+jeff35080 Posted September 4, 2003 Share Posted September 4, 2003 I've got a question that has really been bugging me. Hopefully, someone with more technical knowledge of such matters can provide me with an answer. I am an avid hiker and always take my GPSr with me on my hikes to record my adventure. I have noticed that on my hikes the mileage shown by my GPSr is always incorrect. It always shows less mileage than what I have actually travelled. At first I thought the difference in mileage was due to lapses in reception from the satellite so this past weekend I went out of my way to constantly monitor the screen of the GPSr. On the hike I took this past Monday, the GPSr stated that I had travelled 5.14 miles, but once plotted on the National Geographic Topo! software and my Magellan Topo software it showed that I had travelled almost two-miles further which was a more accurate indication of the hike. Do elevation changes while hiking affect the distance recorded by the GPSr i.e. does the unit not properlly record distance while going up and down hills as opposed to flat ground? FWIW, my GPSr is a Magellan SporTrak Map with the latest software (4.06). I would welcome any insight to this question. Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Quote Link to comment
2oldfarts (the rockhounders) Posted September 4, 2003 Share Posted September 4, 2003 Your unit measures linear distance from point A to point B. If there is a rapid change in elevation along the way it will only see the distance in 2 dimensions. If the change in elevation is say 1500 feet up one side and 2000 feet down the other side of a mesa the GPSr will only read the distance between start and finish and think you went through the mesa instead of over it. John of 2oldfarts ******************************************************* Human beings can always be counted on to assert with vigor their God-given right to be stupid.--Dean Koontz Quote Link to comment
+jeff35080 Posted September 4, 2003 Author Share Posted September 4, 2003 and think you went through the mesa instead of over it. That's what I thought, but wanted someone else to agree with me. Thanks for doing so Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Quote Link to comment
+jeff35080 Posted September 4, 2003 Author Share Posted September 4, 2003 Here is the hike I mentioned in the post, reckon this amount of elevation change would do what I have described?: http://localhikes.com/HikeData.ASP?DispType=0&ActiveHike=0&GetHikesStateID=&ID=4614 Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Quote Link to comment
Kerry. Posted September 4, 2003 Share Posted September 4, 2003 No, there's something more to this one than simply elevation changes but the concept of changes in elevation will always make the GPS trip shorter (all things being equal) than the path the feet followed but certainly not by that much. GPS measurements (trip distace) do not take into account changes in elevation. If you looked at it this way if the GPS trip was 5.14 miles and the actual distance more like 7.14 miles then that would be akin to walking up a continuous slope of about 45 deg (which is quite steep) and at the end be almost 5 miles high. Cheers, Kerry. I never get lost everybody keeps telling me where to go Quote Link to comment
+Jamie Z Posted September 4, 2003 Share Posted September 4, 2003 I want to chime in here because I'm such a math geek. No, the elevation change is not where your error is originating. Read my post here where I think I eloquently described the problem. The error likely has more to do with the fact that the GPS measures a series of line segments, rather than the actual curves you are walking on the trail. Given a 7.14 mile loop with a 1000ft elevation gain gives a angle of 1.5°. [ sin(angle)=(1000/37699) ] This means if you walked 7.14mi (37699ft) your distance traveled over flat ground is about 37686 feet, a difference of about 14 feet. [ 37699*cos(1.5°)=37686 ] Someone check my math. Jamie [This message was edited by Jamie Z on September 04, 2003 at 10:33 PM.] Quote Link to comment
+seneca Posted September 4, 2003 Share Posted September 4, 2003 One possibility. Check to make sure that your units are not set to nautical miles?? (they are about 10% bigger than statute miles). Just a thought. I have never in my life learned anything from any man who agreed with me. Quote Link to comment
+GOT GPS? Posted September 5, 2003 Share Posted September 5, 2003 All my Magellans have come up short. 4000XL, GPS 315, and MeriPlat They come up about 0.85 to the mile, so they are always short. I have not tried the new Sportraks yet, though. Garmin has much better Trip Computer functions and more accurate Trip Distance. When I Download the tracklog of my GPS V, it is very close to the Trip Computer Info. -------------------------------------------------- Pictures of the insides of my Old GPS V My Home Page about what is GPS Geoff - Geocaching, DiscGolfing & Rollercoasters Quote Link to comment
+cache chasers Posted September 5, 2003 Share Posted September 5, 2003 With my GPS it depends on what speed I am traveling at....when I walk my GPS alsways comes up short on mileage buth when I ride my bike over the same trail the milage is right on Quote Link to comment
+GOT GPS? Posted September 5, 2003 Share Posted September 5, 2003 Thats right, AutoAveraging can kick in at times, and cause a little bit of error with the Trip Odom. I wish that AutoAveraging can be turned on or off. The reason I like the MeriPlat so much, is that I can stand still for a bit near the cache, with the compass turned on. The Auto Averaging helps with the Electronic Compass, and It has helped me find caches better than any Garmin. -------------------------------------------------- Pictures of the insides of my Old GPS V My Home Page about what is GPS Geoff - Geocaching, DiscGolfing & Rollercoasters Quote Link to comment
+jeff35080 Posted September 5, 2003 Author Share Posted September 5, 2003 Thanks for all the replies guys! At least I know now that I'm not the only whose unit reads a bit shorter than what is actually travelled. Happy Friday! Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Quote Link to comment
forresth Posted September 5, 2003 Share Posted September 5, 2003 I wonder if you turned on the detailed tracklog option if that would make a difference. I'm kind of grabbing at straws since I'm not an expert. Forrest Quote Link to comment
+Criminal Posted September 5, 2003 Share Posted September 5, 2003 Another way to improve the accuracy is to increase the track-log rate. Moun10Bike and I both logged our tracks on the hike to Gem Lake. He had his GPS V set to “highest” and I was on medium. You can see the results here. His track logs have many more points than mine. I would think his milage will be more accurate. http://fp1.centurytel.net/Criminal_Page/ Quote Link to comment
+jeff35080 Posted September 5, 2003 Author Share Posted September 5, 2003 That's interesting Criminal, I have been using the 'Auto' option, but when I go this weekend I will try the 'Detailed'. I wonder how many miles of track points I could get in Detailed mode.... Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Quote Link to comment
+Criminal Posted September 5, 2003 Share Posted September 5, 2003 quote:Originally posted by jeff35080:That's interesting Criminal, I have been using the 'Auto' option, but when I go this weekend I will try the 'Detailed'. I wonder how many miles of track points I could get in Detailed mode.... Jeff http://www.StarsFellOnAlabama.com http://www.NotAChance.com If you hide it, they will come.... Not sure what unit you have. On the V I would use Auto and highest. http://fp1.centurytel.net/Criminal_Page/ Quote Link to comment
GrandpaCannon Posted September 5, 2003 Share Posted September 5, 2003 I have not checked a lot of distances but my Garmin Vista and a Megellan Handspring GPS module that I recently sold both give me very accurate distance measurement. Uphill, downhill, flat whatever. Besides hiking I have taken the Vista running and driven with it in the car. In the car I get about a 1% difference between the Vista and the car odometer. Quote Link to comment
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