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Old 20-10-2012, 23:39   #1
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Inferring our position from land features

In the long-running thread on whether paper charts are still necessary, the point came up that land topography may not be shown on an electronic chart.

Another poster pointed out that such topography is important to sailors, especially racing, because it affects how the wind flows out over the water.
It occurred to me it's also rather important for selecting an anchorage, and for interpreting radar returns.

But to me there's a more crucial purpose for this information.
It tends to be forgotten, now that we (almost) always know where we are.

There was a time, when within sight of land, that charts were used mainly in order to find out where we were.

When the GPS era, their purpose changed almost overnight: we used them mainly to show us where we were.

I'm guessing that's why land topography is no longer considered to add value to electronic charts.

I think this about-turn might be at the bottom of why GPS keeps sneaking into the paper vs electronic chart discussion. Because (AFAIK) what I'm raising here is a largely un-discussed, and potentially controversial link, I thought it merited a new thread.
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Old 21-10-2012, 00:47   #2
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Re: Inferring our position from land features

You are so correct, knowing hill heights for a good anchorage is so important that is why I use both
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Old 21-10-2012, 01:22   #3
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Re: Inferring our position from land features

I was taught that GPS is an aid to navigation not the only method. A paper chart does not require a power supply.
In sight of land a chart and compass will keep you out of trouble (except in a bar!)
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Old 21-10-2012, 01:23   #4
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Re: Inferring our position from land features

Using conspicuous geographic and architectural features is still a valid and useful way to determine position using a handheld compass when navigating coastlines. It's also nice to see the progression of 'cocked hats' along the chart.

My mooring in directly between two conspicuous points and 75 degrees off a tall communications mast onshore. Using similar features as virtual leadlights, particularly in home waters is something we all should use, especially where currents, tides and wind conspire set and drift which in a busy waterway need to be noted 'on the run' rather than by regular visits to the chart table. That way we can monitor it as it happens, rather than after the fact.

Features along the coast, which are used to navigate traditionally should be transposed to electronic charts, if for no other reason than to confirm in three dimensions our position relative to the hard stuff.
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Old 21-10-2012, 01:24   #5
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Re: Inferring our position from land features

sctpc

Indeed - although I think hill heights (which is all some charts show) is still well short of what we need, especially for eyeball pilotage by identifying terrain features (which is what I'm mainly aiming to explore in this discussion) but even for working out whether and where to anchor.

In the latter case, more important than the hill tops, it seems to me, is the saddles and passes (the low points in the ridge between adjacent basins) so we know where the air will drain through, and which valleys it will flow down, and which bays it will blow out of. Or into.
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Old 21-10-2012, 01:30   #6
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Re: Inferring our position from land features

Thanks everyone for your thoughts

Auzzee:

Conspicuous features are one thing, essential, to my mind - and relatively easy to transfer.

What I'm personally mostly concerned about, though (partly because I sail a lot in places where there are no man-made alterations to the terrain, which is typically hilly or mountainous, or where the only land use is agriculture, so those alterations are generic in nature) is the raw contour line information.

In country like this, these provide the only eyeball-only way of uniquely identifying the prominent terrain features you refer to (from which bearings and sextant angles can be taken), by working out the 3D context which surrounds them.

I personally think it's worth trying to get hold of topographical maps for this purpose, as used by climbers and trampers, if you sail in areas like this. The contour information is much richer and more useful than even the best marine charts.
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