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Old 18-09-2019, 04:57   #16
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Re: Calculating tides at locations without stations

Have a llok at the programm tidecomp, this can be a help for you.
Regards,
Willy
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Old 18-09-2019, 07:13   #17
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Re: Calculating tides at locations without stations

Maybe, this will help you...





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Old 18-09-2019, 20:03   #18
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Re: Calculating tides at locations without stations

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Have a llok at the programm tidecomp, this can be a help for you.
Regards,
Willy
Looks like a nice program. It says it computes the tides using harmonic functions. But when I try to use the program it does not allow me to compute the tide at an arbitrary location. It will allow one to create a secondary port location but you need to have the time offset from the primary tide station. If I knew the offset that would be great, but I don't. That is the problem.
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Old 18-09-2019, 20:19   #19
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Re: Calculating tides at locations without stations

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Originally Posted by kantafig View Post
Maybe, this will help you...





The first location is close to George Town and I'm concerned about the time difference between it and Comer Channel.

The second point is a tide station that I was unaware of off the west coast of Long Island. This has great promise because I can interpolate between Nassau and this location to get the time at Comer Channel. This point is also available on Garmin Blue Chart Mobile.

Thank you very much for this information. I think I will use this data.
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Old 19-09-2019, 04:28   #20
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Re: Calculating tides at locations without stations

perhaps you can use some of those for calculation.....






















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Old 19-09-2019, 10:35   #21
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Re: Calculating tides at locations without stations

Kantafig,

Thanks for the additional information. I saw that one of the locations was the Steventon tide station on the east side of Great Exuma. I was originally hesitant to use tide locations on the east side because the land is a major obstruction in the flow of water. But this morning I did my calculation again and here is what I discovered.

My calculations are for January 15, 2020. My point of interest is the center of the Comer Channel (approx. N23 deg 19.5' W075 deg 24.0').

In my first calculation (which is strictly an interpolation process) using Nassau and Long Island tide stations, I calculated the tide at my POI to be 3.0' at 0848. Using Steventon and Long Island tide stations the result was 3.1' at 0842.

That's pretty good agreement, but that is only "precision", i.e. the data points lie close together. What about accuracy?

The best data I have gotten from online blogs is the high tide at Comer Channel should be 1.5 hours after high tide in Nassau. That would make it 0935. The difference with my calculation is 50 minutes. So which is more accurate?

I don't know how the blogger got the 1.5 hours. I suspect that it was based on someone's observation but without special equipment there is no way to know for sure when the high tide exactly occurs because of the shape of the tide curve. Therefore I feel that my calculation may be at least as good as their comment.

For water depth only, even one hour is not critical given the shape of the tide curve (they are relatively flat at high and low tides). If current was critical then it makes a bit more difference. And remember high and low tides do not correspond exactly with max ebb and max flood.

So the bottom line is for my purposes I think that using about 1 hour after the Nassau high tide will be accurate enough to give me a foot or more under the keel. I can also try to get local knowledge when I arrive at Thompson Bay.

I will post the spreadsheet that I created to calculate the tide at an arbitrary point of interest. But before I do that I need to modify it to make it more universal due to the intricacies of adding and subtracting time in hours and minutes.

Bottom line is that I'm pretty happy with the results I now have and this method of calculating the tide at an arbitrary point of interest.
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Old 19-09-2019, 17:32   #22
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Re: Calculating tides at locations without stations

Looking forward to seeing that spreadsheet.

Thanks.
Joe
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Old 19-09-2019, 21:43   #23
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Re: Calculating tides at locations without stations

I have attached two Excel spreadsheets. The one called "Tide calculation" simply graphs one tide cycle from a set of tide data based on the Rule of 12ths. I wrote this years ago, but with the proliferation of computer-based applications it has a low value except in limited locations where you can obtain tide tables but don't have access to an application that covers the location.

The other Excel spreadsheet is called "Calculate the tide between stations." This is the one I just wrote to estimate a high or low tide at a location physically located between two tide stations. Please read all the notes at the beginning so that you understand its limitations and input required.

The question is how accurate is the method.

I am interested in the high tide in the morning at Comer Channel on January 15, 2020. The spreadsheet estimates the height and time of the tide at this location two times. The first time using data for the tide stations at Steventon and Long Island, and the second time using the tide stations at Nassau and Long Island.

You will see the results are quite similar. The first calculation says the high tide will be 3.1' at 0841 (17 min after Steventon). The second calculation says the high tide will be 3.0' at 0835 (23 min after Nassau). This differs from the one online data point I have that says the offset is 90 minutes after Nassau.

I suspect that the 90 minutes was an estimate based on a transit of the area. If so, the shape of the tide curve at high and low make it difficult to determine the time of a high or low tide very accurately.

If you believe that the high tide at Comer Channel must be between the times for Steventon and Long Island, and between the times for Nassau and Long Island, then it is impossible for the high tide at Comer to be 90 minutes after Nassau.

I'll probably use a 30 minute offset from Nassau. Even an error of 30 minutes will allow for more than a foot of water under my keel during the passage. I also plan to ask locals in Thompson Bay if they know the offset.I have attached two Excel spreadsheets. The one called "Tide calculation" simply graphs one tide cycle from a set of tide data based on the Rule of 12ths. I wrote this years ago, but with the proliferation of computer-based applications it has a low value except in limited locations where you can obtain tide tables but don't have access to an application that covers the location.

The other Excel spreadsheet is called "Calculate the tide between stations." This is the one I just wrote to estimate a high or low tide at a location physically located between two tide stations. Please read all the notes at the beginning so that you understand its limitations and input required.

The question is how accurate is the method.

I am interested in the high tide in the morning at Comer Channel on January 15, 2020. The spreadsheet estimates the height and time of the tide at this location two times. The first time using data for the tide stations at Steventon and Long Island, and the second time using the tide stations at Nassau and Long Island.

You will see the results are quite similar. The first calculation says the high tide will be 3.1' at 0841 (17 min after Steventon). The second calculation says the high tide will be 3.0' at 0835 (23 min after Nassau). This differs from the one online data point I have that says the offset is 90 minutes after Nassau.

I suspect that the 90 minutes was an estimate based on a transit of the area. If so, the shape of the tide curve at high and low make it difficult to determine the time of a high or low tide very accurately.

If you believe that the high tide at Comer Channel must be between the times for Steventon and Long Island, and between the times for Nassau and Long Island, then it is impossible for the high tide at Comer to be 90 minutes after Nassau.

I'll probably use a 30 minute offset from Nassau. Even an error of 30 minutes will allow for more than a foot of water under my keel during the passage. I also plan to ask locals in Thompson Bay if they know the offset.
Attached Files
File Type: xls Calculate tide between stations.xls (41.0 KB, 56 views)
File Type: xls Tide calculation.xls (47.5 KB, 50 views)
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Old 19-09-2019, 21:46   #24
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Re: Calculating tides at locations without stations

It looks like I've attached two Excel files to the above post, but when I try to open they are PHP files. Does anyone know what I'm doing wrong?
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Old 19-09-2019, 21:49   #25
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Re: Calculating tides at locations without stations

Select Port - EasyTide - on-line tidal predictions from the UKHO
There's not more than 15min apart of any stations high tide time in the area. On a shallow area the tide is allways a bit later than sooner... What's more important is the direction the of the tide IMHO, as it makes more difference on your passage than the hight tide time.
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Old 20-09-2019, 16:25   #26
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Re: Calculating tides at locations without stations

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Originally Posted by mlibkind View Post
It looks like I've attached two Excel files to the above post, but when I try to open they are PHP files. Does anyone know what I'm doing wrong?
If you click on it, it will work, if you right-click on it and try and do a save as it won't work.



Thanks for these, I will get a good look at them after I get my new Victron inverter installed.

Joe
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Old 21-09-2019, 16:18   #27
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Re: Calculating tides at locations without stations

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If you click on it, it will work, if you right-click on it and try and do a save as it won't work.



Thanks for these, I will get a good look at them after I get my new Victron inverter installed.

Joe

Joe, thank you for your advice. While that did not help, knowing that you got it to work meant that there might be a problem with my browser. I switched to Firefox and it worked fine. Thank you again.
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Old 21-09-2019, 16:34   #28
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Re: Calculating tides at locations without stations

Now I'm really confused.

1. I have come to realize that my version of Garmin Blue Chart Mobile does not give the correct tide information (at least some of the time). The first thing I noticed was that the graphic and the tabular data did not match when I looked at dates in the future, e.g. 1/15/2020. Unfortunately I don't want to update the program because they no longer support Active Captain within the program and I like having that information handy without the need for internet.

2. I tried AyeTides and those numbers seem to be pretty good for Nassau. But they don't have the tide station on Long Island. The data for Steventon looks very poor ... totally weird for 1/15/2020.

3. On AyeTides I found a tide station at Nurse Channel in the Ragged Islands. The data for this point is almost identical to that of Nassau. That would certainly mean that the times for Comer Channel would be the same as Nassau. I can actually believe that but why do the few blogs related to Comer Channel give a time offset of say 1 hour?

So what's correct? Not sure yet.

One thing I know is that one can't trust these apps when the tides are critical, e.g. the Garmin data is completely wrong. I used to use simple tide tables, but they are almost impossible to find now.
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Old 21-09-2019, 20:22   #29
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Re: Calculating tides at locations without stations

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Now I'm really confused.

1. I have come to realize that my version of Garmin Blue Chart Mobile does not give the correct tide information (at least some of the time). The first thing I noticed was that the graphic and the tabular data did not match when I looked at dates in the future, e.g. 1/15/2020. Unfortunately I don't want to update the program because they no longer support Active Captain within the program and I like having that information handy without the need for internet.

2. I tried AyeTides and those numbers seem to be pretty good for Nassau. But they don't have the tide station on Long Island. The data for Steventon looks very poor ... totally weird for 1/15/2020.

3. On AyeTides I found a tide station at Nurse Channel in the Ragged Islands. The data for this point is almost identical to that of Nassau. That would certainly mean that the times for Comer Channel would be the same as Nassau. I can actually believe that but why do the few blogs related to Comer Channel give a time offset of say 1 hour?

So what's correct? Not sure yet.

One thing I know is that one can't trust these apps when the tides are critical, e.g. the Garmin data is completely wrong. I used to use simple tide tables, but they are almost impossible to find now.
I have determine that I made an incorrect assumption as to the time zone used by the apps I looked at. I looked at four applications: Garmin Bluechart Mobile, AyeTides, realTide and Tides Near Me.

AyeTides specifically says that the times are EDT. That makes sense since I'm looking at tides in the Bahamas. Why would I want it in the PDT zone where I am right now. But that is what Tides Near Me says it is using; it simply looks at the time zone my iPad is in. Okay.

But both Garmin and realTide do not give you any indication of the time zone. I thought it reasonable to assume EDT because that is where Nassau is located and where I would be using the tide information. NO! By adding 3 hours to the times given in these applications one gets numbers very close to the other two apps.

After doing all this the only real big problem I see is that AyeTides gives a very large value for the second low tide (I was doing this for 9/21/29).

I'm now going to see how this all affects my interpolation method and look more closely at the tides given for Nurse Channel. Do they make sense?
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Old 22-09-2019, 04:51   #30
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Re: Calculating tides at locations without stations

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Now I'm really confused.


2. The data for Steventon looks very poor ... totally weird for 1/15/2020.


I hope this can help you.....


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