GPS Explained

Submitted: Thursday, Jan 17, 2008 at 12:19
ThreadID: 53576 Views:2181 Replies:3 FollowUps:10
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This should keep some occupied for a while.

New GPS Page

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Reply By: Member - Graham H (QLD) - Thursday, Jan 17, 2008 at 16:23

Thursday, Jan 17, 2008 at 16:23
Good page

One small thing WAAS is not enabled in Australia and is primarily an aircraft aid anyway.
I realise you have probably cut and pasted some stuff but I constantly see units advertised locally as WAAS enbled and it will give accuracy of ***meters with it.
Not here it wont and as far as I can tell never will.


Still a very worth whle tutorial and pity its not a sticky to help stop all the multiple similar posts
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Follow Up By: Member - Doug T (FNQ) - Thursday, Jan 17, 2008 at 16:53

Thursday, Jan 17, 2008 at 16:53
Graham
Sorry mate , nothing in that page is cut and pasted, all the text is by my hand, the 4 links at the top are just that...links, links to other websites not created by me, as for the screen shots they are from my own GPS Tracks recorded with my GPS in my car and created using Ulead PhotoImpact 11 on my PC, I made no reference to WAAS , and never have , Quest has WAAS/EGNOS capabillity and is mentioned on page 42 in the Quest owners manual.
It may be used by aviation here , I don't fly.

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Follow Up By: Member - Graham H (QLD) - Thursday, Jan 17, 2008 at 21:34

Thursday, Jan 17, 2008 at 21:34
Was in the "Gps Explained" page and looking at it again is obviously a link to the Garmin home page which I presume is American
Johnny Appleseed's site explains about WAAS and DGPS as well and states that WAAS does not work here.
It is an aviation aid and gives altitude and position to aircraft in the USA.
DGPS does however as I had a Lowrance GPS on my boat 4 years ago and bought a DGPS unit from the States. Gave accuracy to about 10 feet. Works from various radio beacons around the coast. My nearest one is at Woorim on Bribie Island.
Lowrance in Aussie quoted me in 4 figures for the unit if they had one. Mine cost $200 US.
Good on you for the page anyway and I hope more refer to it before posting repetitive questions about " what gps do I buy"
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Follow Up By: Member - Doug T (FNQ) - Thursday, Jan 17, 2008 at 21:48

Thursday, Jan 17, 2008 at 21:48
Graham
Is there anything about GPS not American , might be a couple unknown to me , Yes and every thing I do about GPS will be basicly Garmin , the best, I'm on my 5th now, just keep updating every couple of years , The link below will explain WAAS right from the start, My Quest hovers around the 4m to 6m with external Ant'


WAAS

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Reply By: The Explorer - Thursday, Jan 17, 2008 at 16:35

Thursday, Jan 17, 2008 at 16:35
Hello Doug - Good info from various Garmin websites. There are some shortcomings though - I looked in the Glossary of terms and couldn’t find the meaning of "#**#+!s", its doesn’t appear in any dictionary I have either, any clues?:)

Also I am not sure which "#**#+!s" your were referring to but the comment "I have had some #**#+!s try and tell me
a GPS unit does not recognise or take into account corners and hills" is partly wrong and partly right.

Corners are of course recognise/recorded by GPS units otherwise we would all have a collection of straight lines. Corners are also taken into account by a GPS for measuring distance, though settings made on GPS may effect the number of points recorded on a corner and therefore exact distance may not be recorded (i.e. lines drawn between points may "cut" the corner). I am not sure about Garmin’s but Magellan’s have an Auto detailed setting which whacks in lots of points at any significant changes of direction (eg a corner) and less points on straighter sections to improve accuracy of distance measurements and the actual track plot if required.

Altitude is also, as we all know, recorded by GPS units and can be used to show vertical profiles (this was not disputed in previous forum thread) but as has been discussed before slope is not used by your average GPS to measure distance traveled along the ground - this is illustrated by your vertical profile images - the scale along the bottom of your graphs does not change even when steep slopes are encountered. Excluding other variables the distance traveled will therefore be greater than your Graph/GPS indicates whether you like it or not. For example if you were to plot the vertical profile of someone climbing a 1km vertical cliff face the distance travelled would be recorded as 0km on your graphs.

The distance along the bottom of your graphs is correct for the track plot on a flat surface (and takes into account corners) but does not take into account the additional distance traveled when going up and down hills - which may or may not be bugger all in any case.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Reply By: The Explorer - Tuesday, Jan 22, 2008 at 15:24

Tuesday, Jan 22, 2008 at 15:24
Hello Doug, In another possibly vain attempt to convince you that your average GPS (or average GPS mapping program such as Mapsource/OziExplorer) doesn’t take into account the altitude of recorded track points to calculate distance, I have conducted a little experiment which I encourage you to replicate so as to advance your knowledge on a subject you obviously have a great interest.

I opened the original track file you have made available on you “GPS Explained” webpage. Each track point has an altitude and the total distance of the track (calculated by Mapsource) was 253km. I then changed all the altitude readings for each of the 1742 track points to 0 and saved the file under a new name. I then opened this file in Mapsource. Guess what distance the track file then showed in Mapsource? If you guessed 253km you are right.

Can you explain this? My conclusion is that altitude is not used in distance calculations by Mapsource. I did same test in OziExplorer, same result (though OziExplorer shows a distance of 252.09km for both original and edited track files). Initial discussion on this subject was actually about whether GPS units (not gps mapping software as demonstrated here) used altitude when calculating distance but it is apparent that they don’t either.

May pay to change certain comments on your “GPS Explained” page to reflect the facts (both in respect to main subject and the level of experience and knowledge of your fellow GPS users), but that of course is your choice.

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: Member - Doug T (FNQ) - Tuesday, Jan 22, 2008 at 16:09

Tuesday, Jan 22, 2008 at 16:09
Greg
I really thought you had more intelligence than that , A GPS can do a distance A to B straight line ie Brisbane to Melbourne 1373K, reset the the unit for routeing A to B 1679K , therefore the unit must take into account hills, hollows, and corners,
A GPS gets your location by Time, Lat' Long' And Alt' and your trying to tell me a GPS don't use Altitude as well in calculating your location so as to route to a distant point , Now we all know the distances between towns has been established years ago by surveyance, ie over the great divide Grenfell to Penrith for one, say the distance by road is 300K , and thats the distance surveyed over hills and gullies, it's the same distance on paper maps, I coulld park at the Grenfell PO and program the GPS to work out a route to Penrith PO , The GPS will show me 300K,Now if the GPS don't take into account hills etc then something would be wrong with the calculations,
Don't bother me again with your heard tell stories and Mythes and BS written in a book by some prick who does his studies in an office ,I do my tests out in the real world with my 2 correctly set Garmin GPS units which give the same answers,

Doug
-------------------------------------------------------------
GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more.
Cruise Missiles use GPS guidance If they didn't take into account of terrain ......MISSED

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Follow Up By: The Explorer - Tuesday, Jan 22, 2008 at 16:28

Tuesday, Jan 22, 2008 at 16:28
I knew attempting to convince you that you are wrong (or getting you to admit it - not sure if its both or just one of these) would be in vain. I was just putting the facts forward so others dont get mislead by some of your incorrect comments/interpretations.

I have no doubt 99% of others reading the various threads relating to this subject will understand/already know even if you don’t. Its a fact Doug - your average GPS does not utilise altitude to calculate distance. The software/firmware in GPS units simply does not use altitude data for this purpose. Honestly it doesnt. It has nothing to do with my level of intelligence - its just a fact, like 1+1 = 2

Please explain why your track file with no altitude data is shown as the same length as one with altitude data - go on I dare you! Maybe you should contact the Flat Earth Society for some new ideas on how to reply to that question without actually answering it.

Cheers
Greg

PS - you are the only one mentioning "corners" - no one has ever said corners are not taken into account when measuring distance, they are of course used. Your comments become (more) confusing when you slot this in of your own accord.
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: Member - Andrew (QLD) - Tuesday, Jan 22, 2008 at 17:37

Tuesday, Jan 22, 2008 at 17:37
Greg is 100% correct in his statement in regards to altitude. The facts i gained from your tracklog are as follows:

Distance covered (altitude neglected): 253.376km
Distance covered (using altitude point correction, Pythagoras' theorem): 254.281km

As can be seen, there is little difference (~0.3%) using altitude corrected values from your tracklog, and especially given the inaccuracies potentially introduced throughout various areas of route calculation and GPS usage, it is probably easier not to worry about the altitude when calculating these distances (many may cancel others out). However Greg is correct in what he states with altitude.

You have no idea sometimes when it comes to analysing data and the "real world". If you believe the blokes who write some great books on the subject have no real world experience, and that you are the guru of GPS systems, then you are sadly mistaken.

Comparing Cruise missiles' guidance systems to consumer level GPS units is naïve of the complexity available by such a fascinating system. Last time i looked under the hood of my backyard cruise missile, it didn't have a Garmin Nuvi stickytaped to its circuitry.

Your arrogance to certain subjects is astounding especially when others more knowledgable (i would count Greg in that boat) confront your statements.

Take it on the chin, and accept that you can be wrong sometimes. I certainly accept that fact and encourage you to do the same.

Andrew
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Follow Up By: Member - Andrew (QLD) - Tuesday, Jan 22, 2008 at 17:59

Tuesday, Jan 22, 2008 at 17:59
BTW the difference between Greg's total distance and mine is due to the rounding of the original Mapsource data (nearest metre) whereas the OziExplorer software has calculated its' trackpoint distance using 1 decimal place.

Andrew
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Follow Up By: Richard Kovac - Tuesday, Jan 22, 2008 at 22:42

Tuesday, Jan 22, 2008 at 22:42
Greg

Come on, you must admit that a Garmin Nuvi 600-something will calculate the distance from Grenfell PO to Penrith PO to be the same as a paper map, mustn't you?

I would thing Doug is getting confused about the difference between a passive GPS and a in car navigator, of cause the distance will be the same as they are reading the same data.

Just my thoughts

Richard
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Follow Up By: The Explorer - Wednesday, Jan 23, 2008 at 09:41

Wednesday, Jan 23, 2008 at 09:41
Hell gentlemen - firstly, thank you Andrew for some timely back up.

Secondly Richard, Re you request for me to "admit that a Garmin Nuvi 600-something will calculate the distance from Grenfell PO to Penrith PO to be the same as a paper map". Yes, in theory, if both used the exact same roadline information and start and finish points.

And yes you are correct - Doug appears to be confused - discussion (which originated in thread 53040) was about how distance is measured by a GPS when driving along i.e. the distance travelled as shown by the GPS units trip metre and by the recorded/stored track log as calculated from LIVE GPS data NOT how some GPS units calculate distances of road routes which is done using a DATABASE OF STORED information within the GPS units memory - completely diffferent story altogether.

I have just about given up trying to explain the subject at hand to Doug, my head hurts and the brick wall Ive been banging it against is about to fall over:)

Cheers
Greg
I sent one final shout after him to stick to the track, to which he replied “All right,” That was the last ever seen of Gibson - E Giles 23 April 1874

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Follow Up By: Richard Kovac - Wednesday, Jan 23, 2008 at 20:50

Wednesday, Jan 23, 2008 at 20:50
Hi Greg

"I have just about given up trying to explain the subject at hand to Doug"

Don't do that, I enjoy it and it's a neat way to learn new things.. LOL

Always good lean something new at least once a day..

Cheers

Richard
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