{"id":1334,"date":"2016-08-05T03:02:55","date_gmt":"2016-08-05T03:02:55","guid":{"rendered":"http:\/\/blogs.lincoln.ac.nz\/gis\/?p=1334"},"modified":"2023-05-07T00:38:30","modified_gmt":"2023-05-07T00:38:30","slug":"keeping-our-heads-above-water","status":"publish","type":"post","link":"https:\/\/blogs.lincoln.ac.nz\/gis\/keeping-our-heads-above-water\/","title":{"rendered":"Keeping Our Heads Above Water"},"content":{"rendered":"

In this post we discuss the highs and lows of sea level.<\/em><\/p>\n

\"sealevelDeathValley\"<\/a><\/p>\n

So everyone knows where sea level is, right?\u00a0 When we talk about elevation, we usually put it in the context of how far above (or below) sea level something is.\u00a0 Simple, yes?\u00a0 Well….not quite.\u00a0 Sea level is one of those things that’s easily taken for granted but when one delves into it, it becomes increasingly complicated.\u00a0 So in this post we’ll talk about sea level and our brand spanking new vertical datum.\u00a0 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.<\/p>\n

So what exactly is sea level?\u00a0 First off, it’s not an easy thing to measure as it changes continuously.\u00a0 Here’s a track of the tidal levels at Lyttelton over the past wee while from a gauge at the Lyttelton Port of Christchurch:<\/p>\n

\"TideGauge\"<\/a><\/p>\n

Notice that this plot has Forecast and Actual (measured) water level.\u00a0 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).\u00a0 The N<\/a>autical Almanac<\/a> 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?\u00a0<\/em> (If it does, we can get help for you, but as we’ll see below, it shouldn’t.)<\/p>\n

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.\u00a0 Couple that with the rotation of the earth and we experience our high and low tides.\u00a0 On top of that, winds, atmospheric pressure and salinity (amongst other things) can also have an effect on water level.\u00a0 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.\u00a0 Hamish pointed me to this as a possible explanation<\/a>) so quite a lot of variability gets taken into consideration.\u00a0 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.\u00a0 (As we’ll see below, nautical charts use a different datum.)<\/em>\u00a0 “But….what’s that value measured from<\/em>?” I hope you’re asking.\u00a0 It’s a good question; and I find the answer doesn’t really clarify things.<\/p>\n

Let’s take a local example.\u00a0 Lyttelton is what’s know as a standard port, a reference point where tides are measured, predicted and published.\u00a0 There are 16 standard ports<\/a> around NZ.\u00a0 Tides at nearby ports (secondary ports) where tide gauges may not be installed can be estimated based on the standard port values.\u00a0 So what do those tide measurements relate to?\u00a0 Oddly enough, they are all measured against a point on land<\/em>.\u00a0 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:<\/p>\n

\"LytteltonDatum\"<\/a><\/p>\n

Not much to look at, really, but the mighty measure of Mt Cook is tied to that wee bolt.\u00a0 Here’s where that location is on a map if you’re keen for a visit (it’s the grey circle surrounded by yellow):<\/p>\n

\"LytteltonDatumMap\"<\/a><\/p>\n

Now let me see if I can make sense of how this all fits together.\u00a0 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.\u00a0 Sooooooo…we have our reference point at Lyttelton – the bolt in the ground shown above.\u00a0 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.\u00a0 Then, over a period of years (18 for Lyttelton, as it turns out<\/a>) the highs and lows and neaps (and tatties?) and springs are averaged out and a value of Mean Sea Level is calculated.\u00a0 At Lyttelton, that value is (drum roll please…) 1.40 m.\u00a0 As my daughter would say incredulously , “Wait, what?”\u00a0 Why isn’t it zero, pray tell?<\/p>\n

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.”\u00a0 So in a sense all of our land based measurements start from the bottom up, the bottom being this “chart datum” thing.\u00a0 Think of a datum as a point of zero measurement, like 0\u00ba C.\u00a0 It gives us a level where we can talk about things being above or below that level.\u00a0 So if you’re a sailor, one thing you’d like to avoid is running your craft aground.<\/p>\n

\"aground\"<\/a><\/p>\n

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.\u00a0 Here’s an example of one of those charts for just around the inner harbour:<\/p>\n

\"LytteltonHarbour\"<\/a><\/p>\n

As we’ve seen in the tide diagram above, the height of the tide varies with time.\u00a0 To make matters worse, it varies over longer periods of time as well.\u00a0 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).\u00a0 With first quarter and third quarter moons, the range between high and low tide is smaller.\u00a0 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.\u00a0 (And don’t even get me started on tidal bulges…)\u00a0 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):<\/p>\n

\"tide-curves\"<\/a><\/p>\n

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.\u00a0 This figure might help sort some of this out:<\/p>\n

\"tidal-terms\"<\/a><\/p>\n

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<\/a> that Lyttelton’s chart datum: “Has been retained at its pre-earthquake level which has been determined to be 4.508m below\u00a0the position of B.M. UD 40 (LINZ code B40V<\/a>) [our humble bolt above]<\/em> as at April 2012, a stainless steel pin set in a\u00a0concrete 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).\u00a0 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.<\/p>\n

Now I hope you’re sitting down for this next bit (assuming you’re still with me, that is).\u00a0 One would like to think that sea level is something we can rely on, some immutable fixed thing.\u00a0 Well, sadly, it’s not.\u00a0 Sea level changes from place to place (we’ll have to talk about sea level rise another time).\u00a0 It varies around New Zealand – it probably even varies around the Canterbury coast but we’ll probably never know for sure how much.\u00a0 Here’s a table from the LINZ site that sort of shows this.\u00a0 Note that the chart datums at each port are different.<\/p>\n

\"StandardPortsTable\"<\/a><\/p>\n

(Here’s a link to some definition<\/a>s that may help decipher the table.)<\/p>\n

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.\u00a0 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:<\/p>\n

\"geo_levelling-datums\"<\/a><\/p>\n

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<\/a>).\u00a0 And we would never even notice as we move from one zone to another.<\/p>\n

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.<\/p>\n

“Wait, what?” says Islay again.<\/p>\n

Why on earth would sea level vary? to which I would respond, “exactly!”\u00a0 Sea level varies from place to place (perhaps you should lie down for this) because gravity<\/em> varies from place to place.\u00a0 Though you will never be able to notice the difference, gravity changes depending on where you are.\u00a0 Why?\u00a0 The force of gravity is proportional to mass.\u00a0 More mass = more gravity.\u00a0 And here we must look to the ground beneath our feet, or rather the bedrock beneath our feet.\u00a0 Some bedrock is denser, more massive than others and exert a higher force of gravity.\u00a0 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.\u00a0 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.\u00a0 This has in fact already been done on a global scale from the GRACE satellites (Gravity Recovery and Climate Experiment<\/a> – hopefully more to come on this in another post.)\u00a0 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.<\/p>\n

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

C<\/p>\n

GIS Blog Table of Contents<\/a><\/p>\n","protected":false},"excerpt":{"rendered":"

In this post we discuss the highs and lows of sea level. So everyone knows where sea level is, right?\u00a0 When we talk about elevation, we usually put it in the context of how far above (or below) sea level something is.\u00a0 Simple, yes?\u00a0 Well….not quite.\u00a0 Sea level is one of those things that’s easily […]<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"class_list":["post-1334","post","type-post","status-publish","format-standard","hentry","category-uncategorized"],"_links":{"self":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1334","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/comments?post=1334"}],"version-history":[{"count":2,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1334\/revisions"}],"predecessor-version":[{"id":4970,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/posts\/1334\/revisions\/4970"}],"wp:attachment":[{"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/media?parent=1334"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/categories?post=1334"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blogs.lincoln.ac.nz\/gis\/wp-json\/wp\/v2\/tags?post=1334"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}