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Postby OtherHand » Mon Nov 25, 2013 8:31 pm

63ChevyII wrote:When you save a current track on your GPS unit, am I correct in thinking that it removes a large number of points, thus reducing the mileage and accuracy of the file? Is there a way to prevent this from happening?


I'm not aware that a Garmin GPS does any sort of truncation while saving a track. I have seen some software reduce the number of points when importing a Garmin gdb track file or converting it to other formats. But if you bring a track up in Garmin software it should give you all the points that were originally recorded.

When you setup track recording you have a number of options like every so many feet, or so many seconds or auto. I think auto is some sort of black magic and I just leave it on that. That's been fine for the accuracy I need. Remember that there is going to be GPS location drift anyway (sometimes considerable) so a large number of points that aren't accurate from a position standpoint are sorta worthless.
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Postby 63ChevyII » Mon Nov 25, 2013 10:39 pm

OtherHand wrote:
I'm not aware that a Garmin GPS does any sort of truncation while saving a track.


I could be wrong but I think this is something that I've seen. For example, IIRC, when I view 'current track' info on my GPS when I finished my C2C hike last year, it said 22+ miles. After saving the GPX, it became 20.97 miles.

I will see if I can find any documentation to support this.
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Postby 63ChevyII » Mon Nov 25, 2013 10:48 pm

I don't know if this is accurate info or not:

Garmin's Save Function
The "Save" function on current Garmin handhelds is something of a misnomer. It's function is reduce (eliminate some or a lot of data) and then store. The "Save" function may have been primarily intended for the "TracBack®" feature and not actually additional "track" storage. In a sense it creates a type of route that is more detailed than the standard "Route". The active route lists Turn 1, Turn 2, Turn 3, etc. instead of waypoint names since waypoints aren't involved in the TracBack of a "saved" track. It is a blurring of the distinction between a route and a track.
The "Save" function removes many of the active track log points and throws out the date/time information. A "save" function on other brands of GPS receivers may actually save all the data in the current track log - for instance check Lowrance (however note that Lowrance doesn't store date/time information in its track logs). The active track is not automatically deleted. There are some exceptions: Geko 201 and 301. The Geko 201 and 301 retain the date/time information in the processed (saved) track logs but still reduces the number of points to a maximum of 500 (from possibly 10,000). A short active track log of up to maybe 150 points might be "saved" without any points being removed. The etrex series, the 76 series, and the 60 series all throw out the date and time stamps when the active log is "saved" to the "saved" storage areas.
The removal of points is based on an algorithm to pick the most significant points. It does not necessarily reduce to the maximum number of points that can be stored. For instance an active track log of 2016 points might be reduced to 197 points even though there is storage space for 250 points (recent firmware releases have increased this to 750 points in some models). Note that the loss of date/time stamps means the software on a computer won't be able to compute your speed for the various segments


regarding time / distance / auto logging:
Most receivers support logging by time or distance intervals. Garmin also has an "Auto" setting that logs a track point when you deviate from a straight line by a certain amount or speed drops to zero. You set the offset amount from a straight-line with the interval setting values of: "Least Often", "Less Often", "Normal", "More Often", "Most Often". The "Auto" setting will probably give you the best representation of your route while using the fewest number of track points.

Source: http://www.gpsmap.net/GarminHints.html


I will see if I can find more info tomorrow. It appears the info above is at least 7 years old (outdated now?). Like you said though, it doesn't really matter if it's just a bunch of inaccurate points anyway.
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Postby OtherHand » Mon Nov 25, 2013 11:18 pm

63ChevyII wrote:I will see if I can find more info tomorrow. It appears the info above is at least 7 years old (outdated now?). Like you said though, it doesn't really matter if it's just a bunch of inaccurate points anyway.


The info on that web page is from 2006, seven years ago. I suspect much of it is outdated. I have never heard that saving a track truncates it. I've always just downloaded the current track to my PC so if it exists, I never would have seen it as I didn't save it to the handheld. I have a current track in my Garmin I'll try saving and see if truncation occurs. I'm doubly suspicious when he states time stamps are stripped out as that's pretty critical info.

My guess is the new Garmin units can hold so many points for a track that there's no need to do any truncating.
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Postby 63ChevyII » Tue Nov 26, 2013 9:19 am

Thanks for the help OtherHand.


Another thought. My GPS unit has a 'trip computer screen,' which is the screen that I usually view. I have trip odometer, elevation, trip time, avg speed, etc displayed on it.
I have noticed a difference (higher) in the mileage displayed on this screen at the end of a hike and mileage of the gpx file. I always figured this was due to truncating. I am thinking now that maybe the trip odometer is updated more frequently that the GPS sets trackpoints.

:oops: Maybe this is the difference I was noticing in mileage?:oops:

I am hoping to hike Skyline or head out to Joshua Tree Friday or Saturday. I will pay better attn to my GPS to see if I can get it figured out. :?
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Postby Hikin_Jim » Tue Nov 26, 2013 10:23 am

63ChevyII wrote:Image
Baldy from the village is only 6.2 miles? Interesting. I thought somehow that it was more like 8. But dang that's a steep sucker.

What kind of updates have you made to your Skyline graphic?

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Postby OtherHand » Tue Nov 26, 2013 10:26 am

I find the trip computer or odometer always a bit off, for me it typically shows just a bit longer than the recorded track. I don't know which is more correct, and I choose to use the recorded track.

OK, so I checked on truncation, and it turns out my unit does indeed truncate. I didn't know that. But it isn't a problem with new units.

My go-to Garmin is an Etrex Vista HCx. Yeah, it's old and I'd like a new one but the damn thing won't die. Anyway, the maximum number of points it can have on a loaded track is 500. But it can record a track with many more points than that. It just can't reload it.

Example....My GPS has a track I recorded from my last trip in it. I didn't save it, it was still in its main memory. When I loaded it into my PC directly I see it's composed of 1,395 points. Now I saved that same track within the Garmin's internal storage area. When I load that saved track into my PC it only has 500 points. It removed a lot of the less essential points.

Here's why....It's due to the at-that-time 500 point limit. Say you had gone on a hike and generated a 1,200 point track. To use the track back feature, or otherwise make a return route, the Garmin would have to reload that track. But it can only load 500 points, and it does so consecutively. So it would load the first 500 points just fine. But after that for each later point it loaded it would drop off the first point in the track. So by the time you were finished loading a 1,200 point track, you'd only be able to see the last 500 points. That could be a problem.

But with the huge storage capabilities on the newer units there doesn't seem to be a limitation like that so no truncation. I really need to work harder to destroy my unit so I can replace it....
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Postby 63ChevyII » Tue Nov 26, 2013 10:27 am

Hikin_Jim wrote: Baldy from the village is only 6.2 miles? Interesting. I thought somehow that it was more like 8. But dang that's a steep sucker.

http://www.simpsoncity.com/hiking/baldytrail.html

Yup, it's great training for Skyline and with the descent, I actually think it's a tougher hike.

Hikin_Jim wrote:What kind of updates have you made to your Skyline graphic?
HJ

Mileage and more landmarks. I am hoping to re-confirm some of the numbers this weekend.
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Postby 63ChevyII » Tue Nov 26, 2013 10:31 am

Thanks OtherHand, that makes sense to me. I was talking to someone about the mileage discrepancies I was seeing and that's the explanation they gave me. They must've had an older unit.

My GPS unit (Dakota 20) was a refurb.... I may possibly know the reason why it was sent back to the factory now. Playing with it this morning I noticed it has 2-3 lines of dead/dying pixels
:x
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Postby Hikin_Jim » Tue Nov 26, 2013 10:31 am

63ChevyII wrote:Mileage and more landmarks. I am hoping to re-confirm some of the numbers this weekend.
Gotcha. I kind of liked the side by side tables for the Museum Trailhead vs. the Ramon Trailhead on the older version.

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