Drone Flight Planning: Principles and Practices PDF

DIGITAL PHOTOGRAMMETRY (GEOM 2020) Department of Geomatics Engineering and Land Management Planning a Mapping Project: Flight & GCP (Week 5) Dr O.O Aladejana  Outline  Flight Planning  Planning the G...

DIGITAL PHOTOGRAMMETRY (GEOM 2020) Department of Geomatics Engineering and Land Management Planning a Mapping Project: Flight & GCP (Week 5) Dr O.O Aladejana  Outline  Flight Planning  Planning the Ground Control  Selecting Instruments  Estimating Costs and Delivery Schedules 2  FLIGHT PLANNING  For photography to fulfil its purposes effectively, the photographic mission needs to be meticulously planned and executed in strict adherence to the "flight plan."  Flight planning consists of a flight (navigation) map that shows where the aerial photographs are to be taken alongside important parameters. 3  Importance of Flight Planning  Critical for the success of any photogrammetric project.  Flight Plan Components:  Flight Map: Indicates where photos are taken.  Specifications: Defines how to take photos (camera, scale, flying height, overlap, tilt, etc.).  Cost & Timing Considerations:  Aerial missions involve expensive equipment and crew.  Limited time windows due to weather and ground conditions.  Failure Risks: Poor planning leads to costly re-flights and project delays. 4  Flight Planning Procedures  To develop a flight plan, key factors must be determined: 1. Photographic end lap and side lap 2. Purpose of the photography 3. Photo scale 4. Flying height 5. Ground coverage 6. Weather conditions 7. Season of the year 8. Flight map 5  1. Photographic overlap and side lap Aerial photo projects for all mapping and most image analyses require that a series of exposures be made along each of the multiple flight line To guarantee stereoscopic coverage throughout the site, the photographs must overlap in two directions: a) in the line of flight b) between adjacent flights (Overlap) needed for parallax (Sidelap) to avoid missing bits 6 7  Photographic overlap and side lap Aerial photo projects for all mapping and most image analyses require that a series of exposures be made along each of the multiple flight line To guarantee stereoscopic coverage throughout the site, the photographs must overlap in two directions: a) in the line of flight b) between adjacent flights (Overlap) needed for parallax (Side lap) to avoid missing bits 8  The overlapping area between consecutive photographs is 60%.  This degree of overlap is critical for creating stereo pairs  The overlap allows for better accuracy and detail in the final images. 9 10 11  2. Purpose of Photography  The most critical step in planning an aerial photographic mission is identifying the purpose.  Defining the purpose informs decisions on equipment, procedures, and techniques. Metric vs. Pictorial Qualities Metric Pictorial Required for topographic mapping or Essential for qualitative analysis and quantitative measurements. visualization. Uses High precision is needed for tasks like surveying, Suitable for photographic interpretation, creating accurate maps, and photogrammetric constructing orthophotos, aerial mosaics, analysis. and photomaps. Achieved through calibrated cameras (i.e. cameras with known focal length, principal point, Wide-angle or super-wide-angle (short lens distortion coefficient, sensor alignment, focal length) cameras are preferred. Camera image resolution and scaling factors) types Uses fine-grained, high-resolution film or digital This allows for a large base-height ratio sensors with a high pixel count for superior (B/H′), improving the quality of topographic accuracy. mapping. 12  Understanding B/H′ Ratio in Aerial Photography  B/H′ Ratio: The ratio of the air base (B) of overlapping photos to the average flying height (H′).  Impact of B/H′: A larger ratio means:  Greater intersection angles between light rays.  Better accuracy in determining points on the ground.  Figure a:  Lower flying height (H′₁), larger parallactic angle (ϕ₁).  Higher B/H′ Ratio → Better accuracy for mapping.  Figure b:  Higher flying height (H′₂), smaller parallactic angle (ϕ₂).  Lower B/H′ Ratio → Less accurate for mapping. Why B/H′ Ratio Matters:  High B/H′ Ratios:  Lower flying heights, larger x-parallax, leading to greater accuracy in mapping and quantitative analysis.  Low B/H′ Ratios:  Higher flying heights, smaller x-parallax, leading to more errors in position and elevation computation. 13  3. Photo Scale  Average photographic scale is one of the most important variables that must be selected in planning aerial photography.  It is generally fixed within certain limits by specific project requirements.  For topographic mapping, photo scale is usually dictated by the map’s required scale and/or horizontal and vertical accuracy.  In photo interpretation or when preparing orthophotos or mosaics, the ability to clearly see the smallest important ground objects may be the main factor in choosing the photo scale. 14  Deriving photo scale Example  A particular project involves the use of aerial photography to study the centreline markings of highways.  The actual width of painted centrelines on the highways is 100 mm (4 in). A high-resolution film (80 line pairs per millimetre) will be used.  What minimum photo scale is required? Solution  With a resolution of 80-line pairs per millimeter, the smallest objects that could reasonably be resolved with that film would be 1/80 mm = 0.0125 mm in size. Thus, the minimum scale required would be roughly: 0.0125 𝑚𝑚 𝑆 = 100 𝑚𝑚 𝑆 = 0.000125 1 𝑆 = = 8000 15 0.000125  4. Flying Height  Once the camera focal length and required average photo scale have been selected, required flying height above average ground is automatically fixed in accordance with scale where 𝑓 is the focal length of the 𝑓 camera, 𝐻 is the flying height of the 𝑆 = aircraft above mean sea level, and 𝐻−ℎ ℎ𝑎𝑣𝑔 is the average elevation of the terrain. Example Solution 𝑓  Aerial photography having an average scale of 1:6000 is 𝑆 = 𝐻−ℎ required to be taken with a 152.4-mm-focal-length camera over terrain whose average elevation is 425 m 1 152.4 𝑚𝑚 0.1524 𝑚 above mean sea level. = = 6000 𝐻 − 425 𝑚 𝐻 − 425 𝑚  What is required flying height above mean sea level? 𝐻 = 6000 0.1524 𝑚 + 425 𝑚 = 1340 𝑚 16  5. Ground Coverage  Once average photographic scale and camera format dimensions have been selected, the ground surface area covered by a single photograph may be readily calculated.  In addition, if end lap and side lap are known, the ground area covered by the stereoscopic neat model can be determined.  The neat model, as shown in the figure is the stereoscopic area between adjacent principal points and extending out sideways in both directions to the middle of the side lap.  The neat model has a width of B and a breadth of W.  Its coverage is important since it represents the approximate mapping area of each stereopair. 17  6. Weather Conditions  Ideal Conditions:  Clear skies with less than 10% cloud cover.  Clouds above flying height are problematic due to shadows obscuring ground features.  Overcast Weather:  Can be beneficial for large-scale photos (e.g., mapping built-up areas, forests, canyons) as it reduces troublesome shadows.  Unsuitable Conditions:  Atmospheric haze, smog, dust, smoke, and high winds can make photography difficult.  Haze can be reduced using a yellow filter.  Smog, dust, and smoke can't be easily filtered and are best avoided by flying after heavy rains or during cold fronts.  High winds cause image motion, difficulty in maintaining flight paths, and inconsistent heights.  Flight Crew Role:  Must interpret weather and decide daily whether to fly.  Being near the project site helps the crew quickly take advantage of good weather conditions. 18  7. Season of the year  Ground Cover:  For topographic mapping, photos should be taken when deciduous trees are bare (late fall or early spring) to avoid leaves obscuring the ground.  Oak trees may hold leaves until early spring, making the ideal time for photography a short period in spring before new leaves appear.  Forestry photography may require trees to be in full leaf.  Snow Conditions:  Photography is usually avoided when snow covers the ground as it obscures details.  However, light snow can sometimes help in identifying the ground in tree-covered areas.  Sun's Altitude:  Low sun angles (below 30°) cause long shadows, which can obscure details.  Best time for aerial photography is when the sun is above 30°, typically in the middle part of the day.  During November to February, in some northern regions, the sun doesn't reach 30°, making photography difficult.  Shadows:  Shadows can sometimes be helpful, for example, in identifying tree species or locating features like fenceposts and power poles for control points. 19  8. Flight Map  Flight Map:  A flight map Shows the project boundaries and flight lines the pilot must follow for proper coverage.  It is often based on topographic maps or small-scale aerial photos of the area.  Pilot Navigation:  The pilot uses identifiable ground features along the flight lines to ensure accurate coverage.  Alternatively, an airborne GPS can guide the aircraft along predefined flight lines. 20  Flight Line Orientation:  For rectangular areas, flight lines are ideally aligned north- south or east-west, making it easier to follow roads or section lines.  For irregularly shaped areas, flight lines may be aligned parallel to project boundaries for efficiency.  Flight Planning Templates:  Transparent templates show blocks of neat models and are placed over maps to plan the most economical coverage.  Templates help determine exposure stations and are useful when using artificial targets.  Example:  Once the camera focal length, photo scale, end lap, and side lap are set, a flight map can be prepared for a rectangular area. 21  PLANNING THE GROUND CONTROL  Photogrammetric control involves points with known positions in an object-space reference system that can be identified in photographs.  In aerial photogrammetry, ground control is used to relate aerial photographs to the ground.  Ground control is essential in nearly all phases of photogrammetric work.  Aircraft-mounted GPS/INS systems can serve as control in photogrammetric surveys by measuring the camera's position and attitude during acquisition. 22  GCP is any point whose positions are known in an object-space reference coordinate system and whose images can be positively identified in the photographs.  Used for  Geo-referencing products  Accurate determination of processing parameters (intrinsics& extrinsics)  The accuracy of finished photogrammetric products can be no better than the ground control  3 Types –w.r.t measurement  Horizontal Control Points  Vertical Control Points  Full Control Points 23 24 25  COST ESTIMATION AND DELIVERY SCHEDULES  Experience with projects of a similar nature is essential in estimating cost and developing delivery schedule.  In estimating cost, the following main categories of efforts and materials are considered:  labour  materials  overhead  profit 26  Once quantities are estimated as illustrated in the above steps, hours for each phase are established.  Depending on the project deliverables requirements, the following labour items are considered when estimating costs:  aerial photography  ground control  aerial triangulation  stereo-plotting (# of models = # photos -1)  map editing  ortho production  lidar data cleaning 27  Delivery Schedule  After the project hours are estimated, each phase of the project may be scheduled based on the following:  number of instruments or workstations available  number of trained personnel available  amount of other work in progress and its status  urgency of the project to the client 28  Drone Flight Planning: Principles and Practices 29  Drone Flight Planning: Principles and Practice On drones and what it capable of The Why: Do we need Flight Planning? The How: Principles of flight plan design The What: Software to perform drone flight planning Some Notes on Drone Flight Planning 30  On Drones and what it capable of doing (Quad)Copter Fixed-wings Hobbyist, Small-area mapping Map Production Easier to control Covers Larger area Higher Resolution More Stable during flight 31  Drones are commonly used for hobbies:  Aerial Photography  Video Shooting  Filmmaking  etc 32 For more serious areas: Orthophoto Mosaic Point Cloud/3D Models Digital Elevation (DEM) Big question is: How? 33  We need to make sure that our drones captures images at the right time in the right place The Why We don’t want to (and we can’t) control our drones at all times, especially on larger areas or where we Do we need can’t always see the drone flight planning? We want to make sure that the drone is flying safely. That’s why we need to PLAN 34 Short Answer: design a flight plan Designing flight plan is more than just about ‘area of interest’ Another very important aspect is altitude Why? Because of this 35 What differs this images? 36 The higher the drone flies, the larger the area it covers Higher also means = less photo for the same area 37 But higher flight also means less details Ground Sampling Distance (GSD) in a drone flight could be roughly defined as: “How big each pixel is on the ground” Ask yourself: Do I need a detailed view of 3000 px everything or just the global picture? 4000 px 38 Ground Sampling Distance (GSD) is a measurement of accuracy Smaller GSD shows more detailed view For DJI Phantom 4 Pro flying at 80 meters: GSD = 2.36cm/px (each pixel represent 2.36 cm in real world unit) 39  But wait, there’s more: If we want to have 3D Models of AOI A good seamless Or mosaic Then We need to make sure that the images overlapped with each other 40 41 42 If we want to have Coordinate: -7.765o;110.37o Models registered to real-world Or coordinate, Then we need to employ Ground Control Points (GCPs) 43 Ground Control Points are points on the ground, visible in the drone photos, and are defined in a reference system, Before or after the flight GCPs are measured using GPS, or other terrestrial surveying method 44  Most drones nowadays already have onboard GPS, which reduce the need for GCPs However, if you need centimeter accuracy, you either need to have GCPs or improve the accuracy of onboard45 GPS  A final flight plan might looks like this The black dots __ represent Point of exposure or Waypoints It is where the Drone’s camera take the photo 46  In short: We need to set the goal: what kind of end product(s) do you want from the project? The What Then, So what is flight We need to take into account planning? a. Areas (or objects) to cover b. Desired Accuracy (or GSD) c. Flight height and Flight Path d. Ground Control Points (GCPs) e. Overlap and Sidelap f. Kinds of drones g. Regulations, et-cetera 47  We used to calculate and manually draw a flight plan.. but it’s a long time ago. Nowadays, we have apps for that droneharmony.com https://flylitchi.com/ www.dronedeploy.com www.precisionhawk.com/precisionflight https://heighttech.nl/flight-planning-software/ 48  What can these apps do? Automagically design flight plan based on user input parameters (elevation, overlap, etc) 49 What can these apps do? It also calculates flight time, number of images and other calculations 50 What can these apps do? Drone flight path for site inspections 51 What can these apps do? Façade Inspection 52 What can these apps do? Import OpenStreetMap (OSM) Data 53 What can these apps do? Avoiding predefined obstacles And much more 54 … And off we go 55 Questions 56 Self Study 57  Example  A project area is 10 mi (16 km) long in the east-west direction and 6.5 mi (10.5 km) wide in the north-south direction.  It is to be covered with vertical aerial photography having a scale of 1:12,000.  End lap and side lap are to be 60 % and 30 %, respectively.  A 6-in- (152.4-mm-) focal-length camera with a 9-in- (23-cm- ) square format is to be used.  Prepare the flight map on a base map whose scale is 1:24,000  compute the total number of photographs necessary for the project. 58  Solution 1. Fly east-west to reduce the number of flight lines The plane flies in parallel lines east to west to minimize the number of flight paths needed to cover the entire area. 2. Dimension of square ground coverage per photograph [photo scale =1:12,000 (1 in/1000 ft)] is 𝐺 = 9 𝑖𝑛 × 1000 𝑓𝑡⁄𝑖𝑛 = 9000 𝑓𝑡 (2800 𝑚) Size of Area Covered by One Photo: Each photograph taken from the plane covers an area of 9,000 feet by 9,000 feet on the ground. 3. Lateral advance per strip (at 30% side lap) is 𝑊 = 0.7𝐺 = 0.7 9000 𝑓𝑡 = 6300 𝑓𝑡 (1900 𝑚) Distance Between Flight Lines (Side Lap): There’s an overlap between adjacent flight lines to make sure no gaps are left between the photos. This overlap is called a "side lap." At 30% side lap, the distance between two adjacent flight lines is 6,300 feet. 4. Number of flight lines. Align the first and last lines with 0.3G (side-lap dimension) coverage outside the north and south project boundary lines, as shown in Fig below. This ensures lateral coverage outside of the project area.) Distance of first and last flight lines inside their respective north and south project boundaries is 0.5𝐺 − 0.3𝐺 = 0.2𝐺 = 0.2 9000 = 1800 𝑓𝑡 (550 𝑚) Number of spaces between flight lines 𝑓𝑡 6.5 𝑚𝑖 × 5280 − 2 × 1800 𝑓𝑡 𝑚𝑖 = 4.9 (𝑟𝑜𝑢𝑛𝑑 𝑢𝑝 𝑡𝑜 5) 6300 𝑓𝑡 Number of flight lines = number of spaces + 1 = 6 59 Number of Flight Lines: You need six flight lines to cover the entire area. The first and last lines go slightly beyond the project boundary (0.3 times the area a single photo covers) to ensure the entire project is photographed. 5. Adjust the percent side lap and flight line spacing. Adjusted percent side lap for integral number of flight lines (include portion extended outside north and south boundaries): 𝑃𝑆 𝑃𝑆 2 0.5 − 𝐺 + (𝑛𝑜. 𝑜𝑓 𝑠𝑝𝑎𝑐𝑒𝑠) 1 − 𝐺 = 𝑡𝑜𝑡𝑎𝑙 𝑤𝑖𝑑𝑡ℎ 100 100 𝑃𝑆 𝑃𝑆 2 0.5 − 9000 𝑓𝑡 + 5 1 − 9000 𝑓𝑡 = 6.5 𝑚𝑖 × 5289 𝑓𝑡/𝑚𝑖 100 100 𝑃𝑆 𝑃𝑆 2 0.5 − +5 1− = 3.813 100 100 𝑃𝑆 = 31.2% Adjusted spacing Wa between flight lines for integral number of flight lines: 31.2 𝑊 = 1− 𝐺 = 6190 𝑓𝑡 (1890 𝑚) 100 Adjusting Overlap and Flight Line Spacing: To make sure the number of flight lines is an integer (whole number), they adjust the side lap percentage slightly to about 31.2%. This changes the spacing between flight lines to 6,190 feet. 60 6. Linear advance per photo (airbase at 60% end lap): 𝐵 = 0.4𝐺 = 0.4 9000 𝑓𝑡 = 3600 𝑓𝑡 (1100𝑚) Distance Between Photos (End Lap): As the plane flies along a line, it takes photos one after the other. Each photo covers a certain distance, and the overlap between photos along the flight line is called the "end lap." At 60% end lap, the distance between two consecutive photos is 3,600 feet. 7. Number of photos per strip (take two extra photos beyond the project boundary at both ends of each strip to ensure complete stereoscopic coverage): 10 𝑚𝑖 × 5280 𝑓𝑡/𝑚𝑖 𝑁𝑜 𝑜𝑓 𝑝ℎ𝑜𝑡𝑜𝑠 𝑝𝑒𝑟 𝑠𝑡𝑟𝑖𝑝 = +1+2+2 3600 𝑓𝑡 = 19.7 𝑎𝑝𝑝𝑟𝑜𝑥.. 20 Number of Photos Per Flight Line: To cover the entire length of the project (10 miles long), about 20 photos are needed per flight line. A few extra photos are taken at both ends to make sure nothing is missed. 61 8. Total number of photos: 20 𝑝ℎ𝑜𝑡𝑜𝑠⁄𝑠𝑡𝑟𝑖𝑝 × 6 𝑠𝑡𝑟𝑝𝑠 = 120 Total Number of Photos: Since there are 6 flight lines, and each flight line needs around 20 photos, the total number of photos required is about 120. 9. Spacing of flight lines on the map: 𝑀𝑎𝑝𝑠 𝑠𝑐𝑎𝑙𝑒 = 1: 24000 1 𝑖𝑛 = 2000 𝑓𝑡 6190 𝑓𝑡 𝑝𝑒𝑟 𝑠𝑡𝑟𝑖𝑝 𝑊 = = 3.09 𝑖𝑛 (78.6 𝑚𝑚) 2000 𝑓𝑡 /𝑖𝑛 Draw the flight lines at 3.09-in spacing on the map, with the first and last lines [(0.5 – 31.2/100)9000 ft]/2000 ft/in = 0.84 in inside the project boundaries. Spacing of Flight Lines on the Map: The map scale is 1:24,000, meaning 1 inch on the map represents 2,000 feet on the ground. On the map, the flight lines are drawn 3.09 inches apart, and the first and last lines are 0.84 inches inside the project boundaries. 62