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31 Mar 2014

Frequently asked questions

How can I find out tides for a place that is not in the dropdown list?

The dropdown list is not complete, and contains only the more popular places. There is a list of every place we have tide figures for; go to the Other places table for a list of these, and choose one.

Where can I find more information about tide levels and datums?

For most casual uses of the tide graph, the actual height is not really important - the shape of the curve gives a pictorial view of what the tide is doing.
For the more serious user, all our published tide heights and the heights shown on the tide graph are relative to the zero height used in official marine charts. This zero height is known as the datum.

Standard ports have a physical marker that can be used to relate the datum to a real world height. For instance, Auckland has a bronze plaque which is 5.233 metres above the datum. This plaque is set in concrete in the road at the entrance to Captain Cook Wharf.

Land Information New Zealand (LINZ) - who are the suppliers of our tide information - have details about datums around New Zealand, and an explanation of how tide heights are defined, on their website here.

What causes tides to occur?

Tides are caused by the gravitational attraction of the sun and moon upon the eath's oceans, combined with the daily rotation of the earth. The motion of the sun. moon, and earth are entirely predictable, and so we can predict tides well into the future.

Unfortunately, a large number of factors make the prediction process quite complex. As we demand more and more accuracy in the times and heights our predictions, so the prediction process becomes more complex. For any given place, factors that affect tide heights and times include the location of the place on the globe, the depth of the sea, the shape of the coastline, the presence of rivers, bays and inlets, and other nearby land masses. Tides are also affected by non-predictable factors such as wind and the barometric pressure.

Further information may be found at these sites:

The date buttons show the wrong day of the week!

Um .. yes. A very small number of visitors to this site will see incorrect day names. If this happens to you we would appreciate an email, including a mention of the operating system you are using (Windows 98 / ME / NT3 / NT4 / 2000 / XP, Mac, Linux, etc). Please ignore the day names; the day-of-the-month is correct. One day we'll figure this one out, or else we'll just figure out the day ourselves instead of asking the Java Calendar for it. Also see the FAQ question about daylight saving and time zones.

How should I interpret the graph?

The graph shows the tide height for all times over a 24-hour period. The height is relative to Chart Datum which is the zero depth for marine charts. The slope of the curve shows the strength of tidal flows; a steep slope means strong currents, a flat slope (tops of peaks and bottoms of troughs) means slack water. High tide occurs at the top of a peak on the graph, and low tide occurs in the centre of a valley.
Note: The vertical scale is calculated on the fly and it may change as you look at different places or months, thus a curve that looks "big" may in fact be considerably smaller than another that appears smaller, and vice-versa.

Can I find out the exact height for a given time?

Click anywhere on the graph for a display of the tide height for any given time. You can also find the time and height of a high or low tide by clicking the midpoint of a peak or valley.

How do I make the graph show the same place each time I visit?

The Tide Graph applet puts a "cookie" on your computer which contains the last place for which you viewed tides, and when you return to the tide graph page, this place will be selected by default. If you have disabled cookies in your browser or you have cleared the cookie files from your computer, then the applet will show Auckland tides when it starts.

Can I make a bookmark for a particular place?

Yes you can. Locate the place of interest in the List of other places and click on the name. This will take you to the Tide Graph for the chosen place. At this point you can create a bookmark which will always start the Tide Graph for the selected place.

I would like a tide graph for a place you don't list

Obtaining the information for drawing a graph is a complex process and all places for which we have data are present in the List of other places. We will be add more as data becomes available.

Can I use your applet on my website?

We are open to a variety of ways to do this. The easiest is for you to add a link to our Tide Graph, and you are welcome to do this, but bear in mind it is useful in New Zealand only.

If you want the applet directly on your web site then an arrangement is possible, and it is possible to configure the tide graph for almost any coastal region on the planet, if data is availabe. Please see our Linking Policy.

The applet says "FAILED TO READ DATA". What's wrong?

Ensure that your Internet connection is still up and running, and that you are not using an Ad blocker (more info). If you get this message repeatedly then the data you are requesting may not be available on our website. Let us know with an email - and please remember to tell us which month and place you are having problems with.

Are the tide graphs accurate?

Tide heights and times can be affected by several factors, the two most common being the air pressure (low air pressure means higher tides) and by winds (a strong onshore wind will increase the height and bring the time forward, and an offshore wind will do the opposite). After considering these factors, if you feel our data is consistently wrong then please contact us. We put a lot of effort into getting the data correct, but inevitably the occasional error does slip through (and they not all of our own doing, I might add).

How can I print the TideGraph?

The applet doesn't provide the ability to print the graph, and very old browsers generally won't print it either, although later Internet Explorer versions (5.5 onwards) generally work. Try it. If the browser's print fails, the only workaround (Windows only) we know of is to display the graph and then press Alt and PrintScrn. Then start any paint or photo program such as MicroSoft Paint, PhotoShop, etc, and choose Paste as new image from the Edit menu. You should then be able to trim off the bits you don't want and print the graph from the paint program.

What's the story with Daylight Saving time and Time Zones?

The graph is normally corrected for daylight saving, so you should not need to do this manually. We assume a New Zealand Time Zone and Daylight Saving regime so even if you are in another country the times shown will be New Zealand times. When we change to and from daylight saving time there may be a jump in the tide height, and since there is no correct way to show this on the graph (between 2am and 3am) you will have to use your head and figure out the correct height yourself.
Please Note: A small percentage of computers in certain configurations may fail to provide the Tide Applet with Daylight Saving information. If this is the case then you must apply Daylight Saving corrections yourself. If the message "No Daylight Saving applied" appears while Daylight Saving is in force, then you must add one hour to all times.

I only see a grey box or red cross in my browser. What could be wrong?

Why can't I get tides more than a month in the future?

We publish and sell a large range of products giving tide times and graphs well into the future for most parts of New Zealand (and a number of other countries) and we suggest you purchase one suitable for your purposes. If we make this site too durned good, you won't buy any of our products. Visit your local bookshop or take a look at the OceanFun Publishing Ltd online shop.

Can I get a computer virus from this site?

No, you can't, we carefully check all systems for viruses, and in addition our Java applet that runs on your computer does not have access to your disk or other important resources, so you are doubly protected. We can say that our applet is free from computer viruses and cannot infect your computer in any way.
Please make yourself aware of current threats and protect your machine accordingly.
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