Traditionally, aerial mapping required aerial triangulation (AT), which, in turn, required surveying an extensive network of ground control points (GCPs) and extensive post-processing. Acquiring GCPs is a costly, time-consuming, and, at times, dangerous process. The new industry standard, first developed commercially in the 1990s for manned aircraft, is Direct Georeferencing (DG), which requires few if any GCPs and little post-processing.
DG directly measures the position and orientation of an airborne mapping sensor, such as a digital camera or a laser scanner, thereby making it possible to assign a geographical location on Earth to a pixel from a camera image or a digital point from a laser, without any additional measurements referencing the ground. The sensor’s position is obtained by using a GNSS receiver and its orientation by using an inertial measurement unit (IMU) rigidly attached to it. By integrating the data flows from the GNSS and IMU devices, in both real-time and in post-processing, DG can achieve centimeter-level accuracy for the sensor’s position and milli-degree accuracy for its orientation.
The transition from AT to DG was as significant for the aerial mapping industry as that from film to digital media, because maps that used to take months to make could suddenly be created within days or hours. For certain applications, we already have real-time, on-board processing in which maps are made while the plane is still flying.
Compared to AT, DG offers
- a huge cost reduction, due to nearly eliminating the need to survey GCPs
- improvements in collection efficiency, by greatly reducing the need for overlap between flight lines
- the ability to deploy quickly—for example, in a disaster—without first surveying GCPs, and
- the ability to conduct aerial surveys of inaccessible and/or featureless areas, such as deserts or bodies of water.
A few years ago, a substantial investment in micro-electrical mechanical sensors (MEMs) to address the consumer and automotive markets yielded extremely small, low cost, low powered MEMs-based accelerometers and gyros accurate enough to support DG. This dramatic drop in the size, weight, and power (SWaP) and cost of sensors enabled another fundamental transition for the aerial mapping industry: that from manned aircraft to unmanned aerial systems (UAS).
Applanix’ APX-15 UAV, a single-board GNSS-inertial solution that weighs only 60g, is light enough to add to even the smallest UAS without substantially degrading its endurance. The device has been integrated with a wide variety of UAS, both fixed-wing and rotary, and with a wide variety of imaging sensors, including visible-light cameras, lidar, and infrared/multispectral/hyperspectral imagers. It has achieved accuracies of a few centimeters, scaling to accuracy of three to five pixels, with minimal ground control, even in difficult terrain. All APX products include the Applanix POSPac UAV post-processing software to generate high-accuracy carrier-phase differential GNSS-inertial position and orientation.
Some have pointed to RTK as an alternative to DG. However, attempts to produce good mapping results using RTK-only position solutions have been hampered by the need for a dense network of GCPs and the requirement for a dense flight pattern with large overlaps between flight lines, resulting in its limited adoption on UAS.
In addition to requiring accurate knowledge of a sensor’s position and orientation, DG also requires precise and accurate knowledge of its inside geometry. The device must by engineered so well that the parts are stable to a 1000th of a degree and by a pixel size which can be about 5 microns. So, Direct Georeferencing requires serious precision and engineering!