Route planning mileage

[ailsaboat]

I was just wondering if there were any forumites that had a 'best guess' formula for route planning distances.
I'm just planning out this years big trip, which comes out with a charted distance of 3127nm. But as I want to sail as much as possible this will involve the inevitable sailing in the wrong direction method.
Would you add a third? double it - it's not really important, i was just trying to do a rough time line etc

 

[snowleopard]

If the wind is likely to be from astern, 5% is enough allowance. If you have to beat this will go up to 50% or more. It all depends on prevailing winds.

 

[Piers]

Sounds like an exciting trip! Will you be using GPS at all? Do you know if it calculates the track using rhumb line or great circle? Most do the latter which is correct for long tracks (least distance in a straight line), especially when oriented east west.

 

[Guest]

I know I shouldn\'t do this ......

Being an old fogey and ex navigator etc. A great circle is the shortest distance between any two points on the surface of a sphere. It does not care about orientation .... so saying especially East - West is actually not correct .... it could be any direction ...

But of course in practical terms most long distance work is approximating to east / west as that is direction to / from great land masses etc. BUT a trip from say Australia to North Canada ???

Sorry Piers ..... had to get my little bit in .....

Cheers
/forums/images/graemlins/tongue.gif

 

[Anonymous]

Are you sure about that? If so then presumably the 'BTW' figure would vary continuously along the track (albeit a very small variation for passages < 100nm or so). I'm fairly sure that my Raymarine system gives rhumb line, but you've got me wondering.

 

[iangrant]

Err I reckon Nigel is probably about right, sometimes not too hot on alternators but not bad on the old navigation stuff, (he used to drive the big metal ones, lots of tons they were)

Ian

 

[snowleopard]

Re: I know I shouldn\'t do this ......

On the other hand, and the point I believe Piers was making, on a north-south track a great circle and rhumb line are identical but with an east-west track the difference is at a maximum. The difference is again zero on the equator but increases as you go further north or south.

I have never heard of a GPS that doesn't work on great circle bearings. If in doubt, enter a waypoint 180 degrees east or west from your current location. the BTW will be 000, not 090 or 270 (180 in the southern hemisphere).

 

[Piers]

Re: I know I shouldn\'t do this ......

Snowleopard is absolutely right. And he's also right about the fact that almost every leisure GPS uses Great Circle (GC) navigation. However, there are some sets that offer rhumb line such as the bigger Simrads where each leg of a route has the choice.

Note that the charts we tend to use for coastal nav are Mercator charts where a straight line is a Rhumb Line and not a GC.

Hence, when using GPS GC nav, the line on the Mercator chart will not be the GC line followed by the GPS. The start and finish will be the same, but will curve to the north of the rhumb line in the northern hemisphere.

By the way, Kim and I wrote an article on this some years back in MBM...

The three factors that make the error larger are, the closer the track is to east/west; the further the track is from the equator, and the further apart the start and end poiints are. As snowleopard said, a GC is a Rhumb line when on the E-W equator, and N-S meridians of longitude.

If you want a formula that is 95% accurate for east west tracks up to 500nm long at 50 degress north (increasing to 99% accuracy for shorter legs, try this!

[(rhumb line distance)2 x Tan latitudeº] / 28,800 = the error in nautical miles

Latitude degrees is the mid-latitude between waypoints. To convert to metres, multiply the answer by 1852.

Erm - is this enough?

 

[Anonymous]

Re: I know I shouldn\'t do this ......

[ QUOTE ]
..... almost every leisure GPS uses Great Circle (GC) navigation.

[/ QUOTE ]Out of curiosity since it is of little or no practical importance, do you know whether this is based on the spherical approximation or does it account for the actual shape of the Earth?

 

[Piers]

Re: I know I shouldn\'t do this ......

I believe you will find GPSs asume a perfect sphere and do not take into account the fact that Earth is an oblate spheroid which makes the diameter at the Equator some 23.5nm more than through the poles.

 

[snowleopard]

Re: I know I shouldn\'t do this ......

My GPS set can operate in 2D or 3D and in the latter case shows altitude. I haven't checked the reading at the equator but would be very surprised if it showed an altitude of -23 miles! At a guess I'd say the programming allows for deviations from the sphere.

 

[john_morris_uk]

Re: I know I shouldn\'t do this ......

I agree. The software in modern GPS allows for the 'height' of the surface of the earth at any given point. Its a lot more sophisticated than most people give credence for.

 

[Piers]

Re: I know I shouldn\'t do this ......

A good point - and one I will check on. I suspect it assumes a mean sea level of sorts....

 

[LeonF]

Re: I know I shouldn\'t do this ......

Snow please could you explain...how do you enter a way point 180 east or west.. surely this would be south of my position no matter where I was ?? I know I am probably being dense !!

 

[LeonF]

Re: I know I shouldn\'t do this ......

Just realised you meant 180 Long not compass bearing ??