Keeping Our Heads Above Water

In this post we discuss the highs and lows of sea level.

So everyone knows where sea level is, right?  When we talk about elevation, we usually put it in the context of how far above (or below) sea level something is.  Simple, yes?  Well….not quite.  Sea level is one of those things that’s easily taken for granted but when one delves into it, it becomes increasingly complicated.  So in this post we’ll talk about sea level and our brand spanking new vertical datum.  But look on any topographic map and you’ll see elevations shown in various ways, mainly elevation contours and spot heights, all with a numeric value in metres above sea level.

So what exactly is sea level?  First off, it’s not an easy thing to measure as it changes continuously.  Here’s a track of the tidal levels at Lyttelton over the past wee while from a gauge at the Lyttelton Port of Christchurch:

Notice that this plot has Forecast and Actual (measured) water level.  Predictions of tides can be done weeks, months, years in advance given our understanding of what influences the tide (but not by me…but by LINZ).  The Nautical Almanac is an indispensable resource that publishes tide predictions around the country) Does it trouble you at all that all these measurements have values greater than zero?  (If it does, we can get help for you, but as we’ll see below, it shouldn’t.)

Clearly, water level is always on the move, driven primarily by the movement and the proximity of the moon and the force of gravity its mass exerts (gravity will become more important in just a bit). As the moon orbits around the earth, the distance between the two bodies changes, so that gravitational force varies with time.  Couple that with the rotation of the earth and we experience our high and low tides.  On top of that, winds, atmospheric pressure and salinity (amongst other things) can also have an effect on water level.  Sea level is typically calculated at some location by averaging hourly water levels over some time period, typically 19 years (don’t ask me why it’s 19 but it does appear to be a global standard.  Hamish pointed me to this as a possible explanation) so quite a lot of variability gets taken into consideration.  That calculated average of water level is then taken as Mean Sea Level against which all other elevations are measured and this value becomes our zero (or datum) on the topographic maps.  (As we’ll see below, nautical charts use a different datum.)  “But….what’s that value measured from?” I hope you’re asking.  It’s a good question; and I find the answer doesn’t really clarify things.

Let’s take a local example.  Lyttelton is what’s know as a standard port, a reference point where tides are measured, predicted and published.  There are 16 standard ports around NZ.  Tides at nearby ports (secondary ports) where tide gauges may not be installed can be estimated based on the standard port values.  So what do those tide measurements relate to?  Oddly enough, they are all measured against a point on land.  In Lyttelton’s case, it’s an entirely unobtrusive bolt in the ground that you can visit next time your in town – here’s a picture of it from 2010:

Not much to look at, really, but the mighty measure of Mt Cook is tied to that wee bolt.  Here’s where that location is on a map if you’re keen for a visit (it’s the grey circle surrounded by yellow):

Now let me see if I can make sense of how this all fits together.  It starts to get even more complicated because we’ll start having to mix up terrestrial topographic maps with nautical charts – they’re both measuring heights of stuff above other stuff, but the zero point isn’t the same.  Sooooooo…we have our reference point at Lyttelton – the bolt in the ground shown above.  Starting from this location, tidal water level measurements are made from an arbitrary zero point set far enough below the lowest anticipated tide (and the mark) that you don’t end up with negative values.  Then, over a period of years (18 for Lyttelton, as it turns out) the highs and lows and neaps (and tatties?) and springs are averaged out and a value of Mean Sea Level is calculated.  At Lyttelton, that value is (drum roll please…) 1.40 m.  As my daughter would say incredulously , “Wait, what?”  Why isn’t it zero, pray tell?

If you dig deeply into all of the LINZ material on sea level and tides, etc, you’ll see a statement that says: “The above levels, in metres, are referred to Chart Datum, which is the same as the zero of the tidal predictions in all cases.”  So in a sense all of our land based measurements start from the bottom up, the bottom being this “chart datum” thing.  Think of a datum as a point of zero measurement, like 0º C.  It gives us a level where we can talk about things being above or below that level.  So if you’re a sailor, one thing you’d like to avoid is running your craft aground.

Nautical charts are designed to map depths of water; the datum for the charts is the lowest level that water can be expected to reach. Though shown as positive on the chart, they are measured as depths below the datum.  Here’s an example of one of those charts for just around the inner harbour:

As we’ve seen in the tide diagram above, the height of the tide varies with time.  To make matters worse, it varies over longer periods of time as well.  When the moon is full (or new) we have spring tides – the high tides are higher than usual and the low tides are lower than usual (i.e. the range is larger).  With first quarter and third quarter moons, the range between high and low tide is smaller.  Also, when the moon is closer to the earth, these ranges get pushed out; when further away, the ranges are reduced, so it’s a completely moveable feast depending how everything fits together at a given point in time.  (And don’t even get me started on tidal bulges…)  Here’s a figure from LINZ that shows how the highs and lows change over just a single month at two ports (and how they can easily be different at two spots):

Charts use water depth values based on the worst case scenario, the so called Lowest Astronomical Tide (LAT), i.e. the lowest the water would be expected to drop at a particular point and the absolute minimum depths you can expect.  This figure might help sort some of this out:

So the level of the LAT gets adopted as the chart datum against which all water depths are measured and charted. LINZ further tells us that Lyttelton’s chart datum: “Has been retained at its pre-earthquake level which has been determined to be 4.508m below the position of B.M. UD 40 (LINZ code B40V) [our humble bolt above] as at April 2012, a stainless steel pin set in a concrete block adjacent to the main pier of the overbridge to the main wharves.” Given this, our sea level measured at Lyttelton of 1.40 m is 1.40 m above the chart datum or 3.108 m below the bolt (pin).  Nautical charts use the chart datum for zero but topographic maps will take the 1.40 mark as zero and calculate all land based elevations with respect to that zero level.

Now I hope you’re sitting down for this next bit (assuming you’re still with me, that is).  One would like to think that sea level is something we can rely on, some immutable fixed thing.  Well, sadly, it’s not.  Sea level changes from place to place (we’ll have to talk about sea level rise another time).  It varies around New Zealand – it probably even varies around the Canterbury coast but we’ll probably never know for sure how much.  Here’s a table from the LINZ site that sort of shows this.  Note that the chart datums at each port are different.

(Here’s a link to some definitions that may help decipher the table.)

The table above shows how sea level is different from place to place but there’s no easy way to say how much they differ.  What we’ve had for many decades, is 13 different vertical datums (well, data, actually) covering the country based on sea level measurements at some of the ports above:

But there’s no easy way to say how different sea level is between the Lyttelton zone and the Dunedin zone – we’d need to measure them both against another sort of datum, perhaps distance from the centre of the earth – if only there were some common way of measuring sea level across the whole country (tune in later for new about NZVD2016).  And we would never even notice as we move from one zone to another.

So sea level varies within New Zealand but it’s also different from sea level in, say, Tahiti, or Norway, or anywhere else in the world.

“Wait, what?” says Islay again.

Why on earth would sea level vary? to which I would respond, “exactly!”  Sea level varies from place to place (perhaps you should lie down for this) because gravity varies from place to place.  Though you will never be able to notice the difference, gravity changes depending on where you are.  Why?  The force of gravity is proportional to mass.  More mass = more gravity.  And here we must look to the ground beneath our feet, or rather the bedrock beneath our feet.  Some bedrock is denser, more massive than others and exert a higher force of gravity.  Continental bedrock, like that under large continents, is denser than the basaltic bedrock typically making up the sea floor; though the differences may be quite small, they’re enough to exert different gravitational forces that are enough to affect sea level and force it to change from place to place.  It’s the same sort of thing that, even though our bodies or 70% water, we don’t feel a tidal effect, but the sheer mass of all the water in the oceans does – the oceans respond to the gravitational difference even if we can’t feel it. If the earth were uniform in bedrock composition and spherical, and covered entirely with water, we could imagine a single sea level that could be measured, but our non-uniform earth, with continents, and different varieties of bedrock means that differences in gravity can be measured (often with satellites but it can be done from airplanes as well) and mapped.  This has in fact already been done on a global scale from the GRACE satellites (Gravity Recovery and Climate Experiment – hopefully more to come on this in another post.)  It’s really gravity that determines where sea level is so mapping gravity helps us to come up with a “true” measure of sea level.

We’ll have to cover that in another post – I’m exhausted – my brain’s definitely below LAT after all this.

C

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