Digital aerial imaging and mapping with a Phantom Quadcopter

Digital aerial imaging and mapping with a Phantom 1 Quadcopter

by: Paul Illsley

The purpose of this page is to introduce the reader to some of the basic concepts and tools needed to record digital aerial images with a DJI Phantom 1 Quadcopter. It is primarily intended for those wishing to record images for mapping and research applications but it should also serve as a good starting point for those who just want to have fun experimenting.

Author recording images with the DJI Phantom 1 Quadcopter over the Annapolis Valley, Nova Scotia, Canada.
(Photo: Bria Stokesbury)

What is a DJI Phantom 1 Quadcopter and can we use it for mapping?

For many years I've been MacGyvering (if that’s even a word) my own methods of getting cameras into the air to record images (kites, balloons, RC parafoils….) and have had lots of fun doing it. Unfortunately with experimentation comes misfortune and I’ve seen more than one camera take a dive into the ground or bounce off a roof.

While perusing the web I came across a little white wonder called the DJI Phantom 1 Quadcopter and it immediately grabbed my attention. It’s definitely not the only Quadcopter on the market and it may not have the lifting capacity of larger machines but man is it ever packed with some pilot friendly technology.

The group at DJI (and no I’m not sponsored by them in any way) have really done their homework. They’ve taken a very complicated technology and simplified it to a level where literally all you need to do is increase the throttle and the thing flies straight and level the first time (at least it did for me).

The Phantom has a GPS, digital compass, accelerometer and gyro built in and it comes almost ready to fly; all you need to do is to charge the battery, plug in a couple wires, attach the propellers and landing gear then head out to an open area to give it a test flight. This thing will even return to the launch location and land unassisted if it loses contact with the transmitter, or if you lose track of its location in the sky you can just turn off the transmitter and it will return home. The first time I tried this feature was up on a glacier in Alaska and I must say I was nervous to see if it would work. But sure enough the thing stopped where it was, hovered for a couple seconds, started flying back toward the launch location and descended slowly until it landed unassisted within a couple meters of its launch site. I was amazed to say the least.

Now enough about the incredible technology encased in the little white plastic shell (there are lots of great videos online dedicated to this little wonder). What I really want to talk about is can we use it for mapping? You bet we can. Many use this copter for carrying GoPro cameras and it does a great job at that but I want to see if it could be used to carry other cameras like a Ricoh GX200. I chose the GX200 because it is small and light, it has a good quality wide angle lens (not fisheye like the GoPro), good pixel resolution (4000 X 3000), an intervalometer that allows it to record an image every 5 seconds, the ability to set the focus at infinity, manual exposure capabilities, RAW image recording, and the option to stream the viewfinder information out as a video feed (you will see why later). The Ricoh GX200 is now out of production and has been replaced with the Ricoh GR (which has a slightly narrower field of view).

Things to look for in a camera:

    Intervalometer (time lapse option): this option allows the camera to record images at a set interval. Try and pick a camera that will record images at 5 second intervals or shorter. Some cameras have a maximum number of images per time lapse session (99 images) so make sure you have enough time to record the number of images you will need.

    Light weight: I try and keep the weight of the camera to around 300g or lighter.

    Good resolution: go for a camera with at least 12 megapixels, higher if possible.

    Wide angle lens: I try and go for a lens with an angle of view equivalent to about 24mm (35mm equivalent). I have used a GoPro camera and have recorded some very good images with it but I’ve found that it isn’t the best camera for mapping simply due to the fact that it has such a wide angle lens (lens distortion) and the rolling shutter can cause some unwanted geometric distortion in the final image. A rolling shutter (normally used in video cameras) scans the image into memory instead of recording the entire image at once, so the top of the image is recorded before the bottom of the images. This delay in recording the entire image can cause objects to shift relative to their actual location (if the camera is moving) thus creating an image that doesn’t accurately represent the scene. This is only an issue if you are interested in using the camera for mapping purposes. Other than that, a GoPro can be used to record very interesting and useful aerial images.

    Manual exposure: this will help make sure all of your exposures are of similar brightness. If you set the exposure on automatic you could find that each image might be different brightness depending on what the camera is pointing toward (reflections off water, metal roofs,… can affect the auto exposure settings).

    Live video out: this will allow you to use a First Person View (FPV) setup so you can view what the camera is seeing during flight (very useful). I tend to prefer using an FPV system with a monitor instead of the goggle style viewer; it allows me to quickly glance up at the copter whenever I want.

    RAW file option: this isn’t a necessity but RAW files will offer higher resolution and better colour rendition compared to normal .jpg images.

    Manual focus: this allows the camera to record images even though the focusing system might not be able to focus on the ground. In certain situations auto focus has a hard time focusing on water, flat surfaces or areas without contrast or detail.

    Turn off “shake reduction”: this might sound counterintuitive but if you intend on using the images for photogrammetric mapping the lens needs to remain in the same location for each image relative to the imaging sensor. The “shake reduction” system attempts to move the lens (or sensor) to compensate for the movement of the camera, this relative movement of lens and sensor will distort the geometric relationship between the two and cause errors in the final mapping product. If you are just interested in recording images for reference purposes (or scenic shots), then you will find the “shake reduction” option to be quite useful.

UPDATE: if you are looking for an alternative camera, take this list (above) to a local camera shop and ask them to recommend possible newer models. New cameras are released on a regular basis so these photographic professionals should be able to point you in the right direction.

I decided to go real simple and attach the camera to the bottom of the Phantom (for vertical images) with small bungee cords (2 for safety reasons; as if anything about lifting a camera high into the air is really that safe). I placed a piece of foam between the camera and Phantom to absorb vibrations from the spinning rotors. This foam is VERY IMPORTANT. I used a piece of foam intended to absorbed vibrations around electrical components in RC aircraft but you can try any kind of soft foam.

I set the camera on Manual Exposure (to keep all image exposures the same) and a high shutter speed (1/500 - 1/1000 second), set the lens to infinity, set the camera to record images every 5 seconds, set it to record RAW images (for better dynamic range and resolution) and sent the thing aloft. I flew the Phantom across the study area (it’s best to record as many images of the study area as possible and from as many different perspectives as possible). After about 4 minutes of flying I decided to bring it home. Fortunately the batteries are the standard RC Lithium Polymer (LiPo) style so they are pretty inexpensive and rechargeable. I’d pick up at least 5 additional batteries if you can.

After flying it by sight I decided to attach a video telemetry unit (FPV: First Person View) to the camera which allowed me to view what the camera was seeing in real time. I used industrial strength Velcro to attach the transmitter to the camera. This FPV option has proven to be valuable for positioning the camera over specific sites at an appropriate altitude. At the rated DJI transmitter range of 300 metres (984 feet) the GX200 will view a ground footprint of approximately 420 X 315 metres (1312 X 1033 feet) which translates into a ground resolution of approximately 10 centimetres (4 inches) per pixel.

With the GX200 and FPV setup attached (334 gram payload) I found my Phantom preformed well for 4 minutes, between 4 and 6 minutes it was a bit sluggish but still responsive, 6 to 8 minutes it had a decreased ability to gain altitude and at 8 minutes it was unable to maintain a desired altitude (starting with a fully charged battery @ 20 degrees Celsius). For safety reasons I always start bringing it home around the 4 minute mark.

The images below show the camera setup for vertical imaging (both simple and FPV configuration) as well as the process used to create an orthorectified (distortion corrected) mosaic from the images recorded during a flight.

GoProfessional custom case for the Phantom Quadcopter (highly recommended)

It holds everything you need in a safe and easy to access package:

    Phantom Quadcopter (with camera and FPV transmitter attached)
    Quadcopter controller (with extra batteries)
    6 (or more) LiPo batteries and charger
    FPV receiver/screen and battery (same as quadcopter)
    Additional parts and accessories

Side view of the Phantom with the GX200 attached for vertical imaging (notice the foam separating the camera and Phantom body).

Bottom view of the Phantom with the GX200 attached for vertical imaging (281 gram payload).

Bottom view of the Phantom with the GX200 and FPV transmitter attached for vertical imaging (334 gram payload).

Side view of the Phantom with the GX200 and FPV transmitter attached for vertical imaging (notice the foam separating the camera and Phantom body).

Quadcopter controller with FPV receiver/screen and battery

Creating an orthorectified mosaic from the images.

Screenshot showing the relative locations of each of the 35 images recorded (represented with blue rectangles) above the study area (Centre of Geographic Sciences, Nova Scotia, Canada). Software used: Agisoft Photoscan Pro. For this project I did not use the FPV option.

Photoscan will look at the images and determine the relative orientation of each image automatically. If each image has GPS coordinates for the camera location (in the image EXIF data or in a separate .txt file), Photoscan can create a model referenced to real world coordinates. If the images have no camera location information Photoscan will generate a relative model with its own reference coordinate system. This "relative" model can then be georeferenced later if you know GPS coordinates for a number of points on the ground. A third option would be to tag known points in the images with known GPS coordinates (Ground Control Points) before you process the images. With these known points, Photoscan can create an orthorectified image with real world coordinates. It would be good to collect a few extra control points so you can check the accuracy of the final product. These extra points would not be used during the processing stage, they would be used afterward to compare the accuracy of the generated product (3D model or orthophoto mosaic) with known GPS coordinates.

3D point cloud (126861 points) generated from the 35 images (screenshot).

3D model generated from the point cloud (screenshot).

Images draped over the 3D model (screenshot).

Final orthorectified mosaic generated using the 3D model and images.

Single vertical image of Fort Anne National Historic site in Annapolis Royal, Nova Scotia, Canada recorded with the Ricoh camera system (FPV was used for this flight).

Oblique aerial image of Acadia University, Wolfville, Nova Scotia, Canada recorded with the Ricoh camera tilted at an oblique angle (FPV was used for this flight).

To view more oblique aerial images I've recorded with the Phantom Quadcopter click HERE.