Surveying and Aerial Reconnassance using Draganflyer UAV Helicopters and ImageJ Software

ImageJ (Image Processing and Analysis) is a powerful, free, and industry standard image processing library. Designed to run on any operating system, ImageJ is the perfect software package for the analysis of UAV aerial images. Distances, areas, angles, and more complex analysis can all be done with ImageJ and orthophotos – this document will show you how.

An orthophoto is a geometrically correct image -  taken with the camera pointing straight down, at exactly 90 degrees to the horizontal. Orthophotos lack any distortion in shapes and distances, which are a side effect of projecting a 3 dimensional world onto a two dimensional camera sensor. Because they lack distortions, orthophotos can be used to measure the distances, angles, and areas of objects on the ground. Ideally, orthophotos are calibrated against a three dimensional model of the ground. This compensates for changes in ground level, which can cause distortions and changes in scale. Although the scale will change slightly – you can obtain a reasonable estimate of distances, areas, and other measurements without this calibration, just make sure that the camera and aircraft are as close to 90 degrees relative to the ground as possible. Be aware that measurements decrease in accuracy as the distance from the known measurement used for scale calculation increases. This is discussed in depth in the next sections.

The Draganflyer X6 UAV can take aerial pictures in a number of formats, and is fully compatible with the ImageJ software library.

Obtaining the ImageJ Software

ImageJ can be downloaded and installed on any operating system. If you don’t want to install imageJ to your computer, you can also run it as a Java Applet in your web browser.

Assuming that you opt for a hard drive installation, download a copy of ImageJ that matches your computer and operating system. Versions are available for Macintosh, Linux, and Windows, and are displayed on the download page. Windows users don’t need to install the version with Java included, if they already have Java installed.

Once you’ve downloaded ImageJ, double click the installer and select a directory that’s convenient for you to use. C:/Program Files works well on Microsoft Windows.

Image Analysis Basics: Measuring Distance, Angles, and Area

You can use the Draganflyer X6 UAV (equipped with the Panasonic LX3 digital camera), and the ImageJ software to do aerial surveying. ImageJ can measure the distances and angles between two objects on the ground, and compute the areas of selected regions. To do this, you have to provide ImageJ with a known length, or distance between two objects on the ground. The software uses this known distance, and divides it by the number of pixels that the same object occupies on the picture. This gives the picture scale, which is then used to do the calculations.

Finding an object with a known length in the same field of view as the object you want to measure can be challenging. Instead, we suggest this procedure for making aerial measurements:

1. Take the Draganflyer X6 UAV and a meter stick to the location you want to survey.
2. Place the meter stick on the ground, near to the object or area being measured.
3. Fly the Draganflyer X6 UAV over the target area, taking several aerial pictures.
4. Land and download the images to your computer for analysis.

Try to fly the UAV at a moderate altitude. The accuracy of the measurements will decrease with height, but you need the object that you’re measuring to fit into one picture.

Once you’ve obtained the aerial pictures needed, you’re ready to begin analysis with ImageJ. Click on “Start” if you’re using Windows, and then click on the ImageJ menu entry. The initial screen looks like this:

The top toolbar contains all the commands you can apply to an image. and the buttons show the various tools that you can use. Open an image by clicking “File”, and then clicking “Open”. Most file formats are supported, but we’ll use a JPEG file for this example. The file selection dialogue that appears looks like this:

After opening the file, a new window will appear. It may display the image as a thumbnail, scaled down to fit in a small window. Zoom in and out using Control + and Control -, until the image fits comfortably on your monitor. Now that you’ve got the image opened in ImageJ, we’ll go through some of the most common analysis procedures, starting with measuring the distance between two objects.

Calibrating the Image Scale

ImageJ must be calibrated before you can use it for measurements. When you took the images, you set a meterstick (or other object with a known length) on the ground. Find this meterstick in the image that you took, and click the line tool (highlighted in the screenshot below.)

Moving over to your image window, click on one end of the meterstick, and drag the mouse over to the other end, while holding the left mouse button down. Release the mouse button, and click the “Analyze” toolbar entry. Click the “Set Scale” entry in the menu that appears. A small window will appear, which looks like this:

The “Distance in Pixels” is the length of your meterstick, measured in pixels. Enter the meterstick’s actual length in the “Known Distance” entry, check the “Global” box, and then press “OK”. ImageJ will now calculate the image scale, and return you to the picture you’re working on.

A Real Life Example

The picture on the right was obtained using the Draganflyer X6 UAV, which was flown over a farm at a moderate altitude. An aluminium meterstick was set on the ground near the flight area, and can be clearly seen in the image (you may have to click and view the larger version.)

Using the meterstick, we followed the instructions above and found the picture scale to be 7.532 pixels per foot.

Measuring distances with ImageJ is easy – click and drag using the line tool mentioned earlier. Before you release the left mouse button, note the distance in ImageJ’s main window. For example: the distance between the shed and red car was found to be 94.48 feet. This can be seen in the following screenshot:

Measuring angles is also possible with ImageJ. Angles are always measured relative to the positive X axis, and the result is displayed in the ImageJ main window.

Perimeters and arbitrary areas can also be measured with ImageJ. Click any of the selection tools (such as freehand, ellipse, rectangle, etc) and make a selection. View the measurement data by clicking “analyze”, and then “measurements”. The window that appears looks like this:

You can change what information is displayed here by clicking the edit, and then set measurements. Additional information that can be found includes the integrated density, center of mass, and many others. Read the ImageJ documentation for a full description of it’s measurement capabilities.

Disclaimer

Using ImageJ and the Draganflyer UAV for aerial measurements can produce accurate results, but Draganfly Innovations makes no claim as to how accurate this method is in your particular case. Always assume an error of at least a few feet when taking distance measurements, and make sure that the helicopter is level when taking pictures. The camera must be set at 90 degrees to the helicopter for this method to work – do this by turning the camera tilt knob.

ImageJ is a powerful software program, and you can explore more of it’s functionality by reading the documentation.