RPAS Photogrammetric Mapping Workflow and Accuracy

Dr Yincai Zhou & Dr Craig Roberts Surveying and Geospatial Engineering

School of Civil and Environmental Engineering, UNSW

➢ Background

➢ RPAS category and operation regulations

➢ RPAS photogrammetric mapping workflow

➢ Factors affecting product accuracy

➢ Summary

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• Fixed Wing vs. MultiRotor

Advantages• Simple structure• High speed• Long duration

Disadvantages• Large take-off & landing space• Not able to hover• Inflexibility to carry different

sensors

Disadvantages• High mechanical complexity• Low speed• Short flight time

Advantages• Vertical take-off and landing• Flexibility with different sensors• Able to hover and stare

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Weight classes: • micro: gross weight < 100 g• very small: gross weight 0.1 - 2 kg • small: gross weight 2-25 kg• medium: gross weight of 25 - 150 kg• large: gross weight >=150 kg

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RPA selection:• Job nature – mapping, 3D modelling or inspection• Takeoff weight – light weight, less risks• Sensor’s weight or size• Investment budget

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RPA Standard Operating Conditions (SOC):

• VLOS (visual line-of-sight) in daytime only• Under 400ft (120m) AGL (above ground level)• 30m away from people• Not in prohibited or restricted area• Not within 3NM (5.5km) of a controlled

aerodrome• Not over populated areas• Only fly one RPA at a time

*Details in Advisory Circular AC101-01

Operation category:• Excluded RPA – No ReOC or RePL required• Included RPA – ReOC & RePL required

OC – Operator’s Certificate, PL – Pilot’s Licence

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Design flowchart to determine eligibility as an excluded RPA

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1. A company with ABN/ACN

2. An employee to obtain RePL

3. Nominate a chief pilot

4. Prepare an Ops manual (CASA template)

5. Apply for ReOC

6. Chief pilot skill assessment by CASA

7. ReOC granted

8. Chief pilot is responsible for all operations

ReOC Application

RPAS Optical Sensors

RGB compact cameras RGB DSLR cameras NIR cameras

Multispectral cameras Red edge

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Image Distortion and Camera Selection

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eDistortion caused by lens:

• Consumer-grade cameras – large distortion

• Geometric-cameras – small distortion and geometrically stable

Distortion caused by moving camera:

• Electronic (rolling) shutter – large distortion

• Focal plan (mechanical) shutter – small distortion

• Leaf (mechanical) shutter – no distortion (blurry?).

Camera selection:

• Avoid rolling shutter

• Leaf shutter right choice

• Focal plan shutter for low speed flight

Project Planning

• Location (urban, rural or remote?)• Site terrain, vegetation (rich texture on images?)• Area of survey – Google Earth KML• Preferred time frame – weather permits?• Survey in standard operation conditions?• Obtain permission from CASA required?• Geometric accuracy requirement?• GSD (ground sampling distance), terrain, flying height AGL?• Site accessibility? • Launch and landing area?• Land owner consent• GCPs (ground control point) required and survey accuracy?• Flight planning.

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All photogrammetric measurements are based on overlapped images in order to obtain 3-dimensional object geometry

http://www.photogrammetrynews.com/2015/12/planning-of-aerial-photography-overlaps.html

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Flight Planning (1)

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• Check “Can I Fly There?” @ CASA.gov.au

• Apply for permission if the flights are not in SOC

• Weather permit (raining / windy)?

• Flight height / image resolution / GSD setup

• Image overlap – lateral and longitudinal

• Time of the day to fly (image quality)

• Set drone safety features (eg return home, ceiling, geofence)

• Trained observer on site

• RPAS firmware and apps updated

• Pre-flight checklist ready

• GCP survey pre- or post- flight

• Post-flight checklist ready

Flight Planning (2)

Flight Planning (3)

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Image Quality Control

1. RPAS ground speed (motion blur)2. Flight height ( GSD and image coverage)3. Oblique angle ( GSD and image coverage)4. RPAS vibration (gimbal)5. Camera and lens quality (image distortion)6. Camera shutter speed (motion blur)7. Camera sensor resolution (GSD)8. Camera aperture (depth of field – out of focus)9. Light condition (sunny shadow, cloudy under

exposure) 10. Surface reflection (coal stockpile – under exposure)11. Large elevation change (GSD variation affect matching) 12. Time of day to capture images (e.g. high wall shadow)

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Vertical vs Oblique images

Same sensor size➢ Different ground coverage➢ Different GSD

GSD variation due to oblique angle

GSD variation due to terrain changes (1)

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• Constant flight H above take-off location (120m)• Different camera H above ground ΔH=80m (40m – 120m) • Large GSD variation: 2cm at hill top, 6cm at bottom• No oblique images

GSD variation due to terrain changes (2)

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Parallel flight paths (normal overlaps 70%L & 70%)

large image overlaps improve product accuracy

Crossover flight paths (= very large overlaps)

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Image format no effects on accuracy

JPEG format images

Raw (CR2) images

NIR images

RGB images

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Windy condition affects accuracy

Less matched feature points in some areas

Flight tracks off designed paths due to strong wind.

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Water surface

Water surface cannot be mapped or precisely surveyed photogrammetrically

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Multiple flights planning

- By planning software- By mission areas- No gaps or with small overlaps

Rolling shutter (Phantom3)

Electronic 1st curtain (Sony QX1)

Leaf shutter (Canon compact)

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Camera shutter type effects

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RTK (30 sec/epoch)check strings on hard surface areas

Point cloud validation (1)

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Point cloud validation (2)

– Single flight + Nadir images

• Flight H = 120 m• GCPs = 6• Image overlaps = 80%• Number of Images = 110

• Point cloud – RTK string points:o Mean = -6.0 cmo Standard deviation = ±5.7 cm

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Point cloud validation (3)

– crossover flights + Nadir images

• Flight H = 80 m & 120 m• GCPs = 6• Image overlaps = 80%• Number of Images = 220

• Point cloud – RTK string points:o Mean = -5.6 cmo Standard deviation = ±5.4 cm

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Point cloud validation (4)

– 2 flights + oblique images

• Flight H = 120 m• GCPs = 6• Image overlaps = 80%• Number of Images = 330

• Point cloud – RTK string points:o Mean = +2.0 cmo Standard deviation = ±4.8 cm

The following aspects need to be considered for the best practice of conducting a RPAS aerial mapping project.

❖ Accuracy depends on GSD

❖ Optimal σxy = ±1 GSD; σz = ±1.5 GSD

❖ 70% ≤ Image overlaps ≤ 90%

❖ Number of GCPs ≥ 5

❖ GCP survey @ RTK ≥ 30 epochs (best with bipod)

❖ Oblique Images improve accuracy significantly

❖ Time of the day = light cloudy or mid-day (less shadow)

❖ Large elevation variation = oblique images + variable flight H

❖ Use a gimbal to stabilise camera (reduce vibration blur)

❖ Slow ground speed (reduce motion blur)

❖ High resolution optical sensor (small GSD)

❖ Leaf shutter lens (avoid rolling shutter effects)

❖ High quality camera (less sensor or lens distortion)

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Summary