Many factors can impact the YellowScan Navigator’s performance in water penetration. Understanding and mitigating them can help improve the accuracy and reliability of the data collected
Secchi Depth
The water turbidity is one of the main criteria affecting the penetration of the LiDAR in the water.
For Secchi principle and measurement, please refer to the article : Secchi depth measurement
Water Conditions
Water Depth
Deeper water bodies may attenuate the LiDAR signals before they reach the bottom, especially in turbid conditions. The greater the depth, the more the laser signal is absorbed and scattered by the water column, reducing the energy that reaches the bottom and is reflected back. This attenuation is multiplied in waters with higher turbidity levels.
Water Surface
The state of the water surface significantly affects the penetration of LiDAR signals.
Calm, flat water surfaces allow for better transmission of laser pulses into the water column, while rough or choppy water can deflect and scatter these pulses, diminishing the return signals.
However very still, mirror-like water surfaces can also pose a problem by creating total reflection of the LiDAR signal, where the laser pulses are reflected away from the sensor, preventing them from penetrating the water and returning useful data.
mirror-like bodies of water are terrible for data collection
Wind, boat traffic, and other disturbances can create waves and ripples, which can either aid or hinder data collection depending on their intensity and frequency.
Water Color
Colored dissolved organic matter (CDOM) can absorb certain wavelengths of LiDAR signals, reducing penetration depth. Water color, influenced by organic materials like humic and fulvic acids, can vary significantly in different water bodies, affecting the effectiveness of the LiDAR system. Darker waters, typically found in areas with high organic decay, absorb more light, thereby reducing the depth to which LiDAR can effectively penetrate.
Water Currents
Water currents can introduce dynamic changes in water clarity and surface conditions, impacting LiDAR performance. Strong currents can stir up sediments from the bottom, increasing turbidity and reducing the Secchi depth. This turbidity can vary spatially and temporally, complicating the penetration of the signal. Additionally, currents can create surface undulations and internal waves, further scattering the LiDAR pulses and affecting the return signal strength and accuracy.
Underwater Ground Nature
Bathymetric LiDAR is still LiDAR, and behaves similarly to a typical, near-infrared-based topographic LiDAR when surveying different targets: the nature of the underwater ground also plays a role in the ability to capture accurate bathymetric data.
Different substrates, such as sand, silt, rock, or vegetation, reflect LiDAR signals differently. Sandy or rocky bottoms typically provide stronger, more distinct returns, while silty or vegetated areas can absorb or diffuse the signals. Vegetation can create multiple reflections and obscure the true bottom, leading to inaccuracies in depth measurement.
Moreover, varying ground compositions can cause different signal absorption and scattering, affecting the consistency of the data.
Sunlight and Time of Day
Ambient light conditions can affect the LiDAR signal-to-noise ratio. Direct sunlight, particularly during midday, can introduce noise into the LiDAR signals, potentially interfering with the detection of weaker return signals from the water bottom. Early morning or late afternoon flights might provide more favorable lighting conditions for capturing clearer data.
Water Refraction Index
The water refraction index significantly impacts the accuracy of bathymetric LiDAR measurements. Refraction occurs when light waves change direction as they pass from one medium to another, in this case, from air to water. This bending of light can alter the perceived depth of the water body. The refractive index of water varies slightly with temperature, salinity, and the presence of other substances. Applying a correction factor to the depth measurements based on the calculated refractive index will adjust the apparent depth to the true depth, improving the reliability of the bathymetric data.
Multiple factors are to be considered to compute the refraction index:
- Temperature: Warmer water has a lower refractive index, causing less light bending, while cooler water has a higher refractive index, leading to more significant refraction.
- Salinity: Higher salinity increases the refractive index, affecting the laser’s path. This is particularly important in estuarine or coastal areas where salinity can fluctuate.
- Angle of Incidence: The angle at which the laser hits the water surface can also affect the degree of refraction. Too high of an angle can become full reflection. Nadir hits will have a strong specular reflection and saturate the detector, resulting in no point. All points will need to be corrected for refraction.
Influence of Recent-Days Meteorological Conditions
- Secchi Depth: Recent rainfall, wind, or storm events can increase sediment runoff into water bodies, reducing water clarity and thus the Secchi depth.
- Water Surface: Recent wind conditions can create persistent surface waves and ripples, affecting the water surface.
- Water Currents: Meteorological events such as storms can strengthen water currents, stirring up sediments and impacting water clarity.
- Sunlight and Time of Day: Weather patterns, such as cloudy or clear skies, can influence the amount of ambient light.
Always check weather condtions of the day, but also of the past days before a bathymetric LiDAR survey !