Drone mapping with the DJI Mini 4 Pro and Air 3S in DJI Fly: GSD, overlap, flights

Why a mapping grid for these drones is a DJI Fly mission

Mapping apps such as Pix4Dcapture, DroneDeploy, Dronelink and Litchi fly a drone through DJI's Mobile SDK. The current SDK, version 5, is Android-only, and its list of supported products has one of the DJI Fly drones: the Mini 4 Pro. The Air 3, Air 3S, Mini 5 Pro, Mavic 3 Pro and Mavic 4 Pro are not on it, and the SDK for iPhone stopped at version 4. On an iPhone, DJI Fly is the only app that flies any of the six; on Android, only the Mini 4 Pro has a third-party option.

That leaves the waypoint mission. DJI Fly flies a route of up to 200 waypoints from a KMZ file in DJI's WPML format, with a photo at each waypoint, and a mapping grid is exactly that: parallel lines with a photo stop every few metres. The file is planned elsewhere and put into DJI Fly by replacing a placeholder mission's file. The KMZ guide shows how on an iPhone and on a DJI RC 2.

Ground sample distance: how much ground one pixel covers

The ground sample distance, GSD, is the size of one pixel on the ground, in cm/px. It decides what you can see in the orthophoto and the 3D model: at 2 cm/px a manhole cover is 30 pixels wide; at 5 cm/px it is a blob. It follows from the camera and the height: GSD = height × sensor width × 100 / (focal length × image width), with the height in metres, the sensor width and the focal length in millimetres and the image width in pixels.

The table gives the GSD of each DJI Fly drone's main camera in its 12 MP photo mode, the mode mapping is planned in. The 48 and 50 MP photos halve the number on paper, but a quad-Bayer sensor resolves about 12 MP of real detail, and the 12 MP mode allows a photo every two seconds on most of them. The Mavic 3 Pro's 20 MP camera has one mode.

GSD in cm/px with the camera straight down, 12 MP photo, computed from the sensor width and focal length in DJI's specifications. The Mini 4 Pro and Mavic 3 Pro values are confirmed from photo metadata; the others are derived from DJI's field of view or the sensor class and can be a few per cent off.

• Typical choices: 1–2 cm/px for a site or a building, 2–3 cm/px for an orthophoto of a field or a plot, 4–5 cm/px for large areas, all below 120 m above the ground.
• A slope changes the height above the ground, and with it the GSD. A grid over a hillside is planned with the terrain under it, because DJI Fly does not follow the terrain on its own.

Plan the grid

Six decisions, in the order the planner takes them.

1. 01

   Choose the resolution

   Decide the GSD from what the result is for; 2 cm/px for an orthophoto of a field or a plot is the common choice. Read the height for your drone from the table: at 2 cm/px that is about 56 m for a Mini 4 Pro and 57 m for an Air 3S.

2. 02

   Set the overlap

   80% front overlap (between the photos along a line) and 70% side overlap (between the lines). Pix4D's general case is at least 75% front and 60% side; flat fields with little texture get at least 80% both ways, and forests, dense vegetation, snow and water are special cases that want more overlap and a higher flight.

3. 03

   Lines along the long side

   The lines run along the long axis of the area, spaced by the side overlap; the photos along each line are spaced by the front overlap. With 80/70% at 2 cm/px on a Mini 4 Pro, the lines are about 24 m apart and the photos about 12 m apart.

4. 04

   Count the photos and the waypoints

   One waypoint per photo, because DJI Fly does not reliably honour the interval triggers a waypoint file can carry. The 2.8 ha field on the home page took 172 waypoints and 171 photos at 2 cm/px with 80/70% overlap on an Air 3S, and about 27 minutes in the air.

5. 05

   Split into flights

   DJI Fly takes at most 200 waypoints per mission and cannot resume after a battery swap, and a flight is planned within 70% of the rated battery time. A bigger area is split into flights at the line ends, each its own KMZ, up to 1000 waypoints per mission. Fly them in order from the same takeoff.

6. 06

   Check the heights

   Heights in the file are relative to the takeoff point, and DJI Fly has no terrain follow on waypoint missions, so a slope has to be in the file: a grid over a hillside is planned with the terrain under it. Set Max Altitude in DJI Fly at least as high as the highest waypoint.

Fly it and build the orthophoto

1. 01

   Put the KMZ into DJI Fly

   Make a placeholder mission, close DJI Fly and replace its file under the same name, one placeholder per flight. The KMZ guide has the steps for an iPhone and for a DJI RC 2.

2. 02

   Set the camera

   Set the camera before take-off: 12 MP photos, in the format your software reads. The route, the stops and the gimbal angle come from the file.

3. 03

   Light and wind

   Fly in even light: high sun or bright overcast, no cloud shadows moving across the site, and wind under about 6 m/s for consistent overlap.

4. 04

   Take off from the planned spot

   Heights are relative to the takeoff point. Take off where the plan says, or the whole grid moves up or down with the ground, and the GSD with it.

5. 05

   Fly every flight from the same spot

   Land and swap the battery between flights, and fly them in order.

6. 06

   Build the orthophoto

   Copy the photos from the drone and build the orthophoto or the model in a photogrammetry tool such as Pix4D, Agisoft Metashape or WebODM. Skyplanr plans and checks the flight; it does not process photos.

Accuracy without RTK: photogrammetry-ready, not survey-grade

None of the DJI Fly drones has RTK. The photos are tagged with the drone's own satellite fix, and DJI gives its hovering accuracy with satellite positioning alone as ±0.5 m; the absolute position of the finished map is typically about 1–3 m off, more in height than in plan, even when every detail in it lines up. The relative accuracy, distances and areas within the map, is far better, usually within a few pixels.

That is photogrammetry-ready, not survey-grade. For measurements that must tie into a coordinate system (property lines, volumes to be invoiced, as-built drawings), you need ground control points measured with an RTK receiver, or an RTK drone. In Norway you can compare the result with Kartverket's laser height data, the 1 m model Skyplanr checks the route against.

• Relative accuracy (within the map): good, usually a few pixels.
• Absolute accuracy (on the world): about 1–3 m without RTK or ground control.
• Survey tie-in: ground control points measured with RTK GNSS, at least five, in the corners and the centre, or an RTK drone.

A real plan: 171 photos at 2 cm/px

The mapping example on the home page is a real plan from Skyplanr's benchmark: an orthophoto of a 2.8 ha field at Hokksund at 2 cm/px with an Air 3S. The planner laid 172 waypoints with a photo at 171 of them, 80% front and 70% side overlap, and estimated 27 minutes in the air: one flight, within 70% of the battery. The table's height for 2 cm/px on an Air 3S is 57 m.

The limits, stated plainly

• Photogrammetry-ready, not survey-grade: about 1–3 m absolute without RTK or ground control.
• Skyplanr plans and checks the flight and writes the KMZ. It does not process orthophotos or 3D models.
• DJI Fly flies the waypoints. It does not follow the terrain, and it does not resume after a battery swap.
• The camera values for the Mini 5 Pro, Air 3, Air 3S and Mavic 4 Pro are derived from DJI's field of view or the sensor class, not confirmed from photo metadata: a few per cent of GSD.

Questions

Related guides

• Import a KMZ waypoint mission into DJI Fly (iPhone and DJI RC 2)

  DJI Fly has no KMZ import. Swap the file of a placeholder mission under the same name: step by step on an iPhone (RC-N2/RC-N3) and on a DJI RC or RC 2.

• DJI Fly waypoint limits: 200 waypoints, no resume after a battery swap, no terrain follow, Max Altitude

  DJI Fly takes at most 200 waypoints, won't resume after a battery swap, has no terrain follow and fails above Max Altitude. How to plan around each limit.

Sources

What this guide rests on. The third-party guides are linked as published; Skyplanr is not affiliated with them.

1. DJI Developer: Mobile SDK v5 for Android, supported products (github.com)
2. DJI Developer: Mobile SDK for iOS, latest version 4.16 (github.com)
3. DJI Developer: DJI WPML overview (the KMZ format DJI Fly reads) (developer.dji.com)
4. DJI: Mini 4 Pro specifications (camera, image size, hovering accuracy) (dji.com)
5. DJI: Mini 5 Pro specifications (camera, image size, hovering accuracy) (dji.com)
6. DJI: Air 3 specifications (camera, image size, hovering accuracy) (dji.com)
7. DJI: Air 3S specifications (camera, image size, hovering accuracy) (dji.com)
8. DJI: Mavic 3 Pro specifications (camera, image size, hovering accuracy) (dji.com)
9. DJI: Mavic 4 Pro specifications (camera, image size, hovering accuracy) (dji.com)
10. Pix4D: selecting the image acquisition plan type (overlap for the general case and for forests, fields and water) (support.pix4d.com)
11. Pix4D: computing the flight height for a given GSD (support.pix4d.com)
12. Kartverket: Høydedata, the national laser height data (terrain and surface models) (hoydedata.no)