surface accuracy may be reduced

X

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Types of stereo imagery – convergent Images taken with camera angled in towards the object

A model can be produced with this imagery, however the accuracy may be reduced

XIn reality it will be impossible to avoid taking convergent

imagery on some panels due to the stones shape

�But remember base ratio is still

important

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Photo 1 Photo 2 Photo 3 Photo 5 Photo 6Photo 4 Photo 7

Ground

Rock art panel

Multiple stereo pairs – large panel

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2.5m

Plan view of a flat ground level

stone

0.7m

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L

Leave a gap on the left side of the shot to allow for the L to R base movement

Base

Targets placed near the right edge of the SP left image can guide overlap between the stereo pairs

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L R

0.2 – 0.35m

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Area covered by the first stereo pair

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L

Take the next SP by moving the camera / tripod right along the base, remembering to keep parallel to the panel. Aim for a minimum of 20% overlap to the area covered by the previous stereo pair

On the left image of the second SP leave enough overlap to allow for movement to the right

The scale bar (s) and IFRAO scale can be moved to subsequent stereo pairs

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L

Place another set of targets near the

right edge of the left image

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L

0.2 – 0.35m

The 2 SPs will be joined together by this area of

overlap

R

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L R

Area covered by 2nd SP

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L L

Repeat same procedure for the

3rd stereo pair

Remember this movement is

determined by the required overlap not

the base / height ratio

Area covered by 2nd SP

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L RL

0.2 – 0.35m

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L

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987654321Image strip 1

Large panel (plan view)

181716151413121110tape measure

used as a guideImage strip 2

Image strip 3

Targets to aid stitching during processing

Multiple stereo pairs – strips

272625242322212019

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Multiple stereo pairs strips - Hangingstones

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Hangingstones - model

5-10mm accurate surface

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• Maintain the distance between the camera and object on all stereo pairs

• Remember the overlap • Avoid excessive tilt between stereo pairs • Carefully brush off droppings / detritus in a downwind

direction• Soak up any water in the motifs with sponges and allow

to dry• Wet or muddy stones can cause issues during

processing. You’re best coming back later

Image Capture Summary

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PhotogrammetryRecording Form – RF4 RF4 is used to record the photogrammetry image filenames for each panel whilst out in the field•Provides important positional information about the imagery useful when processing •A reference to sort images easily when uploading to the computer• A useful document if image files are misfiled when copied to the hard drive / computer • Forms part of the future record •The form isn’t perfect for every panel, suggestions are welcome

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Photogrammetry Recording Form – RF4 This number is generated by the ERA database once the record has been input

This section records the camera / lens used. Enter 1 or 2

Note: you should have camera 1 and lens 1, or camera 2 and lens 2, not

camera 1 and lens 2.

Enter approx distance (m) the imagery was taken at

Note: if issues are encountered during processing, we may be able to pin down the problem to a distance (too near or too far)

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Photogrammetry Recording Form – RF4 DSC1001DSC1002DSC1003DSC1004DSC1005DSC1006DSC1007DSC1008DSC1009DSC1010DSC1011DSC1012

NContext view looking WContext view looking EContext view looking SContext view looking at HaystackClose up of motifs at S endView of peck marks S end

Roughly sketch an

outline of the panel and a few of the

motifs

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Mark the location and direction

the imagery was taken

from

Enter the image

filenames for the stereo pair in the

table ensuring cross

reference is easy

For small panels fill in the top part of the table

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Continue to mark the

location and direction


from

Note the two SPs taken here were vertical,

hence the column

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Continue to mark the

location and direction


from

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Photogrammetry Recording Form – RF4

654321Row 1

654321Row 2

654321Row 3

DSC2001Same

procedure is followed for large panels

2002 2003 2004 2005 2006DSC2007 2008 2009 2010 2011 2012DSC2013 2014 2015 2016 2017 2018

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Due to the complexity and number of images being captured during this project, the table layout will not work

for every panel

Please feel free to rename columns and rows, as long as we’re able to easily cross

reference filenames to position

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Complete as appropriate. If ‘No’ please indicate reasons in Comment

Enter the surface measurements applied to the

photogrammetricmodel(s)

Enter filenames of processed models & formats used

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Now your turn

csi: rombalds moor photogrammetry photography€¦ · digital camera lower cost digital cameras...

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