Topo vs. GPS distance traveled

[HiknVA]

I just got back from a backpacking trip over the three day weekend and was really surprised to find out according to my GPS (ETrex Legend Hcx) we went 18 miles when we were only supposed to go about 15. I calculated the distance beforehand on my National Geographic Topo software and confirmed my route and distance with the data from the website where I got the destination suggestion when I got home. Which do I trust to give me the correct distance traveled?

 

This is concerning because if I mark a route for a future hike and from my topo map know it is say 3 miles to spring or to a trail junction, if I look at my GPS and add in three miles from my current location, I will get to the GPS location too soon and wonder where my spring/trail is. I was a little concerned the other day when I had (according to my map) 2.2 miles to reach a cabin and trail intersection. I looked down on my GPS, added 2.2 miles to it and started hiking. 2.2 miles came and went long before we came to the cabin. My GPS calculated it being a little over 3 miles. Same with day's hike out. TOPO figured it to be a little less than 2 miles, my GPS calculated it having been almost 3 miles.

 

Am I missing something here? Am I always going to have to assume that my TOPO calculations are off by a few miles? Or is there something wrong with my GPS? (I am rarely ever actually on the supposed trail, sometimes I am above, sometimes below, but always going in the right direction. Could it be that bouncing around that is increasing my distance? I always have good lock on satellites, with 10-80ft location pinpoint depending on if I am in a valley or not.)

[+Team LRS]

I just got back from a backpacking trip over the three day weekend and was really surprised to find out according to my GPS (ETrex Legend Hcx) we went 18 miles when we were only supposed to go about 15. I calculated the distance beforehand on my National Geographic Topo software and confirmed my route and distance with the data from the website where I got the destination suggestion when I got home. Which do I trust to give me the correct distance traveled?

 

This is concerning because if I mark a route for a future hike and from my topo map know it is say 3 miles to spring or to a trail junction, if I look at my GPS and add in three miles from my current location, I will get to the GPS location too soon and wonder where my spring/trail is. I was a little concerned the other day when I had (according to my map) 2.2 miles to reach a cabin and trail intersection. I looked down on my GPS, added 2.2 miles to it and started hiking. 2.2 miles came and went long before we came to the cabin. My GPS calculated it being a little over 3 miles. Same with day's hike out. TOPO figured it to be a little less than 2 miles, my GPS calculated it having been almost 3 miles.

 

Am I missing something here? Am I always going to have to assume that my TOPO calculations are off by a few miles? Or is there something wrong with my GPS? (I am rarely ever actually on the supposed trail, sometimes I am above, sometimes below, but always going in the right direction. Could it be that bouncing around that is increasing my distance? I always have good lock on satellites, with 10-80ft location pinpoint depending on if I am in a valley or not.)

 

Sorry but im not that good yet with these units but does the gps unit measure your distance as the crow flys but when your out walking it will route the miles as per the route you take ?

[+MacFlash]

The etrex HCx series was underestimating mileage up until the last update (2.6/2.6), and from what I have heard,now it is slightly overestimating, but I have never heard it be this bad. From others reports somewhere on the order of 5% Don't forget, the odometer measures total distant traveled. So if you walk forward 50 feet and than back 50 feet, this registers as 100 feet traveled. The other possibility is that under weak signal conditions, the fix is drifting and this is recorded as distance traveled. One thing to check on is time traveled- is it also over estimated- then I would attribute it to drift. All in all I think this is aspect of the etrx HCx line is still a work in progress.

 

Of course you can always measure the distance on the screen from waypoint to waypoint to get a crows flies estimate of distance.

Edited May 27, 2008 by MacFlash

[MtnHermit]

I just got back from a backpacking trip over the three day weekend and was really surprised to find out according to my GPS (ETrex Legend Hcx) we went 18 miles when we were only supposed to go about 15. I calculated the distance beforehand on my National Geographic Topo software and confirmed my route and distance with the data from the website where I got the destination suggestion when I got home. Which do I trust to give me the correct distance traveled?

 

This is concerning because if I mark a route for a future hike and from my topo map know it is say 3 miles to spring or to a trail junction, if I look at my GPS and add in three miles from my current location, I will get to the GPS location too soon and wonder where my spring/trail is. I was a little concerned the other day when I had (according to my map) 2.2 miles to reach a cabin and trail intersection. I looked down on my GPS, added 2.2 miles to it and started hiking. 2.2 miles came and went long before we came to the cabin. My GPS calculated it being a little over 3 miles. Same with day's hike out. TOPO figured it to be a little less than 2 miles, my GPS calculated it having been almost 3 miles.

I view navigation not as distance traveled, but rather as a series reference points. You basically said the same thing, referring to a spring and a junction. When I get there, I get there, regardless of the miles traveled. So if you have a topo or other map in your GPS, or mark these features in your mapping software and transfer those points to the GPS, then it's easy to occasionally refer to your distance to a point and infer time to the point from that distance.

 

For me backpacking is NOT about miles traveled, rather its the journey, with the sights, sounds, smells along the way. Miles are a mear statistics, otherwise meaningless.

 

Congrats on your backpack, still too much snow here to do anything meaningful.

[+Redwoods Mtn Biker]

I just got back from a backpacking trip over the three day weekend and was really surprised to find out according to my GPS (ETrex Legend Hcx) we went 18 miles when we were only supposed to go about 15. I calculated the distance beforehand on my National Geographic Topo software and confirmed my route and distance with the data from the website where I got the destination suggestion when I got home.

 

How did you calculate the distance on NG TOPO? By drawing a route/track? You can't mimic all the twists and turns of a trail doing this and I imagine that most trips will show a greater distance via the actual tracklog.

[HiknVA]

Last night I transferred my track from my GPS to my TOPO. My GPS calculated me having gone 18 miles, the TOPO said I went 16. That's a 2 mile difference.

 

I am mainly concerned about specific points I am interested in like a spring, trail junction, rock outcropping and like to know if I am getting close and need to look for it. We needed to be back by a specific time yesterday and were going off of TOPO saying we had 2 miles back to the car. We made it back with plenty of time but it really was 3 miles. Had we known it was really 3 miles, we'd have left earlier. 2 mile difference over the course of three days isn't a big deal to me, but for future trip planning, am I going to have to add an extra 2 miles onto the trip?

[+Walts Hunting]

When ever I put a route on the Garmin TOPO map I add at least 25% because of all the little meanderings that don't show up on the map. 50% has happened in some really winding areas.

[+Redwoods Mtn Biker]

When ever I put a route on the Garmin TOPO map I add at least 25% because of all the little meanderings that don't show up on the map. 50% has happened in some really winding areas.

 

Yes, the only way to get an accurate distance is to be following an actual GPS track -- your own or one you download from someplace like MotionBased.

[+Orienteering]

I just got back from a backpacking trip over the three day weekend and was really surprised to find out according to my GPS (ETrex Legend Hcx) we went 18 miles when we were only supposed to go about 15. I calculated the distance beforehand on my National Geographic Topo software and confirmed my route and distance with the data from the website where I got the destination suggestion when I got home. Which do I trust to give me the correct distance traveled?

 

This is concerning because if I mark a route for a future hike and from my topo map know it is say 3 miles to spring or to a trail junction, if I look at my GPS and add in three miles from my current location, I will get to the GPS location too soon and wonder where my spring/trail is. I was a little concerned the other day when I had (according to my map) 2.2 miles to reach a cabin and trail intersection. I looked down on my GPS, added 2.2 miles to it and started hiking. 2.2 miles came and went long before we came to the cabin. My GPS calculated it being a little over 3 miles. Same with day's hike out. TOPO figured it to be a little less than 2 miles, my GPS calculated it having been almost 3 miles.

 

Am I missing something here? Am I always going to have to assume that my TOPO calculations are off by a few miles? Or is there something wrong with my GPS? (I am rarely ever actually on the supposed trail, sometimes I am above, sometimes below, but always going in the right direction. Could it be that bouncing around that is increasing my distance? I always have good lock on satellites, with 10-80ft location pinpoint depending on if I am in a valley or not.)

 

Did you leave your GPS on when you took breaks? I have the same issue with my 60CSx. It always wanders when we stop. I ran a simple test and put my old Vista C and my new 60 CSx on a picnic table during a break. I reset the odometer before and was amazed to see how far my 60CSx had travelled while laying on the table. You might want to check that with your Legend HCx. So I have to turn off my 60CSx whenever we stop if I want to get an accurate measurement of my hiked distance.

Edited May 27, 2008 by Orienteering

[+rovers3]

I've been wondering just how accurate the measured distance is with a GPSr also. The other week I did three hikes and on each occasion the GPS was less than the distance on the trail map which had supposedly been measured with a walking wheel. I have a Magellan Explorist XL which registered 19.2km where the map showed 26.1km. Didn't lose satellites as far as I know and the difference was similar on each day.

I was hiking on the Bruce Trail in Ontario.

[+embra]

Tracks tend to be more accurate than GPS odometer readings in my experience. My Explorist 500 was off in its odometer, but if I checked the length of the active track it was pretty good--so clearing the track at the start of a hike provided a good workaround.

 

I concur with the comments above that the turns and curves of the actual hike can get smoothed out in a map program. Delorme Topo allows for creating routes with trails that can give a mileage estimate...but if the topography is steep, it's good to expect more for switchbacks that get lost in the mapping process. Mapped trails tend to be way oversimplified.

[PerryB2]

HiknVA,

 

How were you carrying your GPS? Did you have it in your hand watching the screen the whole time? If not it may not have had good reception at times, which can result in GPS ‘wandering’ which can add to the mileage.

[HiknVA]

I carried the GPS on my sternum strap in its Garmin Etrex carrying case so it was only blocked on one side by my body with a view of the satellites slightly back from my shoulder and all in front of me. Anytime I looked at it it appeared to have really good reception with accuracy varying from 8ft-30ft depending on tree cover, etc.

 

We stopped occasionally but anytime I looked down at it it did not appear to veer of randomly as if we were still moving.

[dayhiker9]

Am I always going to have to assume that my TOPO calculations are off by a few miles?

 

I would say so. I found this to be the case particularly if there are switchbacks. I remember guidebooks saying there would be say 11 switchbacks, while there were only a couple on the map, ie they are not shown accurately on the map. (Too close together? trail changed?). The milder grade somewhat offsets the longer mileage though (both up and down for me, weak ankle). You will need to look at guidebooks , forest service information for actual mileage - though FS trail signs are off to, rounding or what not. I thought you said you confirmed the distance though, but then you said the topo was off saying you had 2 miles to go when it was 3? I suppose the total could be right and just the segment off, rather unusual though.

 

For things you want to find like springs I would think marking them with waypoints would work better than using your GPS as an odometer. I used to use time as an estimate for distance, and maybe an altimeter or landmarks to zero in on a location, I guess the odometer on the GPS is not much better, but I haven't used mine enough yet to judge.

 

As far as transferring your route back to the TOPO software, that is something I will have to try. I wonder if TOPO is dropping waypoints or the GPS is noting distance without saving enough waypoints to reconstruct it. Seems like testing it over trails with known distances would be helpful. I did read in a book about using tracks to map out your route. They said to turn tracking off if you have to leave the trail. Maybe put down your own waypoints a t critical points. It seemed to think you could get a pretty accurate trail (not survey quality but not off my more than a thick line on the map). I returned the book to the library so this is from memory.

[NightTracker]

I haven't done this test yet, but I think the odometer adds distance everytime the accuracy changes. Like I said I haven't done anything to test it yet, but just noticed a few times that it says i've gone X distance when I think I only travelled Y.

[PerryB2]

I carried the GPS on my sternum strap in its Garmin Etrex carrying case so it was only blocked on one side by my body with a view of the satellites slightly back from my shoulder and all in front of me. Anytime I looked at it it appeared to have really good reception with accuracy varying from 8ft-30ft depending on tree cover, etc.

 

We stopped occasionally but anytime I looked down at it it did not appear to veer of randomly as if we were still moving.

 

If you can, you might try carrying it attached to the shoulder strap, face up to see if that improves the accuracy of the odometer.

[Smac999]

the H versions of the gps's have had lots of problems with the trip meter. yes they will probably always be off. the older gps's were alot better. the newest firmware (2.6) is alot better then the first stuff though.

 

so don't use the trip meter to figure how far you have to go... if you had a waypoint at the end you'd know exactly how much further you had to go. (in a straight line, but generally it's not hard to figure it out)

Edited May 30, 2008 by Smac999

[HiknVA]

the H versions of the gps's have had lots of problems with the trip meter. yes they will probably always be off. the older gps's were alot better. the newest firmware (2.6) is alot better then the first stuff though.

 

so don't use the trip meter to figure how far you have to go... if you had a waypoint at the end you'd know exactly how much further you had to go. (in a straight line, but generally it's not hard to figure it out)

 

I will definitely keep that in mind. We hadn't been out in a while and didn't want to overdo it. I had drawn up a route of about 15 miles, so we were surprised we hiked more according to the GPS. I was a little concerned when I got home as to how good my TOPO is for future distance calculations. I will figure adding in an extra mile just to be safe and will make waypoints for important locations and just gauge how far away one is and just use the GPS as a guide and not an exact measure of distance traveled or how far we have left, which is a bummer. Would be nice to know if we are making good time, if we can stop for a longer break, etc. Oh well.

 

Thank you all for your comments and suggestions.

[+Redwoods Mtn Biker]

One thing hand-traced TOPO tracks are good for is estimating elevation gain, using their profile feature.

 

I suggest searching MotionBased.com (which I mentioned before) for any trails you are planning to hike. You can get a GPS track that will often be very precise as to distance. You can also transfer it to NG TOPO for a nice printed map, and to your GPS to display. By doing the latter and navigating the track, you can show exactly how much ground you have left to cover using "Distance to destination."