A Quick Introduction to Drone Photogrammetry
This article provides a quick introduction to drone photogrammetry. We’ll look at how drone mapping makes use of photogrammetric techniques, and how high accuracies can be reached using the right approach and equipment when doing professional drone surveying projects.
What is photogrammetry?
Drone mapping is done using a technique called photogrammetry. Generally, photogrammetry refers to taking measurements from imagery that is captured by drones, airplanes or satellites. Photogrammetrists use techniques to transform imagery from 2D into accurate 3D models: a depth perception is established by combining, correcting and presenting imagery, so that the user is able to view and measure objects in three dimensions. For example, a typical photogrammetric technique is called orthorectification, which refers to removing the effects of image perspective (tilt) and relief (terrain) effects, resulting in a planimetrically correct image with a constant scale that shows all features in their correct location.
The main outputs of photogrammetric surveys are raw images, ortophotomosaics, Digital Surface Models and 3D points clouds. These outputs contain large quantities of images that are combined and processed using specialized photogrammetric software and are used in many different fields for visual assessment purposes, including topographic mapping, architecture, engineering and cultural heritage.
Photogrammetric surveying using drones
Drones now provide a cheaper alternative to aerial imaging surveys. All you need is a camera-equipped drone: the $2.000-$5.000 price range offers a wide selection of professional multirotor devices, whereas RTK/PPK enabled sets are available in the $5.000-$20.000 price range. A second cost item is photogrammetry processing software licensing. Of course you can decide to fly a drone yourself, as drone vendors sell consumer-type drones with cameras and photogrammetric software. However, this does not guarantee good results: a better option is to hire a drone mapping company that will do everything for you, as drone mapping is a highly specialized market led by experienced professionals.
A typical drone mapping project starts with choosing the right contractor for the project at hand. It’s important that the right solution is chosen for the desired output: expertise with the industry vertical is crucial and save you a lot of time and money. After contracting a mapping company that will fly an area of interest, the drone flight is prepared using flight-planning software, that enables an unmanned drone to follow a pre-defined flight path to make sure the whole area is covered. Also, it’s important that a minimum amount of overlap between the imagery is reached so that the imagery can be stitched together using photogrammetric software to produce an orthophoto and enable 3D surface modelling.
To make sure that the data captured is highly accurate and processed correctly, surveyors use large marked targets on the ground in an area of interest, named ground control points (GCPs). Using professional satellite navigation techniques, the exact location of these points can be determined. Drone software will use these points as the basis for positioning the drone imagery, based on their correct real-world location.
Drone photogrammetry allows achieving 1–3 cm level accuracies. However, the achieved accuracies depend on the amount of experience to select the right hardware, flight parameters and data processing. Achieving high absolute accuracies requires the use of RTK/PPK technology and additional GCPs. If you don’t need accuracies that high, using a $500 DJI Phantom-class drone with several GCPs will suffice: it is able to reach 5–10cm absolute accuracy for smaller survey areas without a problem, which might be good enough for many use cases.
Source: Drone LiDAR or Photogrammetry? Everything you need to know