Technical nerdy question!

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Hi

I was wondering if anyone knows what the GPS actually measures when recording a route distance. When you measure a route on the map (bit of string, map measurer etc) it is obviously the flat line distance from A-B (with as many bends and twists as possible of the expected route). It's not practical to allow for the extra distance caused by the height gain, except in allowing extra time.

When out actually walking, and the GPS reset to zero miles/km, does it measure the extra bit due to changes in height or does it simply measure a flat distance like the map?

Is that clear?? If not, think of a triangle. A-B is along the bottom. If you walk up to the top of a hill you end up at B but several hundred feet higher up: C. In effect you have walked along the hypotenuse of the triangle, which will always be longer than the short sides. As experienced hill walkers know, map-miles are shorter than hill-miles.

So what does a GPS measure, given that it's aware of its position in 3 dimensions - does it allow for altitude and measure actual distance covered?

Martlakes

Not a nerdy question at all; that's what a forum is for, to answer questions.

I'm a newbie and am glad you asked, cuz I don't know, either, but will stay tuned for the answer

I am sure the forester will be along with a very long winded explanation soon

Which we may not understand!!

GPS measures distance over ground, it doesn't take into account the height.

Moss T

Thanks for those although some were definitely nerdy in the extreme!!

"Ionospheric nowcasting via assimilation of GPS measurements of ionospheric electron content in a global physics-based time-dependent model ..."

The Wikipedia entry was interesting, and I liked this bit about GPS time:

GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match Coordinated Universal Time (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (19 seconds) with International Atomic Time (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330.

Although I'm a bit miffed my GPS will need to be replaced after 2330 or the time will be wrong!

Not sure I've found an answer yet. Some more explanation from Moss Trooper would help the understanding rather than a bald statement (either way). Basically "why"? When I walk, I walk over the ground so why doesn't it measure the distance between the 'bread crumbs' of the trail it knows it's followed, or does it not know the height of these crumbs?

When I walk, I walk over the ground so why doesn't it measure the distance between the 'bread crumbs' of the trail it knows it's followed, or does it not know the height of these crumbs?

It probably doesn't care....

GPS does triangulation of satellite signals, so it measures distance 'as the crow flies'. As a previous poster noted, it doesn't take altitude into account, so one mile away and a half mile up would appear as 'one mile'.

GPS time and date

While most clocks are synchronized to Coordinated Universal Time (UTC), the Atomic clocks on the satellites are set to GPS time. The difference is that GPS time is not corrected to match the rotation of the Earth, so it does not contain leap seconds or other corrections which are periodically added to UTC. GPS time was set to match (UTC) in 1980, but has since diverged. The lack of corrections means that GPS time remains at a constant offset (19 seconds) with (TAI). Periodic corrections are performed on the on-board clocks to correct relativistic effects and keep them synchronized with ground clocks.

The GPS navigation message includes the difference between GPS time and UTC, which as of 2006 is 14 seconds. Receivers subtract this offset from GPS time to calculate UTC and specific timezone values. New GPS units may not show the correct UTC time until after receiving the UTC offset message. The GPS-UTC offset field can accommodate 255 leap seconds (eight bits) which, at the current rate of change of the Earth's rotation, is sufficient to last until the year 2330.

Not only is it difficult to understand some descriptions for the OP but I'm finding it strange that UTC stands for Coordinated Universal Time and TAI is short for International Atomic Time.

Was the person who made these, dyslexic?

Why do they make dyslexia so hard to spell?

Not only is it difficult to understand some descriptions for the OP but I'm finding it strange that UTC stands for Coordinated Universal Time and TAI is short for International Atomic Time.

Was the person who made these, dyslexic?

No ... French.

TAI = Temps Atomique International

However UTC is more of a rigmarole. The US National Institute of Standards and Technology tells us that

http://tf.nist.gov/general/misc.htm

In 1970 the Coordinated Universal Time system was devised by an international advisory group of technical experts within the International Telecommunication Union (ITU). The ITU felt it was best to designate a single abbreviation for use in all languages in order to minimize confusion. Since unanimous agreement could not be achieved on using either the English word order, CUT, or the French word order, TUC, the acronym UTC was chosen as a compromise.

So UTC is a kind of Franglais

GPS does triangulation of satellite signals, so it measures distance 'as the crow flies'. As a previous poster noted, it doesn't take altitude into account, so one mile away and a half mile up would appear as 'one mile'.

Isn't it going to depend on what GPSr you're using?

Any GPSr with 4 or more locks is capable of giving a 3d fix (calculating height) - so surely some models will also calculate 3 dimensional distance travelled???

This has got nothing to do with the satellite network, and everything to do with the software any particular unit is running.....

Note - the OP asked about distance travelled, not distance to target.

Edited by keehotee

not, think of a triangle. A-B is along the bottom. If you walk up to the top of a hill you end up at B but several hundred feet higher up: C. In effect you have walked along the hypotenuse of the triangle, which will always be longer than the short sides.

Remember that only right-angle triangles have a hypotenuse ....

You had me confused for a while ... especially as I will always avoid a hill that steep

It will always be a right angled triangle, as height is always perpendicular to distance over ground.

Who are you calling perpendicular?!!!

not, think of a triangle. A-B is along the bottom. If you walk up to the top of a hill you end up at B but several hundred feet higher up: C. In effect you have walked along the hypotenuse of the triangle, which will always be longer than the short sides.

Remember that only right-angle triangles have a hypotenuse ....

You had me confused for a while ... especially as I will always avoid a hill that steep

Ok, forget triangles. Think about trying to push an elephant under a carpet!! It's going to pull the carpet in from the edge of the room, therefore showing that it's longer going over the elephant than straight across the flat floor. Any better?!

(As meimur says, a point vertically above the point on the map will form a right angle triangle, but I like the elephant/carpet image!).

Still hoping that someone can give a definitive answer about whether a Garmin Legend measures actual distance traveled or just the flat map equivalent. Since it knows where it is in 3 dimensions, does it take the elephant into account or not? Happy either way, just would like to hear a reasoned view rather than a bald statement of yes it does or no it doesn't. Felt sure some genius out there would have the answer!

Still hoping that someone can give a definitive answer about whether a Garmin Legend measures actual distance traveled or just the flat map equivalent

Get out there and try it.

mark a route out on a map (2d).

Make sure you include lots of hills.

Walk, drive or swim (difficult on a hill) the route, with your GPS.

Compare the results!

Get out there and try it.

Well, I sort of have already, which is why I asked. When I measure a route from a map it is always shorter than the actual recorded distance on the GPS. What I don't know is whether this is just cos it measures the flat line more accurately than my map measurer or whether it does in fact take account of the height affect.

The height affect isn't huge. It should result in about (I think) 45% more distance if the height is the same as the horizontal. Ie if you draw a square 1m by 1m, the diagonal is about 1.45m.

This may sound a lot: for eg on a 1000m high hill you would get an extra 450m horizontal distance if you set off from sea level. So if you set off 1km away from your 1000m hill you should get at least an extra 450m reading. Of course you'd also get another 450m on the way down, so it would almost double your two km walk and read at least 2.9km.

But normally you'd walk several km/miles to get to the top as most hills aren't 45° all the way up and often you don't start at sea level. So over a day's walk the height component could easily be lost in the general error in measuring the map compared with the GPS which records every twist and turn. Consequently, it's quite hard to tell whether the GPS takes the height into account or not. I guess I could try it on a small hillock that's quite steep and see if I can tell over a short distance, and try and walk in a straight line!

Which we may not understand!!

Which we may will not understand!!

Try climbing and descending a ladder 50 times and see if it records the distance travelled.....or at least at the end of that you'll be too knackered to care

As I understand it (probably from reading the Wikipedia pages), vertical position has the worst accuracy of the three dimensions. It may be considered less confusing to only include distance on the flat in travel measurements. If you walk 10 metres along a flat road but the GPS accuracy says you've drifted 20 metres below sea level, you'd prefer it to show you 10 metres. And on a larger scale, if you drive 1000 miles, most hills you climb won't add much difference.

Another nerd here (must be something in the Cumbrian water) - walking along GPSr in hand I've often wondered how it deals with the backwards and forwards motion of my arm .

Another nerd here (must be something in the Cumbrian water) - walking along GPSr in hand I've often wondered how it deals with the backwards and forwards motion of my arm .

Do you do a lot of that sort of movement??

Do you do a lot of that sort of movement??

Cue for another experiment for Martin - a GPSr in each hand with one arm held still and the other waving about . Now what would have happened if I'd been holding the GPSr when swimming breaststroke to a cache last week? I'll stop before the men in white coats come to take me away.

Another nerd here (must be something in the Cumbrian water) - walking along GPSr in hand I've often wondered how it deals with the backwards and forwards motion of my arm .

Well, it usually registers some terrific max speeds - well above walking pace. What it does for the distance is anyone's guess.

It could be something in the water as we seem to have rather a lot of it tonight.

As for experiments, I was hoping for some knowledgeable person to provide a simple, but great sounding answer, thus saving lots of uphill sweating!

walking along GPSr in hand I've often wondered how it deals with the backwards and forwards motion of my arm

Civillian GPS has a minimum error of 3 metres - so unless you've got REALLY long arms, it's not going to make any difference at all.........

Its as the crow flies. FLIES. So i would guess it doesnt count hill miles, unfortuneatly. =D

Civillian GPS has a minimum error of 3 metres

But that's only its accuracy: it doesn't mean it can't detect movement less than 3 metres, even though it's not an accurate measurement.

Civillian GPS has a minimum error of 3 metres - so unless you've got REALLY long arms, it's not going to make any difference at all.........

but Confusius say "Every 3 meters has it's boundary where one 3 meters meets the next". Now if he was walking along the edge of this boundary.........

GPS measures distance over ground, it doesn't take into account the height.

Moss T

I did a 105 k cycle ride at the weekend.

This distance was measured before hand by independent means.

There were 2200 meters of climbing on the route.

My GPS recorded 104.6 k

My cycle computer recorded 104.9 k

A difference of 300 metres.

My cycle computer records the distance by means of wheel revolutions.

Please explain how if my GPS does not take height into the calculation the GPS did not show 4.4 k (including the descents) less than my cycle computer and the distance the course was supposed to be.

On the other hand I would not expect my GPS to record any distance traveled if if I went up and down in a lift.

<snip>

Edited by sTeamTraen

I can not claim to be expert but what I have noticed is that if you download a track recorded by a GPSr and study it on your computer you will find that there are spikes along the way especially if there was poor reception and the GPSr was struggling to find a signal. These can be quite considerable if you lose the signal completely.

Now the GPSr records all these spurious points along the track as the points you have passed through and so your track will always be longer than the distance actually travelled - by how much depends on the quality of signal reception along the way.

The extreme example is to stand still for a while and you will find that your position keeps changing, so you can log a distance travelled without in fact moving. I have found this using a logging program on a PDA linked to a Bluetooth GPSr which claims to record to thousandths of a mile and sitting at my desk it soon starts recording movement. It may be that the better handheld GPSrs allow for this effect but if so I have never seen it stated. While writing this I have left my GPSmap 60CS on my desk which is under a skylight with up to 5 satellites in view but deliberately covering it for a time and have travelled 18ft in 5 minutes! I did not feel the earthquake.

I don't know the certain answer to the original question but have tried to demonstrate why a measured track can exceed the actual travel without the ups and downs coming into the equation. My belief though is that the GPSr only measures horizontal movement as it is comparing locations defined by latitude and longitude (or whatever you use) which do not define altitude.

I hope Forester will come along some time and tell us the truth - and that I can understand his explanation!

Please explain how if my GPS does not take height into the calculation the GPS did not show 4.4 k (including the descents) less than my cycle computer and the distance the course was supposed to be.

There's nothing to stop your GPS measuring distance travelled, including climbs - what it won't do on it's own is measure distance to destination (including climbs)

In theory, with 4 good satellite fixes, the GPSr can place you at a unique point in 3-dimensional space. That's how you get an altitude value even if you don't have a barometric altimeter.

If the software then decides to tell you how far you have come, how fast you're travelling, etc etc, on the basis that every point is to be treated as if it were at the same height, then that's essentially up to the marketing department , or, put another way, what the unit manufacturer thinks you want to see.

It's a tough call. For example, if you are going down a 15% incline on the motorway and you want to know how fast you're going to stay within the speed limit, then you certainly don't want the slope to be ignored. The camera will point along the slope and measure your velocity along the road, not how fast you are progressing along an imaginary flat plane. On the other hand, if you're trying to pick up a cache which is down a steep slope using a fishing rod which is 8 feet long, you probably want to know the horizontal distance to the cache; you can compensate for the drop by unreeling a bit more line.

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