I hope everyone is doing well! I have a question regarding measurements of river through LiDAR or IfSAR. I have IfSAR DTM data of a certain domain, I need to know if the elevation data on a river point is measured as the river bed elevation or the water level elevation at the time the data was taken? Thank you in advance!
Nasir Hameed Thank you for your answer! To add, is there a way of knowing how deep the LIDAR had captured beyond the river water surface? I apologize if my question sounds simple, I am quite new to this topic.
Do you have observed measurements for your elevation in the area? It is good to have observed values so that you can compare. This assists in calibration of LIDAR results.
InSAR data are not good enough for this kind of project. You are really better to use Lidar data in green band. Best of luck
The Bathymetric Lidar is the best sensor to measure the river bed. For more information, you can read the article in the link bellow:
https://www.mdpi.com/2072-4292/7/5/6160/htm
I will highlight the fact that water clarity is what controls the depth to which bathymetric LiDaR can penetrate. Conceptually, if you can see the bottom of a water body from above the surface, then there is a fair chance that bathymetric LiDaR will work. Our eyes perceive light over a range of frequencies (verses the narrow frequency and high sensitivity of the LiDaR instrument), so the visual assessment is only approximate. None-the-less, it gives one an approximate sense of where the LiDaR is most likely to work. There are many water bodies where the LiDaR will not be able to sense the bottom because of turbidity and dissolved material in the water column. Even if the water column is only a few meters in depth.
As for the ifSAR, some tide-gauge/water-level instruments use a radar pulse to monitor the position of the water surface. Therefore, I think that it is likely that your ifSAR data is showing you the water surface, not the bottom. See this question on ResearchGate: https://www.researchgate.net/post/SAR_images_show_water_in_dark_tone_Is_it_due_to_reflection_or_absorption#:~:text=SAR%20images%20show%20water%20in%20dark%20tone.%20Is,gets%20reflected%20off%20from%20the%20radar%20(specular%20reflection).
It should also be highlighted that the frequency typically used for normal/topographic LiDaR (near infrared) works poorly for bathymetry purposes because water is absorptive of near infrared light (and most of the EM spectra). The visible spectrum is where water is most "transparent." Given that life likely started in water, it should not be surprising that vision developed around the spectrum that is transmitted in water and not frequencies for which water is, essentially, opaque. This blog includes a an image of the absorption spectra for water: http://thevirtuosi.blogspot.com/2010/05/why-is-grass-green.html
Matthew Mabey Thank you for taking time to explain really well, I appreciate it! I asked because I have an IFSAR data that says 1.5m elevation within the river surface. The river is far away from the sea and sort of a receiving catchment from a mountainous region, so I was wondering if the river surface is really low (relating to MSL measurements) and probably one of the reason why that area is flood prone?
Lea Dasallas Without knowing more about your IFSAR DTM, I can't offer much except to point out that if one runs an elevation profile across the stream channel, one should be able to tell if the DTM is showing a water surface (which would be an essentially level surface of constant elevation) or a channel bottom (which would usually have an uneven surface. That, of course is a generalization. Some braided streams will have channels that are fairly even and some streams have broad floodplains that are fairly even. In such cases, the water surface might be similar to surrounding land surfaces or associated channel bottoms in regard to how even it is. But in most cases I would think the water surface would still be distinct.
Regardless, in all those cases of flat land and channels, the elevations will be little different from the water surface and would also give you an idea of the elevation relative to sea-level and associated flooding potential.
One final question for you to consider is whether or not the data for your DTM is truly MSL. Other datums could have been used. While Google Earth likely doesn't provide you the precision of your IFSAR DTM, it should allow you to check on the elevations surrounding the stream in your area of interest. An easy check on what the datum used might have actually been.
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