Flight Planning

Center for Geospatial Analytics at North Carolina State University

Corey White adapted from Justyna Jeziorska and Helena Mitasova

Objectives

• Describe the four phases of flight planning, from project definition to flight control
• Apply safety procedures and preflight checklists required under Part 107
• Plan ground control point placement and relate GCP accuracy to ground sampling distance
• Set the flight parameters that shape a mapping mission: altitude, overlap, pattern, speed, and lighting
• Choose a flight planning platform for a given aircraft and project

Topic 2 covered what processing needs from the imagery. Today is how to plan a flight that delivers it, and Wednesday we fly at Lake Wheeler. Two questions keep coming back through the deck: what is the aim of the project, and what resolution does it need. Every parameter later follows from those two answers.

Where this lecture sits

Four questions to keep asking: What is the aim of the project? What resolution does it need? What do the regulations allow? What will the weather do?

Flight planning is the first box of the pipeline, and it is the cheapest place to fix a problem. A flight that was planned wrong cannot be processed right.

Four phases of flight planning

The strip at the top of the next slides tracks where we are: project definition, flight planning, site evaluation, and flight control.

Project definition

Decision	Driven by
Spatial scale: resolution and extent	Project aim, UAS and sensor capabilities, terrain constraints
UAS and sensor	Required resolution, area size, budget
Cost, labor, and time	Area, number of flights, field crew, processing
Terrain information	Existing DEMs, imagery, and local knowledge of the site

Extent and resolution set everything downstream: the aircraft you need, the number of batteries, the number of photos, and the processing time. Collect what you can about the terrain before you commit to a sensor.

Project definition

Legal constraints

• Airspace class and any authorization needed (Topic 1)
• Landowner permission for takeoff, landing, and access
• Local restrictions on the site

Coordinate system

• Chosen by the desired coordinate system of the final products
• Consistent with the coordinate system of the GCP survey
• Record the horizontal and vertical datum

Decide the coordinate system before the survey: a GCP file without its coordinate system is a list of numbers, not coordinates

Two things people forget until it is too late: permission to be on the ground, and the coordinate system. Pick the CRS of the final product first, then survey the control in it, or in something you can transform exactly. Lecture 2B covered why the vertical datum matters here.

Flight planning

• Mission area assessment
• Planning geometric parameters: altitude, overlap, pattern, and camera angle
• Choosing a flight planning and flight logging platform
• Preliminary weather assessment: climate, season, forecasts
• Creating the flight plan in the chosen software

This phase is the desk work. The geometric parameters get their own section at the end of the lecture; the platform choice gets a table. Weather starts here as climate and season, and gets rechecked on the day.

Placing ground control points

• A minimum of 5 GCPs is recommended; 5 to 10 are usually enough, even for large projects
• Use more GCPs when the topography is complex
• Distribute them evenly across the area
• Do not place GCPs exactly at the edges of the area

Even distribution matters more than count. Points clustered in one corner leave the rest of the model free to bow. Points on the very edge sit in weak image geometry and are hard to place on enough photos. Lecture 3B goes into control versus checkpoints.

Surveying the GCPs

Before measuring the GCP coordinates, define:

• The GCP coordinate system
• The GCP accuracy the project needs
• The survey equipment: RTK GNSS rover, total station, or handheld GPS

Target size: 5 to 10 times the GSD, so the center is identifiable on the photos

A handheld GPS gives meters, an RTK rover gives centimeters, and the model can never be more accurate than the control. On Wednesday we survey with the Emlid rover; the GNSS field protocol on the topic page walks through it.

Ground sampling distance

Distance between two consecutive pixel centers measured on the ground:

\[ \text{GSD} = \frac{H \times S_w}{f \times I_w} \]

Symbol	Meaning
\(H\)	Flight altitude above ground level, AGL (m)
\(S_w\)	Sensor width (mm)
\(f\)	Focal length (mm)
\(I_w\)	Image width (pixels)

Larger GSD means lower spatial resolution; smaller GSD means higher spatial resolution

Same formula works with height and image height. Worked example for the Mavic Mini 2 on the later screenshots: 70 m altitude, 6.17 mm sensor, 4.36 mm lens, 4000 pixels wide gives about 2.5 cm per pixel. Halve the altitude, halve the GSD.

GCP accuracy and GSD

Requirement	Rule of thumb
GCP target size	5 to 10 times the GSD
GCP survey accuracy	Better than the accuracy you need in the products; finer than 0.1 times the GSD is wasted, because the target cannot be marked on the image more precisely than that
Accuracy of the final products	Sets the survey accuracy and the GSD, and so the altitude, in the first place

Work backwards: required product accuracy sets the survey accuracy and the GSD, the GSD sets the altitude and the target size

At 2.5 cm GSD the target is 12 to 25 cm across, and there is no point surveying it better than about 2.5 mm. An RTK rover with a good fix gives 1 to 2 cm, and that is what actually limits the control. A handheld GPS at several meters would add error rather than remove it.

Site evaluation

• Terrain check: high obstacles in the takeoff, mission, landing, and alternative landing locations
• Ask the locals about possible air traffic or ground activities
• Weather check: temperature affects battery life, and most UAS cannot operate in rain
• Use checklists, do not rely on your memory

Sample checklists: paper checklist for the Phantom and the RMUS preflight app

Walk the site before the flight day if you can. Look up for wires and trees along the flight lines, and pick the alternative landing area before you need it. Cold cuts battery life hard, so plan fewer minutes per battery in winter.

Site layout

Everything on this picture is chosen on the ground before launch: the operational area, the area of interest inside it, the command center with the pilot, the observer position, the safe area for everyone else, and the primary and emergency landing areas.

Preflight inspection

• A preflight inspection is required under Part 107.49: weather, airspace, people and property on the surface, crew briefing, control links, power, and payload
• The rule lists what to check, not a checklist; the FAA advisory circular AC 107-2 recommends the Remote Pilot in Command (RPIC) follow the manufacturer’s checklist, or develop one when none exists
• Checklists are usually built into the flight software or available from the vendor

North Carolina: no separate N.C. permit is required for commercial and government operators since December 1, 2024

Part 107.49 makes the inspection mandatory; the checklist is how you prove to yourself you did it, and AC 107-2 Appendix E has a sample. Notice the checklist covers before, during, and after: the post-flight column is where data gets copied and the logbook gets written, and that is the column students skip.

Flight control

• The RPIC should launch, operate, and recover from preset locations so the aircraft flies according to the mission plan
• Observation locations should be selected for maximum line of sight throughout the planned flight area (Part 107.31)
• On any failure or loss of visual contact, command the aircraft back to the recovery location or use the built-in fail-safe features

Visual Line of Sight (VLOS): the flight crew must have a clear view of the aircraft at all times, without aids other than corrective lenses

Preset locations means the takeoff, landing, and observer spots from the site layout slide, not wherever the crew happens to be standing. Know the return-to-home behavior and altitude of your aircraft before you need it.

Flight crew roles

Role	Responsibility
RPIC, Remote Pilot in Command	Holds the certificate; responsible for the operation and the final say
PMC, Person Manipulating the Controls	Flies the aircraft under the direct supervision of the RPIC
VO, Visual Observer	Watches the aircraft and the airspace; optional but recommended

Part 107.33: the RPIC, the person manipulating the controls, and any visual observer must be able to communicate with each other at all times

On Wednesday: I am RPIC, some of you will fly under supervision as PMC, and everyone else is a visual observer. Communication means you can be heard without a radio, or with one that works.

Lake Wheeler

Our site for the field flight and for the rest of the course data.

Lake Wheeler Road Field Laboratory

NC State’s agricultural field laboratory, about five miles south of campus. Mid Pines is the field we fly, and the GCP layout you saw earlier is from there.

Lake Wheeler on the sectional chart

Read the chart as in Lecture 1B: Lake Wheeler sits under the outer shelf of the RDU Class C, marked 44/17, so the controlled airspace starts at 1,700 ft MSL and a Part 107 flight at or below 400 ft AGL is in uncontrolled airspace beneath it. Confirm that in a LAANC or B4UFLY app on the day, since the answer is the facility map, not the distance to the airport.

Planning the mapping flight

Now the parameters you set in the software: location, camera, angle, altitude, pattern, overlap, speed, and lighting.

Flight planning software

• Multiple platforms are available; some are dedicated to a specific UAS and sold with the system

Commercial	Open source
DroneDeploy	QGroundControl
Pix4Dcapture Pro	Mission Planner
DJI GS PRO
UgCS
DroneLink

What they share: draw the area, set altitude and overlap, and the app computes the flight lines, photo count, and flight time

The screenshots in this section are DroneLink with a Mavic Mini 2. The parameters are the same in every app; only the buttons move.

Overlap geometry

• Forward overlap: between consecutive photos on a flight line
• Side (lateral) overlap: between adjacent flight lines
• The flight track on the right is the lawnmower pattern the plan produces

Classic aerial photogrammetry got by with 60 forward and 20 to 40 side. SfM wants a lot more, because every point needs to be seen from many angles; that is why the recommended numbers on the next slides are higher than the diagram.

Location and camera

Location

• Fly a larger extent than you need
• Think about the area you need for analysis, plus a buffer
• Edge accuracy is always worse than the middle

Camera settings

General camera settings are usually fine. Optional:

Setting	Value
Mechanical shutter	On
Focus	Infinity
Shutter priority	1/800 s
Aspect ratio	3:2

The buffer matters because the last flight line and the first photos on each line have the weakest geometry. A mechanical shutter avoids rolling-shutter distortion; a fast shutter avoids motion blur at mapping speed.

Camera angle

	Nadir (straight down)	Gently oblique	Oblique (pitched)
Gimbal pitch	-90 degrees	-55 to -75 degrees, 15 to 35 degrees off nadir	About -45 degrees
Best for	Most mapping: orthomosaics and DSMs	A supplemental pass over a nadir block; rough or high-relief terrain	Buildings, facades, 3D models
Trade-off	Simple, efficient	Stronger self-calibration, less doming; more photos	Full 3D detail; more photos and flight time, weaker orthomosaic

Combine them: a nadir grid gives the map, an inclined pass fixes the camera model, and a steep oblique pass adds the facades

Nadir for everything we do this semester. The middle column is the doming fix from Lecture 2B: James and Robson showed that convergent, inclined images let self-calibration separate lens distortion from surface shape. Nesbit and Hugenholtz tested angles from 5 to 35 degrees and found 20 to 35 degrees off nadir gave the biggest gain; James and colleagues in 2020 warned that tilts under 15 degrees are not enough for consumer cameras like the Phantom family. Steep obliques near 45 degrees are for vertical structure, not for the map.

Altitude

• Altitude above ground level (AGL) sets the GSD and the flight path; typical mapping altitude is 70 m to 120 m AGL
• Higher altitude: fewer lines, shorter flight, coarser GSD
• Use a terrain-aware flight path on sloping sites so AGL stays constant
• Stay under the ceiling of the airspace authorization

Altitude 70 m, 75% overlap

Altitude 120 m, 75% overlap

Same field, same overlap, two altitudes. The 70 m plan has about 70 percent more lines and gives 2.5 cm GSD; the 120 m plan flies faster at about 4.2 cm. Part 107 caps you at 400 ft AGL, and the LAANC ceiling can be lower.

Flight patterns

Normal (lawnmower)

• One set of parallel lines
• Standard for orthomosaics and DSMs

Crosshatch (grid)

• Two perpendicular sets of lines
• More detail and stronger geometry, longer flight time

Lawnmower for the field on Wednesday. Crosshatch doubles the flight time and is worth it for 3D models, for oblique work, and for sites where alignment struggled the first time.

Overlap

	Minimum	Recommended for SfM
Forward	60%	75 to 85%
Side	40%	60 to 80%

• Homogeneous terrain (crops, sand, water) needs more overlap: fewer features to match
• More overlap means more photos, longer flights, and longer processing

75% front and side overlap

85% front and side overlap

Question: how will overlap impact your flight path?

Ask them before showing the second screenshot: raising side overlap from 75 to 85 percent at the same altitude adds about two thirds more flight lines, since line spacing goes from 25 to 15 percent of the footprint. Forward overlap costs photos, side overlap costs flight lines and batteries.

Drone speed

• Typical mapping speed: about 30 km/h, set by the app from altitude and forward overlap
• The limit is the camera, not the aircraft

Factor	Effect on speed
Lighting	Low light forces a slower shutter and a slower flight
Camera shutter	Faster shutter allows faster flight
Altitude	Higher altitude allows faster flight for the same blur
Motion blur	Blur is speed times exposure time; keep it under one GSD, under half a GSD for precision work

The app sets a maximum speed so the camera can keep up with the forward overlap. Check the blur yourself: 30 km/h is 8.3 m/s, at 1/800 s that is about 1 cm of smear, under half of the 2.5 cm GSD at 70 m. Slow down in low light or at low altitude; a blurred photo is worse than no photo, because it still gets matched.

Lighting

Conditions	Effect
Overcast, bright	Best: even illumination, no hard shadows
Noon	Shortest shadows on a clear day
Partly cloudy	Exposure changes between photos; avoid if you can
Low sun	Long shadows and glare; avoid

Avoid shadows: shadows move between photos and between flights, and they show up as seams in the orthomosaic and as noise in the DSM

A bright overcast day is the mapping pilot’s friend. If it has to be a clear day, fly near solar noon and finish before the shadows move. Water and glossy roofs give glare that no overlap can fix.

Wrap-up

Assignment 3A: Terrain analysis of the flight site using GIS tools, a flight plan analysis for Lake Wheeler in GRASS

Next: the UAS flight at Lake Wheeler on Wednesday; read the GNSS field protocol before the trip. Lecture 3B covers ground control, checkpoints, and accuracy

Four phases: define the project, plan the flight, evaluate the site, control the flight. The assignment analyzes the terrain of the site you are about to fly. Wednesday we put the site layout, the checklist, and the crew roles into practice.