In normal operations, we are limited by CASA regulations to a maximum altitude of 400 feet above ground level. Operations at higher altitudes are possible with approval from CASA. Our most common operating height is 300 feet above ground level.
The area that can be surveyed is flexible, and depends only on the number of flights that are made. Imagery from multiple flights is incorporated seamlessly into a single dataset. Three 20-minute flights can yield approximately 100 hectares at 3cm/pixel resolution.
Aerial photography generally means the ordinary acquisition of photos from the air, without any specialised techniques. The photos may be either vertical or oblique, and are commonly used for advertising, real estate, and so on.
Aerial surveying, on the other hand, covers a large number of techniques where information (in a broad sense) is being collected from the air. This information can consist of imagery in the human visual range, as well as other bands such as Near Infrared (NIR).
Another technique within the category of aerial surveying is Aerial Photogrammetry, in which accurate geometric measurements are made by advanced 3D processing of overlapping images recorded from different vantage points. This allows the generation of Orthophotos (map-like images that are free of perspective distortion), and 3D terrain data.
A key feature of aerial surveying is that location data is embedded in the files containing the imagery. For example, we supply file types such as ECW, GeoTIFF and Google Earth KMZ. When viewing the data in suitable viewer, the user can instantly read the coordinates of any point in the imagery, and even its elevation depending on the file type.
This technique is known as 3D photogrammetry. The UAV takes images with a certain percentage of overlap, which we specify in control software. As a result, any given point on the ground has been seen from at least two different viewpoints. This technique works in the same way that your brain generates 3D information from the differences between the images from your two eyes. After the flight, specialised software generates height information from the parallax differences between overlapping images. Accuracy in the calculated 3D terrain is enhanced by using multiple Ground Control Points. These are targets placed around the site, whose positions in 3D space have been recorded at centimetre-level accuracy using special survey equipment.
Multispectral imagery is a technique which gathers Near Infrared (NIR) as well as visible light in an image.
Healthy plants reflect far more NIR than visible light, so for any pixel in the image, the difference between the intensity of NIR and visible light can indicate the health of the plants, or the presence of plants versus soil. This can be used to monitor the health of a crop, or to detect weeds.
By calculating this difference for every pixel in the image, we can generate a monochrome (black & white) image where the brightness of each pixel represents the degree to which there is healthy vegetation. To make the differences more visible, the brightness levels can be converted to a colour-based representation of plant health. This is called colour mapping. As an example, green can represent vigorous growth, while red can represent absence of vegetation. Intermediate colours would represent varying degrees of plant health.
This technique, which we use, can be broadly called Vegetation Index. It appeared in the late ’70’s using multispectral imagery from Landsat satellites. There are many different implementations of this technique, which vary according to which visible band is used for the calculation, and which mathematical formula is used. The most well-known is Normalized Difference Vegetation Index (NDVI).
In Agriculture, this technique can identify problem areas in a crop before they are visible to the naked eye.
In Natural Resource Management, this technique can be used to detect weeds, as well as algal blooms in aquatic systems.
Spatial Resolution is described by the Ground Sample Distance (GSD), which is the distance on the ground between the centres of two adjacent pixels. The smaller the number, the finer the detail that you can see. Our equipment achieves 3cm/pixel.
Accuracy refers to the correctness of the location data (coordinates) within an orthophoto or similar product. Ultimately, this depends on the equipment making the measurements in the first place. Advanced survey equipment is used to record the positions of Ground Control Points in the survey area, and after the flight these recorded positions are manually correlated with the GCPs visible in the imagery. The survey equipment has accuracy specifications, and will also report its accuracy from moment to moment in real time.
There are numerous standards used to describe accuracy, but they all specify distance in one way or another. The smaller the distance, the greater the accuracy. A common standard is Circular Error Probability (CEP). This is defined as the radius of a circle, centered on the true value, which will contain 50% of the measurements. For example, our equipment will often display a CEP of 10 millimetres or better. This means that its claimed position can be expected to be within 10 millimetres of the true position 50% of the time. Perhaps a better, and more “demanding” standard is R95, which specifies 95%. In the above example, the distance becomes 21 millimetres.
By using accurate survey equipment and a good number of GCPs appropriate to the task, the accuracy of the final data should be high. This can be verified by placing around the survey area a number of targets whose actual positions are recorded, and compared to the locations claimed by the final orthophoto when you select those points in the image.
UAV stands for Unmanned Aerial Vehicle. An alternative term is UAS (Unmanned Aerial System). The use of the word “System” is intended to convey the fact that there’s more than just a vehicle involved: there’s also a trained operator and a computer on the ground. More recently, the terms RPA (Remotely Piloted Aircraft) and RPAS (Remotely Piloted Aircraft System) have begun to be used, and will probably become standard terminology.