# GPS conversions for the laypeople out there

Submitted by funkybunch on Thu, 2006-02-09 22:48

Does anyone know if it is possible to convert (and if so how) co-ordinates that read 31 xx xx 115 xx xx to that which a Garmin handheld uses ie 31 xx xxx 115 xx xxx. Is there a way of converting the seconds. Is it a matter of multiplying the seconds by 60 or is it simply a matter of adding a 0 to the end/before the seconds co-ordinates ie 31 xx xx0 or 31 xx 0xx.

I'm sure there is a name for both styles but I am not aware of either

Cheers

## Gully

Posts: 963

Date Joined: 04/10/05

## GPS

Yeah mate it shouldnt be that hard. All you do is go into the GPS settings first. Once there you can should find a unit setup icon. Select that and that should give you position formats which are the different ways of inputting coordinates. Just select the one which is 31 xx xx 115 xx xx. Now you can input your coordinates as normal and then once finished it you can go back and return it to 31 xx xxx etc and the points will convert through the GPS

Hope this helps

Gully

## Fly

Posts: 485

Date Joined: 04/02/06

## 3 ways

This is acommon one that trips people up.

There are 3 ways to express co ords lat / long.

(Of course entering waypoints taken on a different datum can also play havoc, but that is a different kettle of fish).

Basically the lat longs are all the same formats up until the seconds.

One format may have the seconds as 1/60th of a degree of arc.

In that format (to two deciomal places) we are dealing with 60ths..so no number should be above 6o. eg. 60/60ths is one Degree...so any larger doesn't make sense

Pretty simple.

Then - we have decimal seconds (1/100ths of a Degree of arc). but only to two decimal places....

For example... .25 seconds in a 2 decimal 1/100ths system is one quarter of a degree of arc (or 15 seconds in a 1/60th format, not 25!)

In this system we can have seconds numbers up to 99!

Confusing eh?

Then there 1/100th to 3 decimal places

These of course are to 1/1000th of a degree of arc and are just more accurate representation of 2nd system above, so as not to confuse people...because you KNOW your going to come across numbers above 6 in a 3 decimal place system...and that they are base 100 (not base 60).

Boy - I think I mighta confuzzlized me now! :rollseyes:

The good thing (say your giving/recieving a lat / long in a mayday message transmission over radio...and not sure what format the transmitter sent it in - or you recieved it in) - is that for finding people at sea - there is NOT a great deal of distance involved in the difference between the two systems... i.e less than 1 nautical mile and usually only a fraction of that..so you needn't be too concerned..

in an emergency situation.

However..

It makes aLOT of difference if plotting a manual course over long distances on a chart, and navigating thru tricky coral passages etc (like the houtman Abrolhos islands for example)...so if you're the skipper of the boat and taking people to sea long distances you need to know the differences and be very good at plotting courses allowing for magnetic variance & compass deviation, allowing for set and drift, and so on and so forth- if you actually want to arrive at your intended offshore destination without the aid of a chartplotter / gps.

Yes you may never need the knowlege, and maybe you even carry a hand held GPS reciever, with spare batteries in case lightening takes out your chart plotter...but - what if the Americans switch off their sattelites (during a war strike for example, as we might see in Iran in March if the news pundits are right?)...

Ever been to sea when the yanks swap all the info backwards and north becomes south on your GPS? Care to guess your chances of getting home from 50 mile out without GPS? (Or worse fnding an island a pin prick in the ocean 50 miles out without GPS?)...

This is why skippers SHOULD know how to navigate the old way - heck even celestial nav if you must(with a sextant from the stars!).

I think one of the maritime colleges are holding a special celestial nav unit commencing in comming days / weeks, if the paper was correct a week or so back. I actually considered going to brush up again on Nav - can't hurt you know!

Whatta they say - Knowlege is power?

Another "Trap" for GPS navigators, ONE clock error in a sattelite can completely throw off the triangulation calculations of GPS.

A sattelite MUST know not only where it is in space but WHEN.

A couple or 3 years back a clever Nasa tech re booted a GPS satelite computer after a shut down for metor showers and got the clock time wrong!

Planes flying down the east coast of Oz were up to 200 nm miles OUT on their proper route (mistake picked up by radar)!

Shyte happens people - usually at the very worst possible time if Murphy the optimist has anything to do with it!

Recognising WHY your gps is telling you porkie pies might save you from a very long trip to visit the penguins in antarctica on your way home from fishing,if you can't tell the difference between when your GPS is right or wrong.

But I've got off the track...sorry I'm just a old phart, likes to impart knowlege!

Cheers!

## funkybunch

Posts: 97

Date Joined: 25/07/05

## Cheers

Thanks Gully and also Flywest... I think!!!!!

I may be a bit slow on the uptake here....It is Friday night after all....

So are you saying that the seconds to 2 decimal places are just less accurate than to 3..... Ie where it goes to 3 it gets you just that bit closer???? ie we dont have to change formats we will just not be as accurate??

Or is there some kind of multiple action I need to take into account??

The forever confused slow learner (and I caught a fish once too...)

funkybunch

## Fly

Posts: 485

Date Joined: 04/02/06

## Ureaka!!.. ahh you no smella so good yaself! ;o)

Indeed...I think you have it in one!

Yea tis not that hard eh?

When you think about it - how accurate is a GPS anyway?

See - it's MORE important to ensure you have the right Datum because that can put you up to 200 meters out! (which in a 100 meter wide channel can ruin ya day tootsweet!).

If you have a paper chart - it will have a datum printed on it - some of the newer ones are ADG 2000 or something from memory (Australian Geodetic Datum 2000) and so on and so forth - so - when doing manula nav calcs from a paper chart - then entering those lat long waypoints into a GPS, it is crucial that the paper chart and GPOS are at the same datum..

Back to your Lat longs..

If you consider that the 1/100ths (3 decimal places) is 1 nautical mile for 100/100ths (or 1 Minute of arc at the equator).....

Then... the third decimal place can only represent 1 up to 9 thousandths of that distance!.

I forget the exact conversions for metrics..so bein an old phart I'll do it in imperial..

Now a statute Miles 1760 yards, (5280 feet?) and a nautical mile is 1 & 1/8th statute miles.. or (old phart clicks on calculator in computer...heck I remember at 15 years of age when the calculator was invented & I chuked out my slide rule!) 1.125 Statute miles..which is 1980 yards or 5940 feet (roughly)

So, that 3rd decimal point - even if it is 9 can only represent 9/1000ths of 5940 ft, or 53.46 feet.

So - without the benefit of the 3rd decimal place a rescuer should theoretically be able to get within about 55 feet roughly speaking..at a WORST case scenario.

Now - finding an underwater opening to a dhui cave...well it might be HANDY to get a little closer than that..

But how accurate is ya GPS?..many are only accurate to within say 6 meters rings a bell (better some days with more sattelites in view at one time..sometimes as god as 1 and a bit meters)...but at 6 meters (20 ft) which is pretty average - the GPS is probably ONLY capable of about half the distance accuracy over 3 digit reference point anyway?

Least I think thats pretty rough calcs but good enough to make the point.

Think it backwards the other way too...

Say you got a 2 digit decimal reference (that miracles of miracles just happend to be all figures less than 6 so you couldn't immediately tell whether it was base 60 or base 100)...

How far out would you be?

Go back to my first post example where 15 seconds in a base 60 is 1/4 nautical mile... (0.25) times (x) 5940 feet = or 1485 feet

But 15 seconds in a base 100 format to 2 decimal places is 0.15 x 5940 feet = (0.15 x 5940 = 823.5 ft

The difference (ampount you would be OUT by if you confised one format for the other?

1485 feet - 823.5 feet = 661 feet (a couple hundred meters!).

Now if your trying to find someone elses secret fishing spot that they have goven you the lat longs of - then a couple hundred meters...is about as big an error as having th wrong datum.

Combine the two errors (wrong format and wrong datum) and you could be 400 meters out, and if the same error is in the Lat as well as the long - then - the distance of the diagonal of a 400 meter x 400 meter square! (say 450 meters for example).

However- if you have say a good sounder - well - you could always cirle around and around in ever decreasing / increasing circles, until you find the lump / wreck / fish or whatever!

The thing is to KNOW the possible errors roughly in your head and not to just give up, when you arrive at the spot - but circle around and look on the sounder until you find whatever it is your looking for.

You know - before the Americans shut off the dithering of their signals from sattelites we were lucky to have 60 meter accuracy anyway (without a land based radio homing beacon to help narrow the margins)...so the accuracy we ave now is unheard of.

Something to onsider tho is that for say navigating narrow passages at night...those 50 meters or so - might be a LOT more error than you can afford...

It is very handy to have all this clear in the back of your noggin while at sea so that - you have some "feeling" for whether what your doing / where your going is pretty much spot on or whether you could be a nats whisker out!

A good example I just heardof (from a couple Perth based delivery skippers - bringing a clients boat back from Singapore...

While stopped over up in the indo islands, someone broke into the boat and stole their hand held backup GPS, mobile phones sunglasses etc etc.

Then when 200 Nm out into 'unfriendly waters' (read pirates) their 240 volt generator broke down..so no power to the electronics on the bridge..

What to do...continue on the 120 NM to Australian waters (Ashmore reef and hail someone local on the radio to follow back into Broome and effect repairs or turn back and navigate manually thru all the narrow straighst back to the indonesian archipellago thru all the pirates in the dark with manual navigation (compass and chart, paralell rules dividers etc etc.

They opted to head for Oz...

They set the auto pilot to run them 20 nautical miles wide of Ashmore reef, by 6 am in the morning. Sadly they arrived ON TOP of Ashmore reef at 4 am!

OOps! he he he..

Someone goofed (badly) with their manual nav calcs..

Either they were well out on their speed estimation (arriving 2 hours early over 120 nautical miles)...and / or, they were "out by 20 Nautical miles" in their allowance for set and drift (the effect of wind and current on the vessel, operating at a tangent to the direction of travel.

Anyway - I think I've strayed from the original question - but I personally find this stuff interesting / fun! My hope is that others learn something from my posts - if not and it's a problem just say so, eh?!

Cheers!