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Author
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Topic: Short Dip Distance Accuracy...How Important?
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Gary Rose
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posted January 05, 2005 05:01 PM
This may sound like a question that the answer is self-evident but here goes.
When having to use a short dip how precise does the distance measured to distant shore have to be? I know that the more precise things are the better the results will be. But I ask this question for a few reasons.
1) When using the dip value from the chart in the Nautical Almanac there is some element of error because the value is picked from a range of heights of the eye and the higher the eye is the larger the range.
2) I live in an area that the rivers and man made lakes are not charted. There are some “Fishing Maps” and I have some mapping programs (highway) for the computer that I can make measurements from. I use these items but they have a margin of error. I think that both the “Fishing Maps” and the mapping programs show the “shore position” at what may be the “Normal Pool” also known as “Summer Pool.”
3) Another problem is that the Army Corp of Engineers controls the level of the water to prevent flooding down stream. The water level fluctuates as much as 15 feet or more. Because the distant shore is not a vertical wall the distance changes with a change in water depth.
The areas that I can get to waters edge at have the distant shore between 900 and 5,000 feet, this is based on the computer generated map. Furthermore one place that I checked this past weekend the waters edge was at least 50 feet out from where it is during the summer. Assuming that the distant shore has the same slope as the side I was on, then there is at least a 100 feet decrease in the distance. Since the probability of the distant shore having the same slope is very slim… Also the parking lot showed a resent trash (high water) line a hundred or so feet up into it.
I realize that the best way to answer this question maybe to go out and take sights from the same location, reduce and plot them during different times of the year as the water level fluctuates to see if my fix changes, but I still would have only a estimation of the distance which goes back to heart of my question. How accurate does the distance have to be?
Thank you,
Gary
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David Burch
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posted January 05, 2005 05:48 PM
Dip short is an excellent way to practice sextant sights using a shoreline that is relatively close to you when you do not have access to a true sea horizon.
for those who have not got to the topic, it is in the back of the book, referenced as a special topic from the back of the book.
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The best way to figure the effect of the dip short input uncertainties is to simply change the input values in the formula and see what dip you get. Ie D= 900 ft = about 0.15 nmi and 5000 ft is about 0.83 nmi. Just use different ones and see the result. You will find that the formula is sensitive to height of eye. ie, try 9 feet then try 11 ft and so on to see the effect.
that is easier than working out the differentials to compute the sensitivity.
With that all said, you may not want to rely on the charts or maps. With a hand held GPS and canoe or kayak or rowboat you can simply go there and measure the lat lon and from the lat lon of the shoreline and that of the sight taking position you can compute the distance. Need to get a good average position at both ends.
also with that method, you can measure the lat lon of a few prominent land marks to use for future reference, without having to take another boat trip.... such as this piling is 0.64 miles off and the water seems to be just beyond that, etc.
and in passing, it is a good question. One must be very careful with the dip short formula. It will work remarkably well, but the input data must be correct. We have a lot of experience with this to back up that advice, and you will learn this by trying a few numbers yourself.
We have found cases where we used our own tabulated distances as a function of bearing along a lake shore measured from a map, only to be thrown off by an all new row of moored boats along that rim of the lake that were not on the chart... at a quarter mile off, that 75-100 feet or so made a measurable difference in our dip short corrections.
--david
From: Starpath, Seattle, WA
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