Sail at Low Tide?


The tide is high out
on the bay

I guess we should hold out
on this day

Lidar collection is gonna have to wait
The shoreline has to be straight

What marshes look like can be worse
Much will be lost of course

Models and imagery makes it clear
It’s best to wait till tidal flats appear

Hydro-enforcing different water elevations
will leave you with some lingering reservations

That’s the long and short of it
put it in the SOW or it could look like …..

In all seriousness – the tidal coordination specifications for lidar collection in coastal areas can be overlooked (and is one of the CSC’s Top Ten Pitfalls of lidar collection) when assessing costs and coverage. It goes hand in hand with specifying collection extents; what one sees on Google Earth or ortho-imagery may not be what is collected during a specific tidal stage and, thus, may fail to meet the anticipated aerial extent (even if the area was collected).

We have been working through this problem as we generate DEMs for a sea level and inundation mapping project. In places we lose parts of wetlands in strips (the flight lines) and it makes any type of modeling output look – well, weird. To a typical user, these ‘artifacts’ are noticeable and do not instill much confidence in the output – as would be expected. There are always constraints with data, but this one can cause more damage than one may expect.

Image of mangroves showing high tide and low tide marks.
Image of mangroves showing high tide and low tide marks.

The level of tidal coordination depends on the expected uses of the lidar data and tidal ranges. An extreme example is tropical to semi-tropical coastal areas where mangroves exist. The surface they grow on is only exposed at lower tides, if at all.  When not strictly tide coordinated the resulting modeled lidar ground surface will likely be a reflection of the elevation of roots not the sediment surface they are growing from.

Capturing the ground surface in marshes is typically less sensitive. The majority of the habitat will be exposed when the water surface is below the mean-tide-level, as most species grow at slightly higher elevations.  Significant research exists to suggest capturing the ‘real’ ground surface is difficult in this habitat even when tide coordinated, so if modeling the marsh surface is important it is advantageous to set-up the collection parameters to maximize the potential to capture the ground surface.

Data with flight lines collected at different tide stages
Data with flight lines collected at different tide stages

The downside to requiring tidal coordination is, yeah you can guess, the cost. It requires tighter windows in a setting that can be difficult to begin with given the weather (fog, clouds, waves, etc.) and higher level of sensitive air-space around airports and large cities.

Not requiring tidal coordination is a gamble, sometimes it is not a problem (whew) and other times it looks like a noticeable blunder (insert expletive here). There are fancy ways to ‘fudge’ the elevations using the existing species and the range of elevations that they are found in (assuming there is some elevation data in some areas) to recreate a probable surface. This is how SLAMM (sea level affecting marshes model) used to work before lidar came along and fixed everything – kinda, sorta?

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