[Resolved] Predict tides.

[brainlag]

I'm trying to figure out a way to predict tides using PHP. The time between a high tide and the next high tide is roughly 12 hours and 25 minutes.

Here's the problem. I need to predict the tides based on a known tide in the past eg: 05/22 12:39pm was a high tide. But I need it to displays today's tide.

How can I base a clock on that time and date, add 12 hours and 25 minutes to the next tide and still have it display today's correct tide time.

While any help is greatly appreciated, I would rather understand the solution than it being handed to me. I read the http://php.net/date, but I was unable to get the correct information.

Thank you.

[mtmosier]

First I'd convert all your numbers to seconds.

$known_tide = strtotime('2005-05-22 12:39:00');
$current = now();
$interval = (12 * 3600) + (25 * 60);

Then it should just be a matter of some simple mathematics.

$num_tides_passed = floor(($current - $known_tide) / $interval);
$last_tide = $known_tide + ($interval * $num_tides_passed);

And so on.

[id10t]

Yup, convert your times into a unix time stamp format and just do some simple math. Also, there are date functions that use the hebrew calendar which is lunar based, so you can also plot moon phases along with the tides

[brainlag]

Thanks for the replies and the heads-up about the Hebrew calendar, but I'm stumped. I can't seem to get a proper readout. I keep getting -numberrs, which I assume are the UNix time? I really have no idea at all.

I'd post what I've tried, but I've tried many, many different things. I just don't understand enough about the basics to make it 'work'.

[brainlag]

OK, I'm getting closer, but I'm still off and I'm not sure why....

<?php

$knowntide = strtotime('1117284300');
$current = date(U);//the "now" gave errors
$interval = (12 * 3600) + (25 * 60);


   $num_tides_passed = floor(($current + $knowntide) / $interval);
   $last_tide = $knowntide + ($interval * $num_tides_passed);

   echo ($last_tide + $num_tides_passed + 7200)
?> 

It's realy close, but not 'on time' based on the $knowntide. I added 7200 to offset the time difference between my time zone and the server's.

Any ideas? I've been banging my head for 12 hours on this. Maybe I'm doing the wrong math, I don't know, I've been looking at it so long I can't see anymore.

[mtmosier]

$knowntide = strtotime('1117284300');

When I run this I get -1 back, meaning that strtotime can't determine what date you are specifying. This number, 1117284300, doesn't mean anything to me. Perhaps it's already a unix timestamp? In that case skip the strtotime.

$current = date(U);//the "now" gave errors 

My fault. now() is a mysql function. You really wanted time().

echo ($last_tide + $num_tides_passed + 7200)

This doesn't make much sense to me. $last_tide is a unix timestamp, and $num_tides_passed is an integer representing the number of tides passed (ie, the number of interval values to add). When adding them together you're treating $num_tides_passed as seconds, which doesn't seem to have any meaning. (If you're trying to get the next tide try adding $interval instead, since that's the number of seconds between tides.)

[brainlag]

OK, I tried the suggestions and still, the timees aren't working for me no matter what I try. I'm sure it has something to do with my poor math skills, but I changed the knowntide to a number that ends in 5 and therefore, I would assume, the number should always end in 5 or 0, but it doesn't.

<?php

$knowntide = strtotime('2005-05-28 12:45');
$current = (date(U) + 10800);
$interval = (12 * 3600) + (25 * 60);


   $num_tides_passed = floor(($current + $knowntide) / $interval);
   $last_tide = $knowntide + ($interval * $num_tides_passed);

   $next =  $last_tide + $interval;

   echo $next
?> 

Returns:"3352041300" which is Sat, 21 Mar 2076 18:35:00 GMT. That would be a long tide 😃

Biology major... math eludes me.

[mtmosier]

$num_tides_passed = floor(($current + $knowntide) / $interval);

That needs to be $current - $knowntide.

[brainlag]

Originally posted by mtmosier

$num_tides_passed = floor(($current + $knowntide) / $interval);

That needs to be $current - $knowntide. [/B]

Output is: Mon, 30 May 2005 21:25:00 GMT

Changed a few things and it's close enough.

Thanks for the help. I think I finally 'see' the math behind the problem now. now on to step 2 😃

Again, thanks for the help.

<?php
    $knowntide = strtotime('2005-05-30 02:09');
    $current = date(U);
    $interval = (12 * 3600) + (25 * 60);

$num_tides_passed = floor(($current - $knowntide) / $interval);
$last_tide = $knowntide + ($interval * $num_tides_passed);
$next_high = $last_tide + (12 * 3600);
$next_low = $last_tide + (6 * 3600) + (13 * 60);

   echo date('m/d g:i a', $next_low + 25200) ."\n";
   echo date('m/d g:i a', $next_high + 25200) ."\n";
?>