Flight planning is a critical initial stage in photogrammetric surveying that directly influences the accuracy, coverage, and quality of aerial imagery. For land survey professionals in Kenya, meticulous planning ensures that the diverse terrain, from the undulating highlands around Nakuru to the expansive plains of Narok, is effectively mapped, supporting precise land parcel delineation and infrastructure development. This process involves defining the mapping area, selecting flight parameters tailored to the survey objectives, and addressing operational challenges such as weather conditions and regulatory compliance. Effective flight planning optimizes resource use, enhances data reliability, and safeguards safety during aerial missions.

2.1 Flight planning

Flight planning is the preparation of an aerial survey mission before anyone goes to the field: defining the area, choosing flying height and overlaps, laying out the flight lines, checking terrain, weather and airspace rules, preparing equipment, flying and monitoring the mission, and processing the images into maps and models. Each decision affects the next, so a mistake in planning (too little overlap, wrong height, missing permission) can make a whole day's flying useless. A good plan delivers complete coverage at the required ground sample distance, safely, legally and within the battery and time available.

Example: before mapping a 120-hectare irrigation scheme in Mwea, a survey team plans the boundary, a 2.5 cm ground sample distance, four flights and the ground control layout in the office, and confirms KCAA authorisation and landowner consent before travelling.

2.1.1 Determining the area to be mapped

Determining the area to be mapped means defining exactly where the aerial survey will take place, how large it is and what features it must include, before any other flight parameters are chosen. The area is usually outlined as a boundary polygon on a base map, satellite image or the client’s plan, and a buffer of about 10% to 20% is added around it so that the edges are fully covered with good overlap. The size and shape of the area decide the number of flight lines, the number of flights and batteries, and the processing time, so getting it right avoids gaps and wasted effort. Example: a land surveying firm mapping a SACCO’s 54-hectare property for subdivision drew the boundary on a satellite image, added a 30 m buffer and confirmed the beacon positions with the client before planning the flight.

Information Needed to Define the Area

• Project purpose: Whether the map is for subdivision, topographic design, volume measurement or monitoring, which decides the accuracy and coverage required.
• Boundary and extent: The coordinates or plan of the boundary, including any corridor width for roads, pipelines or power lines.
• Required accuracy and resolution: The ground sample distance and positional accuracy the client needs.
• Features of interest: Buildings, rivers, tall structures and vegetation that must be captured or that may obstruct the flight.
• Access and take-off points: Safe, open places from which the drone can be launched and kept in sight.
• Ownership and permissions: Landowners and neighbours whose permission or notification is required.
• Reference materials: Topographic maps, cadastral records (registry index maps, survey plans) and recent satellite images used to confirm the extent.
• Stakeholder information: Land use plans and local knowledge from the county planning office, the client and landowners, which can clarify unclear boundaries.

Steps for Determining the Area to Be Mapped

1. Agree the purpose with the client: Confirm what products are needed and how they will be used.
2. Obtain boundary data: Collect plans, beacon coordinates or a sketch of the area of interest.
3. Draw the polygon: Outline the area in the flight planning app, GIS or Google Earth.
4. Add a buffer: Extend the boundary outward to ensure full coverage and overlap at the edges.
5. Calculate the area: Compute the area in hectares to estimate flight time and data volume.
6. Identify obstacles and take-off points: Mark tall structures, power lines and safe launch sites.
7. Confirm with the client: Share the planned extent and get approval before final planning.

Worked Example: Area to Be Flown

A rectangular site measures 900 m by 600 m. A 30 m buffer is added on every side. Find the area of the site and the area to be flown in hectares.

Site area = 900 × 600 = 540,000 m² = 54 ha

Flown length = 900 + 2 × 30 = 960 m

Flown width = 600 + 2 × 30 = 660 m

Flown area = 960 × 660 = 633,600 m²

Flown area = 63.4 ha

2.1.2 Selecting appropriate flight parameters (altitude, overlap, etc.)

Flight parameters are the settings that control how the drone flies and takes photographs, mainly the flying height (altitude above ground), forward overlap, side overlap, flight speed, camera settings and flight line direction. They determine the ground sample distance (GSD), which is the size of ground represented by one image pixel, as well as the accuracy of the final products and how long the mission takes. Lower altitude gives finer detail but needs more photos and flight time, while higher overlap gives better 3D reconstruction but increases the number of images. Example: for a quarry wanting accurate stockpile volumes, the surveyor chose a height of 80 m, 80% forward overlap and 70% side overlap to achieve about 2 cm GSD.

GSD = (Sensor width × Flying height) ÷ (Focal length × Image width in pixels)

Footprint width = GSD × Image width in pixels

Photo scale = Focal length ÷ Flying height

Air base (distance between photos) = Footprint along track × (1 − Forward overlap)

Flight line spacing = Footprint across track × (1 − Side overlap)

Typical Flight Parameters

Worked Example: GSD, Footprint and Spacing

A drone camera has a sensor width of 13.2 mm, focal length of 8.8 mm and image size of 5,472 × 3,648 pixels. It flies at 100 m with 80% forward overlap and 70% side overlap, with the short side of the image along the flight direction. Find the GSD, footprint, photo spacing and line spacing.

GSD = (Sensor width × H) ÷ (Focal length × Image width)

GSD = (13.2 × 100) ÷ (8.8 × 5,472)

GSD = 1,320 ÷ 48,153.6

GSD = 0.0274 m = 2.74 cm per pixel

Footprint across track = 0.0274 × 5,472 = 150 m

Footprint along track = 0.0274 × 3,648 = 100 m

Photo spacing = 100 × (1 − 0.80) = 20 m

Line spacing = 150 × (1 − 0.70) = 45 m

Photo scale = 8.8 mm ÷ 100,000 mm ≈ 1 : 11,364

Factors Influencing Parameter Selection

• Required accuracy and GSD: Engineering surveys need small GSD and therefore lower altitude.
• Type of terrain and features: Forests, water and uniform surfaces need higher overlap to match images.
• Legal height limit: Altitude must remain within the maximum height allowed by regulations.
• Battery endurance: Parameters must allow the mission to be completed with available batteries.
• Processing capacity: Very high overlap and low altitude produce large datasets that need powerful computers.

2.1.3 Planning the flight path to ensure coverage and image quality

The flight path is the route the drone follows over the area, usually a series of parallel flight lines (strips) in a back-and-forth "lawnmower" pattern, with photographs taken at regular intervals along each line. Good flight path planning ensures complete coverage with the required overlap, consistent image quality and the shortest practical flying time. Flight lines are usually oriented along the longest side of the area to reduce the number of turns, and for 3D mapping of buildings a double grid (cross-hatch) or orbit pattern may be added. Example: for a road contractor mapping a 6 km corridor, the planner used two long lines along the road instead of many short cross lines, saving battery time and reducing turns.

Common Flight Patterns

Steps for Planning the Flight Path

1. Load the area: Import the boundary polygon into the flight planning app.
2. Set parameters: Enter height, overlaps, speed and camera settings.
3. Choose line direction: Orient lines along the longest side of the area, or across slope in hilly ground.
4. Generate the lines: Let the software create flight lines and photo positions.
5. Check coverage: Confirm that the lines extend beyond the boundary to cover the buffer.
6. Check terrain following: Enable terrain following in hilly areas so that the height above ground, and therefore GSD, remains constant.
7. Estimate time and batteries: Review flight time, number of photos and battery changes.
8. Plan take-off and landing: Choose safe home points and plan battery swaps between flights.
9. Allow turn-around zones: Extend lines beyond the area so that the drone (especially a fixed-wing drone, with its wide turning circle) is flying straight and level before the first photo of each line.
10. Check the GCP layout against the lines: Make sure the planned ground control points fall well inside the image coverage and not on the outermost edge of a line.

Worked Example: Number of Lines, Photos and Flight Time

Using the spacing from 2.1.2 (photo spacing 20 m, line spacing 45 m), a site 900 m long and 600 m wide is flown with lines along the length at 8 m/s. Estimate the number of lines, photos and flying time.

Number of lines = (Width ÷ Line spacing) + 1

Lines = (600 ÷ 45) + 1 = 13.3 + 1, so 15 lines

Photos per line = (Length ÷ Photo spacing) + 1

Photos per line = (900 ÷ 20) + 1 = 46 photos

Total photos = 15 × 46 = 690 photos

Flight distance = 15 × 900 + 14 × 45 = 13,500 + 630 = 14,130 m

Flight time = 14,130 ÷ 8 = 1,766 s ≈ 29.4 minutes

With about 20 minutes of usable flight per battery, 2 batteries are needed.

2.1.4 Considering factors such as terrain, weather, and airspace regulations

Before flying, the planner must consider the terrain, weather and airspace regulations, because each can make a flight unsafe, illegal or unsuccessful. Terrain affects the actual height above ground and therefore the GSD and overlap, weather affects the drone’s stability and image quality, and regulations control where, how high and under what permissions drones may operate. In Kenya, drone operations are regulated by the Kenya Civil Aviation Authority (KCAA) under the Civil Aviation (Unmanned Aircraft Systems) Regulations, which cover registration, pilot licensing, operating limits and permissions, and the current requirements should always be checked with KCAA before a mission. Example: a team mapping a tea farm on steep slopes in Kericho used terrain following and planned the flight for the morning, before the usual afternoon rain and wind.

Terrain Considerations

• Changes in elevation: In hilly areas, height above ground varies, changing GSD and overlap, so terrain following or higher overlap is needed.
• Tall obstacles: Trees, masts, buildings and power lines must be cleared by a safe margin.
• Uniform surfaces: Water, sand and dense crops have few features, so higher overlap helps image matching.
• Take-off and landing areas: Flat, open, firm ground away from people is needed.
• Line of sight: Hills and buildings may block sight of the drone and the radio signal.

Weather Considerations

• Wind: Strong or gusty winds cause drift, blurred images and high battery use; fly within the manufacturer’s wind limit.
• Rain and moisture: Most survey drones are not waterproof, and water droplets spoil images.
• Light conditions: Even light, such as bright overcast or near midday, reduces long shadows; avoid very early or late flights.
• Cloud shadows: Moving cloud shadows cause uneven brightness in the orthomosaic.
• Temperature: High heat reduces battery performance and air density, shortening flight time.

Airspace Regulation Considerations

• Registration and licensing: The drone must be registered and the pilot appropriately licensed or authorised as required by KCAA.
• Operational permission: Approval may be required for commercial survey flights and for specific locations.
• Height limit: Flights are generally restricted to a maximum height above ground, commonly about 120 m (400 ft).
• Restricted and prohibited areas: Airports, military sites, national parks, government installations and crowds require special permission or are no-fly zones.
• Visual line of sight: The pilot must normally keep the drone in direct sight at all times.
• Privacy and landowner consent: Permission from landowners and respect for privacy must be observed.
• Wildlife and protected areas: Flights over national parks, reserves and conservancies need approval from the Kenya Wildlife Service or the managing authority, and low flying disturbs animals and birds.
• Contingency planning: Alternative landing sites and an agreed abort procedure are planned in case of sudden storms or equipment failure.

2.1.5 Preparing for mission execution, including equipment setup and safety procedures

Mission preparation is the work done before take-off to make sure the equipment is complete and working, the ground control is in place, the team understands the plan, and risks to people, property and the drone are controlled. A thorough pre-flight checklist prevents common failures such as flat batteries, missing memory cards, uncalibrated compasses or wrong camera settings, which can waste a whole field day. Safety procedures include a site risk assessment, briefing, establishing a safe take-off zone and agreeing emergency actions. Example: before mapping a hospital’s new site, the survey team placed and surveyed ground control points, cordoned off the take-off area and briefed hospital security about the flight.

Equipment for a Drone Mapping Mission

Steps for Preparing for Mission Execution

1. Confirm permissions: Check that authorisations, landowner consent and notifications are in place, and inform the relevant authorities (air traffic control near airports, local administration or police where required) of the flight times.
2. Charge and check equipment: Charge batteries, controller and tablet, and check propellers, camera and memory cards.
3. Load the mission: Upload the flight plan and offline base maps to the tablet.
4. Place ground control points: Lay out GCP targets evenly across the area, including corners and centre, and survey them with GNSS.
5. Carry out a site risk assessment: Identify hazards such as people, power lines, livestock and weather.
6. Set up the take-off area: Place the landing pad in an open area and cordon it off.
7. Brief the team: Assign roles such as pilot and observer and agree emergency procedures.
8. Run the pre-flight checks: Calibrate the compass if needed, check GNSS lock, return-to-home height and camera settings.

Safety Procedures

• Keep a safe distance from people: Do not fly over crowds or people not involved in the operation.
• Use a visual observer: An observer watches for aircraft, birds and obstacles while the pilot monitors the screen.
• Set return-to-home correctly: Set the return height above the tallest obstacle on site.
• Handle batteries safely: Store and charge lithium batteries in fire-safe bags and never use swollen or damaged batteries.
• Have an emergency plan: Agree what to do if the drone loses signal, has a low battery or must land immediately.

2.1.6 Conducting the flight mission and capturing aerial imagery

Conducting the flight mission is the stage where the drone takes off, flies the planned lines automatically and captures the aerial images, while the pilot and observer supervise. Most survey missions are flown in automatic mode by the flight planning app, which triggers the camera at the planned positions to achieve the correct overlap, but the pilot must be ready to take manual control at any time. Good practice is to take a short test photo, confirm image quality, and keep records of each flight for processing and quality checks. Example: a team mapping a flower farm’s greenhouses near Naivasha flew in the late morning when the wind was calm, completing the mission in three flights with battery changes between them.

Steps for Conducting the Flight Mission

1. Power on in sequence: Switch on the controller and tablet first, then the drone, and wait for GNSS lock.
2. Check the camera: Take a test image and check focus, exposure and that the image is saved.
3. Announce take-off: Warn the team and confirm the area is clear.
4. Take off and hover: Take off, hover at a few metres and check stability and controls.
5. Start the automatic mission: Upload and start the mission, watching the drone climb to flying height.
6. Monitor the flight: Watch the drone and the telemetry for battery, signal and photo count.
7. Change batteries when required: Let the drone return, land, replace the battery and resume from the last point.
8. Land safely: Bring the drone back to the landing pad and switch off the drone before the controller.
9. Record the flight: Note date, time, weather, flight number, battery used and any problems in a flight log.

Good Practices for Image Capture

• Use fast shutter speeds: Prevent blur from movement and vibration.
• Keep exposure consistent: Avoid big changes in brightness between images.
• Avoid flying during changing light: Fast-moving clouds create patchy images.
• Fly the same height above ground: Use terrain following for even GSD.
• Capture extra images at edges: Make sure boundary areas have full overlap.
• Keep the team in contact: The pilot, observer and ground crew agree signals or use radios so that decisions are made quickly and safely.

2.1.7 Monitoring and adjusting the flight plan as needed during the mission

During the mission, the pilot continuously monitors the drone, its telemetry and the site conditions, and adjusts the plan if conditions change or problems arise, so that the data remains complete and the operation stays safe. Monitoring covers battery level, signal strength, GNSS status, wind warnings, photo count and the drone’s position relative to obstacles and people, while adjustments may include pausing the mission, changing height or speed, returning early or adding a re-flight of a missed area. Example: while mapping a county market site, the observer noticed a crane being raised nearby, so the pilot paused the mission, increased the safe height in that area and resumed once clearance was confirmed.

Items to Monitor During the Flight

Steps for Adjusting the Flight Plan During the Mission

1. Detect the problem: Identify the issue from telemetry, warnings or the observer.
2. Pause the mission if needed: Hold the drone in hover to assess the situation safely.
3. Decide the action: Choose whether to continue, change parameters, return home or land immediately.
4. Apply the adjustment: Change height, speed or route in the app, or bring the drone back.
5. Resume from the last point: Use the app’s resume function to continue without leaving gaps.
6. Record the change: Note what was changed and why in the flight log.
7. Plan re-flights: Mark areas that need repeating and fly them before leaving site.

On-site Data Checks Before Leaving

• Count the images: Confirm the number of images matches the plan.
• Check sample images: Look for blur, overexposure or wrong camera angle.
• Check coverage: Use the app or a quick preview to confirm there are no gaps.
• Back up the data: Copy images to a laptop or second card on site.
• Confirm GCP records: Make sure all GCP coordinates and photos of targets are saved.

2.1.8 Post-flight data processing and analysis to generate accurate photogrammetric products

Post-flight processing converts the aerial photographs into accurate photogrammetric products such as orthomosaics, digital surface and terrain models, point clouds, contours and volume reports, using photogrammetry software. The software finds common features in overlapping images, computes the camera positions and orientations (aerial triangulation), uses ground control points to fix the model to the correct coordinates, and then builds dense 3D data from which the products are generated. Accuracy is then checked against independent check points and reported as a root mean square error (RMSE). Example: images of a water company’s treatment works site were processed into a 3 cm orthomosaic and a terrain model that the engineers used to design new pipelines.

Photogrammetric Products

Steps for Processing Drone Data

1. Import images: Load all images and their positions into the photogrammetry software.
2. Check image quality: Remove blurred, duplicate or take-off and landing images.
3. Align images: Run aerial triangulation to match features and compute camera positions.
4. Add ground control points: Import GCP coordinates and mark each target in several images.
5. Optimise the model: Re-run the adjustment to fit the model to the GCPs.
6. Build the dense point cloud: Generate detailed 3D points from the aligned images.
7. Generate surfaces and orthomosaic: Create the DSM, classify ground points for the DTM and produce the orthomosaic.
8. Derive products: Generate contours, profiles and volumes as required.
9. Check accuracy: Compare model coordinates with surveyed check points and compute RMSE.
10. Export and report: Export products in required formats and write a processing and accuracy report.

RMSE = √[Σ(error²) ÷ n]

Worked Example: Accuracy Check Using RMSE

Five check points show height differences between the model and the ground survey of +2, −3, +4, −1 and +3 cm. Find the vertical RMSE.

Σ(error²) = 2² + 3² + 4² + 1² + 3²

Σ(error²) = 4 + 9 + 16 + 1 + 9 = 39 cm²

RMSE = √(39 ÷ 5)

RMSE = √7.8

RMSE = 2.79 cm

Factors Affecting the Accuracy of Products

• GSD and flying height: Smaller GSD generally gives better accuracy.
• Number and distribution of GCPs: Well-spread GCPs control the model across the whole area.
• Image overlap and quality: Sharp images with good overlap give better matching.
• Terrain and vegetation: Dense vegetation hides the ground and reduces DTM accuracy.
• Processing settings and operator skill: Correct settings and careful GCP marking are essential for reliable results.

Integration with GIS and survey data

The finished products are brought into GIS and survey software together with existing data: cadastral boundaries, ground survey points, utility records and land use layers. Overlaying the orthomosaic on registry index maps shows where boundaries, fences and buildings differ, and terrain models are combined with ground survey levels for design work. Products are delivered in the client's coordinate system (for example Arc 1960 UTM, or WGS 84 where specified) with metadata stating date of flight, GSD and achieved accuracy.

Chapter Summary

This chapter covered the essential process of conducting image acquisition in photogrammetric surveys, beginning with detailed flight planning. It emphasized the importance of accurately determining the area to be mapped and selecting suitable flight parameters such as altitude and image overlap to ensure comprehensive coverage. Planning the flight path took into account terrain features, weather conditions, and airspace regulations to optimize image quality and mission safety. Preparation for mission execution included setting up equipment and implementing safety procedures, followed by the actual flight mission where aerial imagery was captured. The chapter highlighted the need for real-time monitoring and adjustment of the flight plan during the mission, as well as post-flight data analysis to produce accurate photogrammetric outputs. Environmental conditions crucial for successful image acquisition were discussed, focusing on clear weather, good visibility, minimal cloud cover, stable atmospheric conditions, and favorable lighting. Finally, the roles of ground control points, image quality, and ground truthing were examined as fundamental components for ensuring the precision and reliability of photogrammetric surveys.