Windy vs. Current Atlas for slack times

kilroy

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I've noticed major discrepancies between the times windy predicts for slack and the iOS Current Atlas app. Which is the better of the two to rely on and why? I'm just learning this stuff and read the recent "Slack Times" thread with interest.
 

eriktheviking

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Prince George, BC
Which is the better of the two to rely on and why?
My understanding of the "why" part is that the tide and current tables are derived from measurements so based historical data that are very specific to the station. Overall the tides (and hence tidal currents) are related to the relative motions of the Earth, Moon and Sun (see tidal theory) so are periodic on a variety of timescales (Metonic Cycle). Many apps and other compressed data sources base the values they report on periodic harmonic models of varying complexity- less data needs to be stored, then the values are computed. These can be in error due to the sophistication of the calculation, the input data (the model parameters for each station), and then due to local effects that are hard to capture in these general models.

One issue I have not seen much discussion of is the impact of sea-level rise on the tide and current tables, which are based on historical data (decades worth or more).
 
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Timfrick

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There may be a few bugs in the iOS Current Atlas app. I’ve noticed some discrepancies.
It’s probably not designed for predicting slack times anyway but rather water movement patterns. You could cross reference the app with Borsboom tables and the hard copy of the Current Atlas when things don’t match up.
 

cougarmeat

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kilroy, I see this is your first post; Welcome to the forum. I try to find three sources with the goal of having two agree. If I find the first two agree, I stop there. But even so, it’s all just an estimate.

We have become very dependent/believing in techology. Today, I return from errends and knew it was way hot. The thermometer in the shade was in the mid 90’s and one not shielded was at 100 degrees. I was talking with a friend in Eugene and mentioned how hot it was and she said, “No, it’s only in the low 90’s - that’s what my phone says.” I told her that I didn’t care what her phone said - I was here now, I had just been outside, and it was definitely nudging 100 degrees.

Every winter we have some people getting stuck in a back forest service road because instead of actually looking around at what they are driving into, they blindly follow what there GPS is telling them.

A local snow park has a trail map at the parking lot, it has a UTM coordinates around the perimeter of the map. Trouble is, someone made a mistake in setting up those coordinates and they put you somewhere in the midwest. I let the forest service know about two years ago and I don’t think they corrected the numbers.

My point is, research is good. But people make mistakes, typo’s are made (I should know). Computer programs - which are just instructions written by humans - have been found to contain miscalculations or a misplaced decimal point in an equation.

Predictions are given by algorithms whose inputs are based on past experience/history. And you might have noticed, especially in the NorthWest, that recently very little in the climate arena is matching past experience.

So when DeepZoom says there will be a North flowing current of 2.4 knots, I don’t think, “There will be a current of 2.4 knots.” I think, “There will be a current stronger than I want to paddle against about this time.” and adjust my plans accordingly.

If one table says slack will be at X time and another says it will be 15 minutes later - that’s close enough. It gives me a window to shoot for.

So it is good to find out as much as you can about what you are going to be paddling into. But don’t get too locked into the what some “prediction” says. Sometimes it helps to add the word, “suggests”. "This table suggests the winds will be xxx.” If you get to different predictions that are close, that is probably as good as it gets. For fun, you can keep a log comparing what various apps predict and what you actually find when you are there.
 
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nootka

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I wrote a program in 2014 to calculate tide heights & current speeds from harmonic constants.
Here's a code snippet:
/* convert degrees to radians */
epo = epo * M_PI / 180;
eqa = eqa * M_PI / 180;
speed_rps = speed_dph * M_PI / 648000.0;

deg = speed_rps * t_seconds + eqa - epo;

// value or first or second derivative
if (d012 == 0) ss = amp*node * cos(deg);
else if (d012 == 1) ss = amp*node * speed_rps * sin(deg);
else if (d012 == 2) ss = amp*node * speed_rps*speed_rps * cos(deg);

sum += ss;


The algorithm is exact, it is not guesswork. The harmonic constants are calculated from observations - good results are from a year's worth of observations, one per hour, and best results are from multiyear observations. So the predictions at any one station might be really good or they might be just okay.
I wrote another program that tells me what time to launch from Campbell River if I want to paddle through Seymour Narrows at 8 knots current speed. The two are 24 kilometers apart, in a channel with non trivial currents. On any given day there may be wind or waves or seaplanes or ferry traffic, yet I arrive at the Narrows within 5 minutes of the desired time. I don't consider my computer predicted launch time to be a suggestion.
 
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cougarmeat

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nootka, the computer code itself is exact. It will run the same calculation steps every time. It’s the quality of the input data to formulas that is variable. Even the definition of “predicted”, says“… stated or estimated as likely to happen in the future; forecast …” It does not say, “exact”.

We’ve all had our life experiences. I recall one summer when friends wanted to visit me in Bend to go rock climbing. It seemed every weekend they wanted to come down, the local new’s predicted weather reported rain. And each time, while my friends stayed away, we had a nice sunny weekend.

On one stay at the north end of Vargas, the high tide kept coming and coming and coming. It was much higher than the tables declared it would be. That’s because there had been some local storm and a guide who had a group on the same beach said it was the highest he had ever seen. So the tables said it would be a high tide - and it was. But it was higher than the numbers said it would be.

If you pull your boat up above what some chart/program says the tide will be and think that therefore you don’t have to tie it off. A large tanker could come by and cause a shore surge that would reach your boat if it were just above what you read was going to be the high water mark.

At night we listen to the local weather. And they report wind speed of 0 (zero), as we look out the window and see tree branches swaying in the “zero” wind.

It’s not the algorithm, the instructions, that are guesswork. It’s the input data to those instructions. And I wasn’t saying it was going to be radically off. I was saying that you might have different sources giving different “exact” numbers but if those numbers are close to each other, That’s probably good enough.
 

nootka

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nootka, the computer code itself is exact. It will run the same calculation steps every time. It’s the quality of the input data to formulas that is variable.
Did you even read my post ??



Plot of currents expected in Discovery Passage today. Purple lines are for ebb. Red lines are for flood.

Jul_11_2021.gif
 

AlphaEcho

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Quadra Island, BC
@kilroy - I'm a technologist and I've had the privilege of working in mining and commercial transport, two industries where Geomatic Information Systems (GIS) have exploded in the last 30 years. My entire career has focused on the transition from paper, analog systems to digital systems. I've seen your question posed a thousand times by executives, managers, and every day users: what's the difference between X system and Y system and which is better? I've answered those questions in conferences, on email, and over many, many beers in an ex-pat bar in the jungle.

The short answer is: there's no short answer. If you want to focus on the short answer, I (or the rest of us) will try to find out more about your level of skill and understanding, and then feed you an answer that will imply which solution is better for you where you are now. You'll pick one, KNOWING it's the best, and go away. Thus saving us all from a lot of sturm und drang.

The medium answer is: even if we had the programmers behind the two applications you ask about, it would not be possible to get to an answer that made sense to a layperson in just this discussion. If you don't want to dig deeper than this, then just pick one of the applications in question and see how accurate it is for the various locations you visit regularly. You can then come back and tell us how things worked out.

The long answer is: the good people on this forum are all trying to say the same thing - tools are only as good as the person using them. Whether a shovel, scalpel, or survey grade HPGPS, it takes time to learn how to use them well and to know which version of that tool is most appropriate for which situation. Because they want you to engage in that process, they have given you key tactics to employ:

- understand the basis for how the data is collected, collated, and presented
- any system (and data model) is just an approximation of the real world and it is vital that you understand what those factors of approximation are
- use more than one tool to learn which is best in which situation
- understand the value of getting to a 'good enough' approximation and plan accordingly

I for one would love to have someone from each of the programming teams from Windy and Current Atlas come and talk to us about their design process and what choices they made to adjust for holes in the source data, transposition errors, etc. We would all probably learn a lot. However, that's likely not going to happen. The best we can do is put our heads together, compare notes and experiences, and come up with our own 'good enough' ranking of which tools are probably OK for average Joes like us paddling in the best playgrounds on the planet.
 

kilroy

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Jun 21, 2021
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Thanks for the replies, very helpful. Good points about using multiple models, model compression/simplification, and correlating these tools to your own real world observations.

FWIW, it looks like the Current Atlas is based on CHS data, although it isn't clear if the interpolation between current stations is their own algorithm or if they get that data from CHS as well.

 

cougarmeat

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nootka, maybe it’s the heat - which I’m about to escape - assuming the prediction of a 30 degree difference from what the whole PNW has felt will be exact - and I was not questioning your efforts nor the accuracy of your predictions in that narrow area. I’m glad it works for you and thank you for sharing it with us.

kilroy’s question was about the predictions of a Current Atlas covering a large area. It’s my experience that the tables for that atlas are calculated and distributed for the year, at the beginning of that year. And there are weather unknowns that can greatly influence what a chart says something will be.

I was trying to address the issue where a “program”, tide/current table says the current will be 2 knots but on the specific date a hurricane has moved into the area. I wouldn’t put much stake on that 2 knot prediction. Now that’s extreme to make my point. But I’ve seen the exact dynamics on a smaller scale - too much belief is afforded to what “the book says” rather than what is actually happening at the time. Hence my example of my friend telling me the temperature was in the low 90’s because that’s what her phone predicted while I was actually there, in the heat, looking at a gauge that said 99.

So I’m was not at all addressing your calculations of current though Seymour Narrows. I was referring to something that makes a yearly prediction over a large area.
 
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