A typical UAV project involves these basic steps:
• Plan flight.
• Get ground control points surveyed at site (optional).
• In field, perform and pre-flight calibration.
• Fly mission taking photographs.
• Review photograph quality and overlap on site.
• In office, load images and any GPS data onto computer.
• Load images and GPS data into PhotoModeler.
• Have PhotoModeler process the images to produce camera positions and initial ground model.
• Possibly add ground control points and reprocess if they are available and the project needs them.
• Produce extra analyses in PhotoModeler for export such as volumes, orthophotos, DSMs, and PDF reports.
• Use data alone or in external programs (such as CAD or GIS) to make decisions.
If flight management software is being used (preferred to manual flying), define a region to be captured, the number of photos, the overlap, the flight altitude, and the direction of flight lines.
For a good photogrammetric project, the overlap should be at least 60% and preferably 70%. When there are trees or other tall vegetation, higher overlap may be required. The larger the overlap the more photos produced - although more photos leads to longer processing times. The flying height affects the number of images produced for a given area, and also influences the amount of detail captured on the ground surface. We also recommend against single flight lines (a linear set of photos in one row) – at least two parallel overlapping flight lines is a good aim.
Aerial projects can often be processed with no need for any additional data, for generating visual representations of ground features. Including a basic linear scale between any 2 visible ground features can be sufficient for many types of basic measurements that don’t have high accuracy requirements. See Setting Scale.
For other projects that have higher accuracy requirements, or that need to be in a specific defined geographic coordinate system, you may need to have ground control points (GCP) surveyed. There would typically be 6 to 20 points depending on your accuracy requirements and size of the project and with a good spread across the photographic area.
These GCP are identified on the ground with a large cross, a large circle, or similar large high-contrast feature that will be visible in the UAV images. The targets are placed before the flight but can be surveyed before or after the flight (as long as they have not moved). These points are measured with a survey instrument (total station, or GPS/RTK unit for highest accuracy) and the locations recorded. Also note what coordinate system has been used and note the horizontal and vertical datums.
If the UAV has high accuracy GPS then good accuracy can still be achieved with a smaller number of ground control points, or even sometimes with no ground control.
The project is then flown and images captured. A review of the image data in the field can be helpful to ensure the quality is good.
The images and optional GPS data are downloaded from the UAV and loaded on your computer.
If the UAV has a GPS system, then one has to consider its accuracy and how to use the data. Some UAVs have a crude GPS system that might not be suited to accurate use in photogrammetry. Also, other UAV may store the height / altitude record as height above take off point instead of the GPS standard (height above the WGS84 ellipsoid). One has to be careful to review and compare ground control and UAV GPS data to ensure they are correct especially in making certain both data sets use a consistent vertical datum. See Define Geographic Coordinate System Dialog.
In addition, if ground control points are available one has to consider whether they are accurate enough to be of benefit in the process. The Typical Details Steps below describe one typical (also usually efficient and accurate) method of using ground control points (GCPs).