When processing bathymetric laser data, additional processing steps have to be applied since both, the laser beam’s refraction at the air-water interface and the lower propagation speed within the water column must be taken into account in order to obtain a geometrically correct point cloud. Those specific processing steps – usually called "refraction correction" – have to be carried out for each flight strip right after the standard ALS processing workflow.

Refraction correction of the points below the air-water interface (i.e. both points on the water bed and within the water column) requires a geometric Water Surface Model (WSM). WSMs can be generated from the laser data by one of the tools offered with RiHYDRO. An alternative option is to import already existing water surface models. In the first case, the water surface points must be classified before. For this task, a specific point classification tool in RiHYDRO is provided, which allows classifying the points on horizontal water surfaces automatically. 
The user can specify the index of refraction of the water and also a depth bias value to be taken into account during refraction correction.