Two important notions must be distinguished about accuracy for an aerial mapping project, absolute versus relative accuracy.
1. Absolute Accuracy
Absolute accuracy is related to the accuracy of the entire reconstructed model positioning in relationship with its actual position on Earth. The absolute accuracy can be somehow assessed looking at model positioning on the background base maps. If the model needs to be integrated with a GIS for studying its interactions to surrounding environment and assets, it might be required to reach a high absolute accuracy.
2. Relative Accuracy
Relative accuracy is related to the accuracy of positioning and distances of the features within a reconstructed model, regardless of how well positioned is this entire model versus its true location in actual world. A high relative accuracy will enable precise geospatial measurements within the model. When focus is on studying the internal model features, relative accuracy matters, and a high absolute accuracy might not be required.
- A building model can be reconstructed with shapes and dimensions true to actual size, but is located 50 m off its street adress, with this offset visible from the base map. It has a high relative accuracy, but a poor absolute accuracy.
- A building model can be well centered on actual, real-world location, but presents distortion : a high absolute accuracy, but low relative accuracy.