The International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences, Volume XLII-2, 2018 
ISPRS TC II Mid-term Symposium “Towards Photogrammetry 2020”, 4-7 June 2018, Riva del Garda, Italy 
This contribution has been peer-reviewed. 
https://doi.org/10.5194/isprs-archives-XLII-2-961-2018 | ©Authors 2018. CC BY4.0 License. 
963 
Figure 2. Empty wave pool (a) and intensity coded reference 
point cloud (b) acquired by terrestrial laser scanning. 
v .»o-v-*v- a 
V.'CUtôK'MMi 
(b) 
Figure 3. Swimming pool with artificial waves generated by a 
wave machine (a) and ALB point cloud (b) with water surface 
points in blue and water bottom points in grey. 
teristics of the height held can be influenced by parameters of the 
Fourier grid (width, height, mesh size) as well as wind conditions 
(wind speed, wind direction). In order to achieve comparability 
to the experimental validation, we aim at reproducing the wave 
pattern like it is actually present in the measurement data. For 
this purpose, we analyzed the wave pattern represented by the 
measured water surface points to derive its amplitude and wave 
length. The wave amplitude refers to the vertical distance from 
mean level to crest and the wave length specifies the horizontal 
distance from crest to crest. Subsequently, we choose suitable 
simulation parameters to obtain a water surface model with simi 
lar properties. 
The bottom surface modeling is focused on the plane character 
istic of the actual pool bottom. We deliberately omit the slope 
down, since the predicted measurement errors will be specified 
in percent of the water depth. Therefore, the simulated water bot 
tom is generated as a horizontal plane surface. 
The ray path modeling is realized by dividing the incident laser 
pulse into a large number of subbeams representing a finite foot 
print at the water surface. The intensity distribution within the 
incident laser pulse follows a Gaussian intensity profile. The re 
fraction effects at the air/water interface are modeled by Snell’s 
law for every individual subbeam. Our simulations are limited to 
identical forward and backward laser pulse paths here. Effects of 
diffuse reflections at the water bottom with fractions of the dif 
fusely reflected signal accidentally being projected towards the 
receiver aperture are neglected. The final ground reflections are 
represented by the intensity-weighted centroid of all individual 
subbeams. 
In order to quantify the total effect of waves, the simulations com 
pare laser pulse paths resulting from the refraction at the local 
wave-induced water surface (fig. 1, blue) to paths resulting from 
the refraction at the horizontal (fig. 1, purple) or local tilted (fig. 
1, red) water surface assumed in conventional correction meth 
ods. The assumption of a horizontal water surface is realized 
by local horizontally oriented water surface elements at differ 
ent heights provided by the water surface pulse echoes. For the 
locally titled water surface we perform a Delaunay triangulation 
for all water surface points. The water surface point density is 
adapted to the distribution actual present in the data set. The inci 
dence angle a t n t required by Snell’s law is calculated with respect 
to the surface normal of the triangle intersected by the incoming 
laser ray. 
4. EXPERIMENTAL VALIDATION 
The experimental validation is based on the LiDAR bathymetry 
data as well as the terrestrial laser scanner data, which serve as 
reference for the following tests. The bathymetry data, acquired 
in the airborne survey campaign, is provided as uncorrected 3D 
point cloud, i. e. no refraction correction and run time correc 
tion was applied on the raw data set. An accurate time stamp is 
available for each 3D point in addition to the classification in wa 
ter surface and water bottom points. Furthermore, information on 
the sensors trajectory and manufacturer specifications regarding 
the refractive indices of air and water are accessible. 
Based on these information we apply a runtime correction and the 
simple refraction correction method assuming a horizontal water 
surface as well as the more complex refraction correction method