When talking accuracy, it’s important to know your point of reference; i.e. accurate relative to what? Points within a survey model may be accurate relative to other points within the same model which is known as internal accuracy. Alternatively, the same survey may be accurate with relation to its true position on the earth, otherwise known as absolute accuracy.
Internal accuracy is the accuracy of measurements between points in an inspection or survey. This is important when you want to make measurements only on that survey — getting a volume or measuring a length.
Absolute accuracy is critical for comparisons of multiple datasets across time.
There are a few things you can do to achieve good internal accuracy:
- Use an integrated system that directly connects the camera triggering with the GPS. If you only use EXIF tags later, the timestamp is stored in whole seconds, which means the GPS position can be wrong by many meters.
- Make sure that your camera is properly calibrated and synced with your GPS clock to log positional information at the same time when the photo is taken. Clock errors are the most common source of internally inaccurate data that we see at Propeller and are usually one of the first issues encountered by pilots when flying new custom-built drones.To give an example of scale, a clock error of a tenth of a second on a fixed wing traveling at ten meters per second introduces an error of one meter to the captured data by itself.
- Capture lots of data. If you only take a few dozen photos of a small area, the GPS inconsistencies can lead to large errors. More GPS data helps the errors to average out.
Unfortunately, even a perfectly calibrated camera and standard GPS unit can still produce errors of several meters or more depending on flight conditions.
A proven method of improving GPS accuracy is to use an on board RTK module to provide additional corrections. RTK comes with many benefits including, the convenience of not having to rely on ground control and the ability to produce consistent results accurate to several centimeters.
However, like any technology, RTK has its limitations which can make it unsuitable for some applications. RTK requires a strong, uninterrupted cellular connection between a base station and the module in order to stream its corrections. When surveying sites that are particularly large or with poor reception, a drop in this connection makes your corrections unusable until it is recovered.
To achieve consistent internal and absolute accuracy, ground control points are the best possible option.