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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #31  
Old 02-28-2010
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Quote:
Originally Posted by Boasun View Post
If the chart is 1:80,000 that means that you have 80,000 square inches representing land & water area squeezed into 1 square inch of the chart.

But when using the latitude scale on the chart for measuring distances, I have found that there can be a slight difference from near the bottom of the chart compared to the top of the chart.

Mmmmm. I was a cartographer for 17 years at the old Defense Mapping Agency, and I have to take issue with your "80,000 square inches representing land & water area squeezed into 1 square inch of the chart" comment.

A chart with a scale of 1:80000 means that one LINEAR inch on the chart = 80,000 LINEAR inches in the real world. If you are looking at one square inch of chart you are looking at a piece of water and land 80,000 inches on a side, or 80,000 inches X 80,000 inches: an area of 6.4 MILLION square inches, not 80,000.
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  #32  
Old 02-28-2010
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You are anchored in Tod Inlet (Lat 48° 34’N, Long 123° 28’W), planning to go to Maple Bay (Lat 48° 48’N Long123° 36’W) via Sansum Narrows. The turn at Sansum Narrows is at 1200.
The distance to Sansum Narrows from Tod Inlet is 14 miles. Assuming a speed of 5 knots, at what time should you leave Tod Inlet to make the turn?
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  #33  
Old 02-28-2010
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And I was a Navigator for USN for about as long and it seems as if we both learned it differently. But checking bowditch I find that it is linear in any Direction.
But we both learned our data differently and sometimes it needs to be brought out so that we may refine our knowledge.
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  #34  
Old 02-28-2010
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I'd point out that what Boasun said would have been correct if he had said "SQUARED" instead of "SQUARE".
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Old 03-01-2010
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Tides and Currents

I want to be at point A in a narrow channel at slack water, but I have no current tables. I know that high tide is at 0300. At what time should I be at point A and why is slack water not at the high tide mark?
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  #36  
Old 03-01-2010
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I use Canadian Chart 3462 in my navigation class; it is 1:80,000 chart. However, the 1:80,000 is measured at 49˚ 30'N which is not on the chart. The top of the chart is 48˚ 54'N. As this is a Mercator projection, it seems to me that converting distances measured using the scale would not be exact.

In you view, would this be a correct interpretation?
I also posed this question to the Canadian Hydrographic Service. Here is their response:

Quote:
Thanks for asking this question which many mariners never notice or have no idea what this is all about.

The challenge when making a map is representing a spherical object (the earth) on a plane surface (piece of paper). There are many potential way of doing this (different projections) and they all have some inherent distortions. In the CHS, for navigating purposes at this latitude, we generally use a Mercator projection since for navigation it has desirable qualities, such as a straight line plotted on a Mercator projection represent a true course or constant bearing, and the shapes of features are preserved.

When we do a series of charts we want some consistency between each chart in that series. When we derive a base projection, we need to determine where the scale is true, and we call that the scaling latitude or mid latitude. That scaling latitude is mentioned in brackets in conjunction with the scale stated on the chart. On chart 3462 you will find this as 1:80,000 (49 30'N). What this means is that the place where you find the scale true is at this latitude. The rest of this chart or any other chart in the series with this latitude will have some distortion in the scale.

The reason we choose one latitude for the series of charts is so that they will all overlap each other with consistency, allowing mariners to transfer there position from one chart to the other with ease. If each chart in a series has a different scaling latitude, then the areas of overlap would not match each other, making it difficult for use.

As I noted earlier, there are always compromises when using different projections. In this case the Mercator projection is used since it has desirable qualities for navigating, such as plotting bearings, and a series scaling latitude is used since it allows easy transfer of positions between charts in that series.

To answer your question directly, nowhere on chart 3462 would the scale be true, though it is close. It is only accurate at the latitude which it is stated, and in this case it would be on the adjoining chart at 49 30'N.

I hope this answers your question. If I can be of more assistance, please just ask.
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  #37  
Old 03-01-2010
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Quote:
Originally Posted by lancelot9898 View Post
I want to be at point A in a narrow channel at slack water, but I have no current tables. I know that high tide is at 0300. At what time should I be at point A and why is slack water not at the high tide mark?
First some terminology: slack water actually refers to a lack of current. Standing water means the depth is not increasing or decreasing.

The reason slack water does not coincide with standing tide is that the horizontal motion of water is due to its being accelerated by differential tides. The point of highest tide moves as the day goes on. Say the tide is flooding and it's almost high tide. But suppose high tide upstream of me happens later in the day, even if only a few minutes later. Obviously the current will have to keep flooding, even when it's high tide where I am, because the water level upstream has to keep rising.

In the given example, the exact amount of time difference between high tide and slack current depends on the upstream volume difference between the time of high tide at A, and the farthest upstream high tide time. If there's a big reservoir upstream of you, it will take a while after high tide before the water at A is slack, but if you're just a mile from the head of an inlet, slack water will coincide almost exactly with stand.
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  #38  
Old 03-01-2010
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A Tide problem...
The height of the tide at low water is 0.0 ft. the range of tide is 9.0 ft
The duration is 06h 00m. The height of the tide 02h 12m before high water will be ____________??
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  #39  
Old 03-01-2010
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Quote:
Originally Posted by Boasun View Post
A Tide problem...
The height of the tide at low water is 0.0 ft. the range of tide is 9.0 ft
The duration is 06h 00m. The height of the tide 02h 12m before high water will be ____________??
By the rule of 12ths:
3 hrs before high tide, the height is 54/12 ft or 4.5 ft.
2 hrs before high tide, the height is 81/12 ft or 6.75 ft.
Interpolating:
12 minutes before that is 0.2 hrs. The tide rose 2.25 ft in that time.
0.2 hrs * 2.25 ft/hr = 0.45 ft.
6.75 ft - 0.45 ft = 6.3 ft.

The height of tide at 02h 12m before high water is 6.3 ft.
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Your vessel is on a course of 297° T at 11 ktnots. At 0019 a light bears 274.5°T and at 0048 the light bears 252°T. at what time and at what distance off will your vessel be when abeam of the light?
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