Map coordinates confusion - UK Based

[docramage]

Researching different map co-ordinate systems suggests there are multiple ways of expressing Lat & Long in figures alone. There is no advice saying which format is default for Sat Navs (mine is Garmin - which offers 7 options).

Furthermore, if I enter a pair of numbers (in any of the formats) into Google Maps, or BaseCamp, and several others, the locations displayed are nowhere near where I expect - sometimes they are many miles off.

Can anyone advise me:

Which format is most likely to be default. So far I have tried h ddd* mm' ss.s", h ddd.ddddd*, h ddd* mm.mmm' (where * represents the character for Degrees. None works well

Should I expect a correct format to display within say 50 feet ?

Many thanks for anyone helping

Philip

[sussamb]

The correct format will show the correct location. How close it is to the actual position will vary as some formats allow truncation of digits. There is no 'default' that everyone adheres to.

Your Garmin has no 'default'. Not only can you select a number of lat/long formats but many others as well, eg British National Grid
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Where there's a will ... there's a way.

[DennisN]

In some systems, it's not necessarily WHAT you enter, more HOW you enter it.

e.g. Ndd mm sss, Wdd mm sss. Those are just guesses because I use the decimal format. And in MY useage, I don't enter N or W, entering a plus figure gives a North location, minus would be southern hemisphere. Similarly, positive W is East of 0, so for example a lot of locations in UK (west of Greenwich) require a minus entry. As an ignoramus in these matters, I don't know what your h stands for (please don't explain, I won't understand).

Furthermore, in the systems I use, there must be no space between the two figures, only a comma.

If you are getting results nowhere near where you expect, you are entering the data wrongly.
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Dennis

If it tastes good - it's fattening.

Two of them are obesiting!!

[Kritou]

Because our Earth is not a perfect sphere different datums have been, and still are, used to best define the local area

For the Global Positioning System the WGS 84 (World Geodetic System 1984) datum is used. Many GPS receivers will offer a choice so it is important to ensure your device has the correct one selected in the settings menu

[sussamb]

Choosing a certain lat/long format in your Garmin will select the correct datum of WGS-84
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Where there's a will ... there's a way.

[docramage]

Many thanks

Philip

[jad12]

Hi all

As for plot/input of a GPS lat/long not showing where you really are Kritou is dead right. Different regions have different datums based on different best fit spheroids. Many designed years and years ago.


GPS works on a WGS84 global best fit spheroid that clever guys at sums have worked out. In the UK we have OSGB on the Airy 1830 spheroid. Best fit for UK.


if you plot WGS84 lat/long on OSGB you will be out. It varies over the UK but around London it's about 100m different.


I would expect/hope that a GPS position input to your satnav would put you where (within accuracy specs) you really are. UK maps are likely originally based on Ordnance Survey maps so either some software in you satnav transforms or datum shifts WGS84 to OSGB or the OSGB map is shifted to match 84 and hence the likes of Google Earth and Maps. The sums are horrible. If this difference is not sorted somehow I would say your satnav map is a broken pencil......pointless.


As for input formats, yes everyone right. Most mapping stuff will take many ways of input.

However the h. Like DennisN, I'm lost. I've never seen it in 40 years of navigating. I may be missing something so would be grateful for an explanation.

Ladies/Gentlemen, this type of stuff has come up before. If there is interest I could come up with and post a short pdf to explain these things a bit more.

cheers
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