Why have UAVs brought photogrammetry back in fashion?
Laser and LiDAR are well recognised as survey techniques for capturing data for mapping and modelling but before their advent, photogrammetry was much more widely used.
What is photogrammetry?
Photogrammetry is a technique which uses aerial photographs for map making and surveying and has been around for over 100 years. Essentially it is the science of obtaining reliable measurements using photographs. The output is typically a map, a drawing or a 3D model of a real-world structure or land mass. It was initially widely used for military purposes; during World War II photogrammetric technique were used extensively to meet the growing demand for maps and it has always been particularly beneficial for mapping inaccessible areas.
The quality of the maps produced via this method has steadily increased as technology improved both the quality of the photographic images and data processing tools. At the same time though advances in other methods have been made and techniques such as laser surveying and LiDAR have grown in popularity.
Photogrammetry was then seen as old fashioned and there is no doubt that prior to the advent of UAVs it fell out of favour for a while. UAVs (with their on board GPS) are the perfect tool to bring this technique to the forefront once again as an accurate and more importantly cost effective tool for surveying.
Why do UAVs make a difference?
Historically the images required for photogrammetry were captured by plane. However, because of the height at which they needed to fly this has an impact on image quality and consequently the map or model quality produced from the image data. Before the use of UAVs better quality maps and models were being produced from methods such as laser and LIDAR surveys but the use of UAVs has brought 2 fundamental changes.
1. They can be flown considerably closer than previous airborne data capture, vastly improving the quality of the data
2. They are significantly cheaper to mobilise than an aircraft with far fewer restrictions on where they can take off and land
Furthermore the capability of the image processing software has seen significant improvement and post flight processing of captured images can result in multiple extremely high quality data products such as meshed 3D point clouds, digital surface models and orthomosiacs
With GPS-equipped UAVs, the latest in digital imaging equipment and sophisticated software extremely high accuracy levels can be achieved at a fraction of the cost of other methods such as laser.
The quality of any model built from the data is entirely dictated by the quality of the image captured irrespective of any software technology. UAVs can carry quality cameras with high megapixel data capture.
Multiple overlapping photos (80 to 90% overlap) of the ground or model are taken as the UAV flies along an autonomous programmed flight path called a waypoint. To overlap photos of an object or land by 80 to 90% would be impossible to complete accurately by pilot navigation. N.B. It is essential to have a UAV which has waypoint Navigation technology.
What data can UAV photogrammetry produce?
The aerial imagery obtained via UAV photogrammetry can be used to produce a variety of different data products e.g.
- DEM / DTM / DSM (surface models)
- Orthophotos (geospatially corrected aerial images)
- Georeferenced Maps & Models
- Meshes & Point Clouds
- 3D Building Models
- Contour Maps
- NDVI Model/3D Vegetation Modelling
- Volumetric Surveys
The processed data provides accurate area, linear and volumetric measurements.