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#162200 - 02/15/12 09:06 AM Dumb question on GPS
Gershon Offline
member

Registered: 07/08/11
Posts: 1106
Loc: Colorado
Let's say we hike a mile straight up or down a 30 degree slope. I know this is almost dangerously steep, but the numbers work easy.

If I accurately pace my distance. But on the map, I've only moved .87 miles. Which will the GPS show? My impression is it will show the mile.

If I marked the start as a waypoint and I hiked in a straight line, will the distance to the waypoint show as .87 miles or 1 mile?

This only matters over long distances. The Appalacian Trail is 2,187 miles long. Is that according to the map? Or is that according to miles walked. If the average slope is 7 degrees, there is a 10% difference.

Maybe I woke up too early this morning.
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#162213 - 02/15/12 11:04 AM Re: Dumb question on GPS [Re: Gershon]
wandering_daisy Offline
member

Registered: 01/11/06
Posts: 2661
Loc: California
I would think horizontal distance. Based on several satallites the GPS should be capable of measuring the distance on the slope, but I am not sure all GPS units are that sophisticated. If it gave you slope distance than that would not match the map that is shown on your GPS screen.

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#162218 - 02/15/12 12:10 PM Re: Dumb question on GPS [Re: Gershon]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
I would be really surprised if the GPS did not give you the full mile. GPS locates you 3-dimensionally. It would be shear laziness on the part of the programmers not to use the info. I would also think reported trail miles are actual miles and typically not numbers measured off a map. What about the topo map tools some people use? It too should also be able to use elevation change to calculate actual miles covered.

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#162220 - 02/15/12 12:22 PM Re: Dumb question on GPS [Re: wandering_daisy]
Gershon Offline
member

Registered: 07/08/11
Posts: 1106
Loc: Colorado
Originally Posted By wandering_daisy
I would think horizontal distance. Based on several satallites the GPS should be capable of measuring the distance on the slope, but I am not sure all GPS units are that sophisticated. If it gave you slope distance than that would not match the map that is shown on your GPS screen.


But if it does not give you the slope distance, it will not match the distance you hike. With a GPS in a car, the distance traveled matches the odometer with a correction for the percent the odometer is off.

I think I'm going to have to go climb a mountain and see. Yet, as you say, if it does it that way, it will not match the map.


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#162222 - 02/15/12 12:56 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
A GPS, does not add elevation profiles to your distance, it measures point to point.

You could climb a ladder that's a mile high and still not have moved a bit from your starting lat/long, and your GPS will tell you that have traveled zero feet, as it should.


Edited by billstephenson (02/15/12 12:58 PM)
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#162224 - 02/15/12 01:06 PM Re: Dumb question on GPS [Re: Gershon]
lori Offline
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Registered: 01/22/08
Posts: 2801
The GPS measures what you have traveled. I can go down a series of switchbacks and drop 1,000 feet and it will tell me how far I've gone on the trail - it's more than it would tell me if I had not walked all those switchbacks and just slid down a pole. It leaves breadcrumbs and measures speed as well as time.

I suppose I could do an experiment - throw the GPS off a hill and go get it at the bottom, and see what it tells me, then walk back up the slope and compare the two. But it's not going to short you because the distance is not linear.
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#162226 - 02/15/12 01:15 PM Re: Dumb question on GPS [Re: Gershon]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
To calculate actual distance (d) off of a topo map. Determine the horizontal distance taveresed (x), count up the number of contour lines you cross (n), and the vertical distance between each contour line (h). Plug into this equation:

d = x/sin(arctan(n*h/x))

if you don't like arctan and sin's you can use the alternate formulation:

d = square root(x^2 + (n*h)^2)

With topo map software you could easily develop a cubic spline algorithm to get a more accurate estimate of distance traversed. I would be surprised if commercially available topo software didn't include that feature.


Edited by BZH (02/15/12 01:31 PM)

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#162227 - 02/15/12 01:18 PM Re: Dumb question on GPS [Re: lori]
wandering_daisy Offline
member

Registered: 01/11/06
Posts: 2661
Loc: California
I doubt you have driven your car straight down a 30 degree slope! So that is not a good test.

Switchback vs straight down is not the question. Switchbacks will show up as horizontal variation with associated distance.

The question is the gometry of slope distance (triangle hypotenes) vs horizontal distance (long limb of triangle adjacent to the right angle).

If the "bread crumbs" are recorded as lat/long or equivelent, you get horizontal distance- the shorter distance.

By the way I am not a GPS user, just have done surveying in the past. A GPS seems to have the capability to triangulate and calculate slopes, but I do not think that is necassarily what you get when it tells you the distance. It is NOT what you get if the distance is shown on the screen (flat surface). Does your GPS have an option to give you both? You can probably find out if you read your users manual in detail or call the manufacturer.

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#162229 - 02/15/12 01:29 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
Originally Posted By billstephenson
A GPS, does not add elevation profiles to your distance, it measures point to point.

You could climb a ladder that's a mile high and still not have moved a bit from your starting lat/long, and your GPS will tell you that have traveled zero feet, as it should.


GPS most certainly does measure elevation (its not as accurate as longitude and latitude though!). Whether you are delivering bombs to a target or airplanes to a runway, the elevation is very important. See how smooth of a landing a plane would have trying to land at sea level at Denver International Airport. Horizontal distance traversed has very little meaning outside a 2-dimensional map.

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#162242 - 02/15/12 03:57 PM Re: Dumb question on GPS [Re: BZH]
Gershon Offline
member

Registered: 07/08/11
Posts: 1106
Loc: Colorado
WanderingDaisy,

Men do NOT read directions. We fiddle with it until we break something.

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#162244 - 02/15/12 04:07 PM Re: Dumb question on GPS [Re: Gershon]
Gershon Offline
member

Registered: 07/08/11
Posts: 1106
Loc: Colorado
I got my ladder out. Marked the bottom as a waypoint and climbed the ladder. It showed the distance to the waypoint as 9 feet. I climbed up and down 3 times and it showed my distance traveled as 31 feet and the distance to the waypoint as 31 feet even though I was standing at the waypoint.

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#162245 - 02/15/12 04:15 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
W_D has it right. I've worked with the data files a GPS records, distance is measured with a series of lat/long points.

You can adjust the distance between recorded points on most GPS units, and the finer grain you select provides a more accurate account of the distance you've traveled.

A GPS can also record an elevation profile. This will show you the ups and downs you traveled, but that is not the same as the distance you traveled.

Distance is measured on a two dimensional plane. If you view an elevation profile on a graph you'll see the miles shown on the graph are not adjusted to elevation.

They are not adjusted on for elevation on your GPS track either, but that's as it should be. If you buy a square mile of land the boundaries would be pretty wonky if they took terrain into account when measuring the mile. There are old jokes about getting more land for your money here in the Ozarks based on that.

Many GPS units have a "Measuring tool" built in. It will show you the distance between your location and the curser on the screen. You can use that to see how the topography is not accounted for when displaying the distance between to points.



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#162250 - 02/15/12 04:48 PM Re: Dumb question on GPS [Re: billstephenson]
OregonMouse Offline
member

Registered: 02/03/06
Posts: 6168
Loc: Gateway to Columbia Gorge
I've been having fun with distance estimates in local guidebooks (Mt. Hood and Columbia River Gorge). Most of the printed guidebooks agree within 0.2 mile, but the distances shown in the PortlandHikers.org Field Guide definitely vary from the printed guidebooks, many up to half a mile less. Most of the distances in the online Field Guide were measured with a GPS.

Hard telling who's right, but some of the folks who did the measuring for the Field Guide admit that GPS readings underestimate the distance!

I personally don't and won't use a GPS. It's just another gadget that needs batteries and can malfunction. I've gotten along just fine with map and compass (plus looking for landmarks, old USFS blazes, etc.) for 67 years now. Besides, I'm still trying to figure out the directions for my 4-year-old digital camera!


Edited by OregonMouse (02/15/12 04:50 PM)
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#162251 - 02/15/12 04:56 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
You should have read Gershon's reply above you before you replied. When you buy a 40 of land it is 1/4 mile by 1/4 mile on a 2-d map (or aerial projection). If you walk your entire boundary it will be farther than 1 mile and the surface area of your 40 will be more than 40 acres. In deed I once looked into purchasing a piece of land. It was a 40 with a small chunk cut out where somebody built a house. So it was less than a proper 40 but it was advertised as 42 acres.

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#162252 - 02/15/12 04:57 PM Re: Dumb question on GPS [Re: Gershon]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
Quote:
I got my ladder out. Marked the bottom as a waypoint and climbed the ladder. It showed the distance to the waypoint as 9 feet. I climbed up and down 3 times and it showed my distance traveled as 31 feet and the distance to the waypoint as 31 feet even though I was standing at the waypoint.


That sounds right. My GPS gives an estimate of accuracy for the lat/long it displays. On a good day it's about 12 ft.

Same with elevation.

If you stand still and watch it, you can see the accuracy readings fluctuate as satellites come in and out of range.

These fluctuations do affect your trip meter too.
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#162254 - 02/15/12 05:11 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
Originally Posted By BZH
You should have read Gershon's reply above you before you replied.


What is it you think am I missing?

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"You want to go where?"



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#162256 - 02/15/12 05:24 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
Originally Posted By billstephenson

What is it you think am I missing?


Wow... so if I understand you correctly, you believe Gershon's reading are a result of inaccuracy in his GPS? Is that correct, so I assume you believe his results would have been the same if he had stood at the bottom of the ladder for the same amount of time as it took him to climb up and down 3 times? You sure are stubborn Bill.

Gershon, would you mind repeating your experiment? Apparently Bill is still unconvinced that GPS's calculate actual distance traversed. Set a way point and stand their for a couple minutes. Report the distance traveled. Climb up and down the ladder three times and report the distance traveled. Then stand there a couple more minutes and report the distance traveled. Here are my predictions for the three readings: (1) 0-3 feet, (2) 25 - 31 feet, (3) 0-3 feet.

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#162257 - 02/15/12 05:32 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
Originally Posted By Gershon
...and the distance to the waypoint as 31 feet even though I was standing at the waypoint.


Originally Posted By BZH
Originally Posted By billstephenson

What is it you think am I missing?


... so if I understand you correctly, you believe Gershon's reading are a result of inaccuracy in his GPS? ...

You sure are stubborn Bill.

Gershon, would you mind repeating your experiment?


Gershon's experiment shows the inaccuracy, he pointed it out.

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"You want to go where?"



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#162259 - 02/15/12 05:46 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
Originally Posted By billstephenson

Gershon's experiment shows the inaccuracy, he pointed it out.



He walked 30 feet to get there!!! He is climbing up and down a ladder. As he is climbing up and down his distance traveled is increasing, even though he hasn't moved inch in terms of lat. and long.

At the end of his experiment the GPS said he had traveled 31 feet. It didn't say he was 31 feet from his original way point. Ooooh... that would be another way to verify what you are saying is wrong. If his results were due to inaccuracy, it would say he is 31 feet from his way-point. I bet it says he is right on top of his way-point.

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#162261 - 02/15/12 06:10 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
Quote:
If his results were due to inaccuracy, it would say he is 31 feet from his way-point.


That is what happened.

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"You want to go where?"



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#162262 - 02/15/12 06:20 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
your right I read that wrong.

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#162266 - 02/15/12 06:53 PM Re: Dumb question on GPS [Re: BZH]
billstephenson Offline
Moderator

Registered: 02/07/07
Posts: 3836
Loc: Ozark Mountains in SW Missouri
Originally Posted By BZH
To calculate actual distance (d) off of a topo map....

d = x/sin(arctan(n*h/x))

if you don't like arctan and sin's you can use the alternate formulation:

d = square root(x^2 + (n*h)^2)


Let's go back to that.

That math is way over my head but writing the code to use is not. I'll have to take a look at the data file my GPS exports, but if it does record elevation with the lat/long in the track logs then could you write a formula to account for that?

I should point out that I don't think it's beyond the GPS/GIS technologies capability to do what Gershon wants, it's just not how it is approached and implemented. There are some logical reasons behind the approach used.

But for our purposes, the total distance walked would be more accurate if it included the difference in elevation between the points. And knowing the total ups and downs, which I can do if the elevation is there, would also be interesting to know.

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#162267 - 02/15/12 07:19 PM Re: Dumb question on GPS [Re: billstephenson]
BZH Offline
member

Registered: 01/26/11
Posts: 727
Loc: Torrance, CA
Originally Posted By billstephenson

Let's go back to that.

That math is way over my head but writing the code to use is not. I'll have to take a look at the data file my GPS exports, but if it does record elevation with the lat/long in the track logs then could you write a formula to account for that?

I should point out that I don't think it's beyond the GPS/GIS technologies capability to do what Gershon wants, it's just not how it is approached and implemented. There are some logical reasons behind the approach used.

But for our purposes, the total distance walked would be more accurate if it included the difference in elevation between the points. And knowing the total ups and downs, which I can do if the elevation is there, would also be interesting to know.



Sure... The equation for linear distance between two points in 3-d space is:

d = sqrt(deltax^2 + deltay^2 + deltaz^2)

you would calculate deltax as the difference in latitude between points, deltay as the difference in longitude, and deltaz as the difference in elevation.

That would be the linear approach (assuming you walked in a straight between successive points). I could develop an algorithm the plots a smooth curve over all the data points and then calculates distance traveled from their. I'd have to think about that a bit more. In 2-d space it would be called a cubic spline.

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#162271 - 02/15/12 08:10 PM Re: Dumb question on GPS [Re: BZH]
Gershon Offline
member

Registered: 07/08/11
Posts: 1106
Loc: Colorado
I just repeated the experiment. Sometimes it measured the distance up the ladder, sometimes it did not.I went up and down about 10 times.

This time, the distance to the start stayed the same with normal 2 or 3 foot fluctuations.

Meanwhile, the neighbors are probably starting to wonder about me.

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#162273 - 02/15/12 08:29 PM Re: Dumb question on GPS [Re: Gershon]
wandering_daisy Offline
member

Registered: 01/11/06
Posts: 2661
Loc: California
Found this :

http://www.gpsaustralia.net/forums/showthread.php?t=18681

If the GPS has two lat/long coordinates, each with an elevation, and stores this data it capable of calculating the actual distance walked from point A to B, but apparently Garmin does NOT. You get horizontal distance. Then you download the data and convert to the hyoptonuse distance as a post processing function. Of course you only get the distance between measured points, so if you are not on continuous data gathering, you get a choppy set of numbers. There are tons of calculation algorithms out there on the internet that convert.

There may be some GPS units that have a chip inside that converts, thus allowing you to choose how you want distance presented. I suspect most do not simply because there appear to be many different ways to calculation depending what you want to measure. Anyway Gershon needs to look in his manual and see if there is a setting that allows one or the other.

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