simple online way to measure sea distances

[AntarcticPilot]

I was going to suggest measuring it in km, then converting it as I thought I remembered that it was a very simple conversion, i.e. 2:1. Before posting I looked it up in a reference book I use frequently and was surprised to see that a UK nautical mile = 1853.2 metres whereas an international nautical mile = 1852 metres. New one for me.

And if you use the latitude scale of a chart, the length of a nautical mile varies with latitude!

Originally, a nautical mile was defined as one minute of latitude; convenient for navigation, and if the Earth was perfectly spherical, the same everywhere. In a similar way, the length of the metre was originally defined as 1/10,000,000 of the distance from the equator to the Pole (it's close enough to be a useful approximation).

Unfortunately for those who like units of measurement to be fixed quantities, the Earth is not a perfect sphere, but approximates (pretty closely) an ellipsoid - in other words, it is a bit flattened, so the diameter between the Poles is less than the diameter measured across the equator. That means that the length of a degree of latitude is less at the equator than it is at the pole, so that a minute of latitude (i.e. a nautical mile) varies by 18.66 metres between the equator and the pole.

The difference between the Uk and International nautical mile is a matter of choices made historically; both are reasonable mid-range figure that will be "close enough" wherever you are.

It's only in these modern, GPS days that errors of measurement of a few metres have become relevant.

 

[chartnavigator]

It's only in these modern, GPS days that errors of measurement of a few metres have become relevant.

Surely this is why so many boaty people bump into things that they know are there and were sure they were going to miss. Any sensible navigator leaves room for error and gives known obstacles a wide berth. Just because you see a picture of your boat and a rock on your sat nav, you can't assume that the rock is accurately charted, (or has not moved, if it is a sand bank, spit or the like), it all depends still on when the chart was last updated, particularly in really out of the way places on the globe. Some chart information is still based on 19th, even 18th century surveys in the places private yachts are inclined to sometimes go. It was exactly this kind of cock up with GPS that got a £multimillion frigate on a rock and a shamefaced commander in the rattle a couple of years back.

On the thread topic - for £0.50p you can download a PDF preview of my book on how to use Google Earth for route planning at routelist.co.uk.

 

[AntarcticPilot]

Surely this is why so many boaty people bump into things that they know are there and were sure they were going to miss. Any sensible navigator leaves room for error and gives known obstacles a wide berth. Just because you see a picture of your boat and a rock on your sat nav, you can't assume that the rock is accurately charted, (or has not moved, if it is a sand bank, spit or the like), it all depends still on when the chart was last updated, particularly in really out of the way places on the globe. Some chart information is still based on 19th, even 18th century surveys in the places private yachts are inclined to sometimes go. It was exactly this kind of cock up with GPS that got a £multimillion frigate on a rock and a shamefaced commander in the rattle a couple of years back.

On the thread topic - for £0.50p you can download a PDF preview of my book on how to use Google Earth for route planning at http://routelist.co.uk.

Tell me about it! 30+ years of mapping Antarctica makes me VERY aware of the potential deficiencies of charts. We only recently achieved consistent mapping at the 100m accuracy level for most of the coast of Antarctica.

Incidentally, Google Earth has pitfalls for navigators; mainly that the images may be quite old, especially in areas with a lot of cloud-cover. That coupled with an area of strong tides and sand/mud-banks may mean that the images are to be used with caution. Further, the projection used by GE is not Mercator, so there are (small) discrepancies between charts and GE. In the polar regions, the problems become acute.

 

[chartnavigator]

Tell me about it!

I obviously don't need to. Pity there still seems to be so many boaties out there still that put the same kind of blind 'faith' in their sat nav as Martin Luther put in God. I'm sure there are many who are unaware that the data for sat nav imagery is often derived from good old real paper charts, (a few of which are very old indeed and may even have mermaids and sea monsters in the blue spacy bits.)

Regards Chris.

 

[ghostlymoron]

Back to the original question. Google earth is without doubt the simplest, cheapest and available-ist facility for approximate measurement of routes and distances.

 

[chartnavigator]

Agreed; and good enough for the purpose intended, of getting a rough idea of 'how far will it be ?'. One thing though. You can use Google earth like that on a PC or lap top but not on a tablet. Tablet GE does not have a measuring facility, (at least iPad didn't last time I looked).