Position format?

[Student Pilot]

What format do most use for Lat/Longs on GPS? A lot seem to use Lat, Long, digital minutes these days. There are quite a few different one's quoted with some being similar but a lot different on the ground. A few years ago I was out North Western NSW and the police/National Parks/Forestry and RFS all used different position format, surely we can agree to use the same one? One of the only drawbacks to GPS use is finger trouble, if we can agree to use one format then it would be one less thing to worry about.

 

 

[Guest davidh10]

...There are quite a few different one's quoted with some being similar but a lot different on the ground..

If you convert between formats, they will mark the same place, but you cannot enter Degrees, Minutes, Seconds in an input field that accepts only Degrees and Decimal Degrees and expect the same location without conversion. You can usually set your GPS input format and change it before entering a location, if you cannot be bothered converting between formats.

 

Separately, be aware that there are a number of different Geodetic Datums in use, worldwide. WGS84 (World Geodetic Datum) is probably the most common and is used by Airservices Australia for their charts, however be aware that there are others such as NAD83 (used in the USA) ED50 (in Europe), GDA94 (Australia), AGD96 (Australia)... so it depends on the map you are using. It should say which Datum it is using.

 

Some will give similar locations in some parts of the world, but others won't. I'll let you Google it to learn more, but you always need to ensure you know the Datum used with reference to any Lat / Long that you want to use.

 

In Australia, WGS84 and Degrees, Decimal Minutes seem to be the most useful currently.

 

* Have a look at this reference and it will explain a few things about WGS84, GDA94, the differences and reasons..

 

As explained at this page, Australian maps used in bush walking etc., are likely to be based on AGD66 or GDA94.

 

 

[Student Pilot]

Yip, got all that and have been using them since GPS came out. Now can you get Government bodies to all work on the same format?

 

 

[flying dog]

Well, that is a good question.

 

Alas the answer isn't as simple as it seems.

 

Ok, my understanding of why we have different datums.

 

I may be wrong, but I am sure the idea is correct.

 

OK, maps originally didn't have Lat/Long. This was a later addition.

 

And there are MAPS and there are CHARTS - I am still coming to understanding the specific differences but anyway.

 

I'll call them maps when they could also be CHARTS, but for the sake of simplicity....... MAPS shall suffice.

 

Say it is WWII. Your side have maps and the last thing you want is the enemy to know where you are.

 

This can be reduced by using a custom DATUM. Yes, I know: Millitary maps use a different system - GRIDs etc.

 

Indulge me.

 

So unless they know the datum and where it is, your maps are not as useful to them as they could be.

 

Ok?

 

Now, Let's say someone wants to map Oz. They use/pick/create a datum and use it.

 

Someone else in Yank land wants to make a map of America. It would be "silly" us using their datum, and/or them using ours.

 

So they have their own datum.

 

Likewise for Europe.

 

Then there are the POLAR areas. Normal Lat/Long don't work too well there.

 

So, let's say everyone used ONE datum. It is 1900. Or when ever.

 

The accuracy of the datum is technology dependant. As "back then" it isn't as good as it is today, don't get ahead of where I am going.

 

So, everyone uses that datum and make their maps with that datum.

 

There will be inherant errors with any map/datum system. Nothing is PERFECT. I don't undertand the shortcomings of different datums and/or the errors between them.

 

Then technology improves.

 

Errors are detected. A new datum is defined. Everyone starts using the new datum on the newer maps.

 

Therefore all maps MUST have their datum specified on them. As well as that the dates for which the map is valid.

 

There are other things which MUST/SHOULD be on a map but that's outside the scope of this post - and my knowledge at this point in time.

 

Another example:

 

Say you want to map out your property - you have a BIG farm out the back of NSW.

 

It isn't for anything legal. It is just for your benefit.

 

You can set your house/home as the datum and refere all points to it.

 

Getting/using the "official" datum is too much work and/or not applicable to your needs on the map you are making.

 

Sorry I can't go into all the specifics, but it has been too long and some things I never learnt.

 

But I hope this kind of explains why we have so many datums.

 

:)

 

 

[flying dog]

Oh, P.S.

 

I also think it is to do with the different STYLES of maps.

 

There is the "Flat" one, where the world is simply a FLAT, RECTANGULAR shape.

 

Then there is the one where the world is a zig-zag shaped thing which is more accurate to the actual size/shape.

 

I forget the names, but Lambert conformal conic comes to mind.

 

The polar areas are "distorted" with the rectangular maps and look bigger than they are due to how the map is drawn.

 

Sorry, getting off topic.

 

If you are interested:

 

http://en.wikipedia.org/wiki/Map_projection

 

 

[flying dog]

Sorry, but this also may be a good read.

 

http://www.geowebguru.com/articles/199-map-projections-and-coordinate-systems-part-2

 

 

[Guest davidh10]

Another reason for maintining different datums and not having just one, was disclosed in one of links I posted above...

 

Continental drift causes continents to move with respect to one another. Australia moves about a centimetre a year in a northerly direction, however Australia moves as a whole. Thus the Australian Government uses an Australian Datum and all places stay put with respect to that, while they drift with respect to the World Datum WGS84, which is reviewed every few years in terms of its model.

 

The creator, who or whatever you believe that to be, did not create Earth as a perfectly spherical planet. It has bulges due to land mass as well as near the equator due to centrifical force. The surface is somewhat flexible and thus there are actual tides in the earth's crust due to the gravitational force of the Sun and Moon. Mathematical models just approximate a representation of the surface of the Earth, and as technology has improved, so has the model. This means that every model has some inherent inaccuracies.

 

Obviously every continent will want its own Datum for the same local purposes.

 

Land surveys are with respect to Survey marks, which are concreted into the ground and don't move, so here's another form of Datum that is used for legal purposes on land titles, mining leases etc..

 

Maps are usually produced by governments and land maps are often only updated every 20 years, so there will be the technology of the day as well as depending on purpose, applied to the Datums used.

 

So now there's a geospacial reason as well as a historical or time dimension to the reason.

 

There's probably no danger of an outbreak of world peace either.

 

 

[Student Pilot]

Yes well Continental drift and a Mercator projection verses Lambert comformal is all very impressive and I'm sure EXTREMELY interesting to some but I think empire building by bureacrates and local Government is more to blame. You still haven't given a good reason why we all can't work on the same system? Surely if it's good enough and accurate for one it's good enough for others? Yes you can convert it but that is my point, the more human involvement, the more chance of finger trouble and error.

 

Put it another way how many here use Lat/Long/Minutes/Sec?

 

or Lat/Long/Digital minutes?

 

or Lat/Long/Digital?

 

0r Other?

 

Doesn't anybody think standardisation with this sort of stuff is a good idea?

 

 

[Guest magcheck]

Actually there is some standardization

 

In Australia the current AIP states that positions for aviation is DD MM.dd or DD MM.ddd

 

I the maritime environment it is DD MM.dd although there are still references to DD MM' SS" because it will take along time to change everything. Remembering that SS" have been used for hundreds of years

 

The key is to check whether or not a decimal point has been used. If it has it is in decimal if not it is probably in seconds

 

 

[flying dog]

David, "Obviously every continent will want its own Datum for the same local purposes." Isn't that what I said in my example?

 

 

[Yenn]

If you want to be able to go from map / chart to the GPS do what magcheck suggests, otherwise you will be dividing degrees ino tens or hundreds with a scale.

 

Horses for courses and i use UTM/UPS for bushwalking as that is what most of the maps used are based on. Also it is easy to work out distances as the answer is in Km and decimal Km.

 

 

[Guest magcheck]

An excellent tool is at

 

http://rumkin.com/tools/gps/degrees.php

 

 

[Guest davidh10]

David, "Obviously every continent will want its own Datum for the same local purposes." Isn't that what I said in my example?

Yes, it was, and I thought your posts were very good on the subject.

 

I did however believe that you were making the case for individual continental datums based on convenience or nationalistic interest. What I believe I added was the case for maintaining national as well as international datums for specific purposes based on the purpose, technical or legal requirements.

 

...You still haven't given a good reason why we all can't work on the same system? Surely if it's good enough and accurate for one it's good enough for others? ...

Doesn't anybody think standardisation with this sort of stuff is a good idea?

Generically, standardisation is a wonderful thing and used correctly helps users in the applicable field. I agree that some standards are created simply due to bureaucratic reasons, however there are good reasons for others. It would probably be good if everyone in the world drove on the same side of the road, and I recall one country actually successfully changed some years back, but lets confine ourselves to mapping and datum standards.

 

I actually think that standardisation is occurring...

 

WGS94 is the current "World Standard" for use by GPS units. You may be uninterested in continental drift, as I am sure are most people, but if your property title was based on WGS94 co-ordinates, then after 10 years, your property boundary, as represented by your fence would be 10cm out of place by WGS84. There have been court cases over property boundary encroachment of as little as an inch, so clearly WGS84 as a global standard is unsuited to this purpose.

 

On the other hand, it would be ridiculous if aircraft and shipping had to change the datum used in their GPS as they crossed national boundaries, so a world system is the only one that makes sense. Aircraft and shipping don't need centimetre accuracy and aviation maps are updated frequently (Garmin aviation updates expire and are re-issued every 28 days. Airservices Australia expires and re-issues charts every six months.).

 

I hope this has illustrated the need for national datums in addition to a world datum.

 

No one datum serves all purposes with the required accuracy over time.

 

While national datums could probably be rationalised to one per non-distorting tectonic plate segment, that is where national govenments like to control their own. At least Europe has adopted one for all its constituent countries.

 

The other reason is historical, in that maps are updated less frequently than technology, so we have a lot of older standards in use where the maps have yet to be updated and revised to the latest standard. I don't think anyone wants to pay more taxes to speed this up.

 

In fact GPS units are subject to errors that can make the location shown innacurate by varying amounts. If you engage in geocaching, I'm sure that you don't just take an instantaneous reading of position, but watch the range over which it changes and publish an averaged location. TSO'd GPS use RAIM updates to mitigate such inaccuracies, which can be due to satellites being taken out of operation for maintenance, ionospheric propagation issues or local terrain issues. GNSS navigation (ie. navigation by GPS) can make use of RAIM prediction information to mitigate such inaccuracies. The references below may assist.

 

Wikipedia RAIM page.

 

Airservices Australia RAIM info.

 

Quite separately to GPS, I was amazed, when I started flying, at the mix of different measurement systems in use, just for distance and speed. Let me say that it is confusing when international glider pilots bring their gliders to Australia and call altitude in metres, when the standard here is feet. Mind you, why is our standard feet, when Australia went metric back in the 80s?