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I'm modeling sea level rise, which entails adding the projected sea level rise for a specific decade to the current MHHW height (I think. I am new to this.)

My question is very general--what is MHHW in terms of datums? for example, WGS84?

I have projected Sea level rise in centimeters, and have created a raster from it. But I need to add it to the MHHW for the region.

Where do I get MHHW? Do I look at tide charts and average them? The procedure I'm following seems to indicate that MHHW should be considered 0, or just a baseline, ground level.

How can this be right? isn't MHHW above 0? Also I know in some systems sea level is not defined as 0.

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Mean Highest High Water is a reference surface corresponding to the height of high tides, averaged over a certain period of time (usually 19 years, a value close to a Metonic Cycle and a Lunar Node cycle). Data from tidal gauges, satellites, etc. are usually processed and stored in grid files, which contain the difference in elevation between different height datums for each cell of the grid, and can then be used to convert from a height datum one to another.

Some tools like VDatum come with these files built-in, and can be used to transform between several Vertical Datums including MHHW, NAVD88. The software contains grid files for several height reference surfaces, like geoids, the height of tides, etc. and can convert from tidal to orthometric or ellipsoidal datums. For example, you can convert your Projected Sea level rise raster from MHHW to NAVD88, and the resulting raster will have the projected mean high water elevations expressed in NAVD88, which can then be compared to another DEM in NAVD88 to create a map of the extent of mean high tides in the future.

Behind the scenes, what happens in that VDatum will use the grid file containing the height of MHHW relative to Local Mean Sea Level (LMSL), then will use the grid containing the height of NAVD88 relative to LMSL, in order to perform the correct transformation.

Because the Earth is dynamic, everything changes over the years. Sea level changes, the crust moves up or down, so any reference surface related to the ground or the sea move relative to each other over time, up to several centimeters a year, so when working with high accuracy data, it becomes important to know the Epoch in order to perform accurate transformations. Depending on the desired accuracy of your analysis as well as the accuracy of the grid files in VDatum, this may or may not be relevant to your specific project.

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  • Thank you for your response. So if I am starting with points at 0 elevation, and I input them into software (such as VDatum) as MHHW and output them at another datum, like WGS84, will the result be the MHHW line in WGS84? -I am surprised that this would work correctly. I would need to input the current year 2020, correct, so the software knows where the MHHW is? – Jeff Jun 1 at 1:07
  • For question #1, yes, the result would be the MHHW elevation expressed in WGS84. Though, I doubt converting to an ellipsoidal height like WGS84, ITRF, or NAD83 would be of much use in your case, because those aren't gravity-based, which is bad for flood analysis. DEMs will have a gravity-based vertical datum (most likely NAVD88 in the US). Note that the horizontal datum can be WGS84 or NAD83, but the Vertical Datum will most likely be gravity-based. – FSimardGIS Jun 1 at 3:39
  • For your question #2, from what I read on VDatum's website, the current reference tidal period that they use is 1983-2001, so MHHW will correspond to the mean height during that period. So it is based on data from around 30 years ago. I don't think you can input years (epochs) in VDatum, so you might have to take into account the rise that occurred between +-1992 and your specific reference year. – FSimardGIS Jun 1 at 3:44

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