Measuring lat and long on a chart

M

max_power

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A lot of the fun with GPS is making your own waypoints and giving them personalized names.

But all mine seem to be in the centre of the chart and difficult to get right. Many charts do not seem to have measurements other than top/bottom and sides now.

I would be interested to know what ‘tool’ or method other sailor’s use.


Regards
 
Although I was taught to use a Portland plotter on my DS course, now I am putting it actually to use, I find a 18inch ruler very useful as it's long enough to go across most charts (which the PP isn't) and give a light pencil line in the area I want.
Now starting to play with electronic charting (Oziexplorer) so medium term won't be an issue any way!

Jim
------
 
Hi! Use your dividers to measure the distance from any close by, lat or long line which runs across the chart and then move across to the scale on the side or bottom of the chart (which ever ) and read off your position.
Regards
Rob

robinwdunham
 
short rules?

take it your problem is that the plotter, parallel rules or whatever won't reach to the scale at the edge of the chart?

simple solution (for longitude in this example) is:

a) measure with dividers the distance from the point in question to the nearest meridial on the chart.

b) follow the meridian down to the bottom of the chart

c) mark off with the dividers the same distance along the scale: this is your longitude.

i find this much more reliable than trying to walk the rules across the chart or interpret the skeleton scales printed in the middle of some charts.
 
Hi Wishbone !

Thanks for the response.

Bit thickheaded myself at the moment so apologies for the unclear post. Am sitting here noting that sailing seems lightyears away. The suntan's gone ( windtan ?) two more inches round the gut and am watching too much TV at the expense of doing something positive - like sailing. Do those colonial types in Oz get bored with sailing all year round ? Adsence makes the heart grow fonder ?

Back to the post. My home made places on the charts never seem to be near a bit where it's easy to measure. I like the idea of an 18 inch ruler. Bit too obvious for me though.
 
G'day max,

Yes use the dividers, but make sure you measure the bottom or side of the chart at the same point as the line used, some chart formats are not uniform: you can check this, measure 5 nm at the corner and transfer this to half up the page, 5nm will outside the divider points.

And, no, we don't get sick of sailing all year round in Oz.....

Avagoodweekend Old Salt Oz /forums/images/icons/cool.gif
 
I hate to disagree with Oz but don't use the bottom of the chart at all. Lines of longtitude converge at the poles, so as you move away from the equator you would end up with some very funny length miles. Lines of latitude, however, are evenly spaced. So the scale at the side of the chart is always right. It may be that in Oz you're using charts which are close to the equator and it wouldn't matter much. But in Europe it does.
 
careful!

perfectly legitimate to step off along the bottom of a chart to get longitude. of course you can't get a distance in miles that way.
the other proviso is that you're using a mercator projection so the meridians are parallel.
 
Re: careful!

perfectly legitimate to step off along the bottom of a chart to get longitude>>
Of course that's right SL. But I think Oz's technique implied he was measuring distances rather than degrees. If he wasn't, I apologise. You're also right about mercator v other projections. But the woman's doing a day skipper class, not routing across the Atlantic or walking to the N pole! I expect it will be some time before she comes into contact with non mercator charts.
 
Re: careful!

Thanks everyone, the dividers method certainly seems to be the best idea, particularly for kneecap navigation in the cockpit where it’s hard to unfold the complete chart. I never got on that well with dividers after stabbing myself in the thigh in an unexpected gust but I will get them out again.

For fun only, I got some charts out on the floor. I never realized it before but many are three feet wide. I got two plastic rulers and made them into a right angle with the aid of a screw. Not bad for an ‘instant’ lat and long if you have the place of interest at the corner of the triangle but you need pretty much a flat surface.

Just as a matter of interest with Stanfords charts, why do some scales have minutes divided by four units ?
Very difficult to use with GPS and decimal places.

John - Tell us more about this woman you can’t get off you’re mind.
 
Re: careful!

For someone whos way points , and I quote "Are mostly in the middle of the chart" you didn't realise a chart was 3 feet wide ??????? Who are you kidding . in the words of the chap on the television "I dont believe it!!!"
 
Re: careful!

I new they were wide, but never actually measured them until tonight.

Stop having a go at me you horrible person !

or.........

I will let loose more of my Panthers in the West Country !


GRRRRRRRRRRRR!!!!!!!!!!!!!!!!
 
JohnM is correct, and I was wrong. Dont take measurements from the bottom of your chart, only from the side.

John, Thank you pointing out the error.

(Reminder to me, don't post at 2am, the brain is sleeping)
 
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