Odometer vs. active track vs. saved track

[+EraSeek]

I recently finished a 21 mile beach hike along Puget Sound. I noticed, as I had before, that my odometer varied from my track log distance. When discussed before people usually state that their odometer reads lower than their track log. Mine is the other way around. I had my track log set to "Auto-update most often".

This bugs me so I looked into why it does this. Here's a few thoughts and things I found:

 

I assumed that the odometer does one second updates so I set the track log to "Time-One second". On a trip to work this morning both recorded 6.49 miles. (Only one test.)

 

I am curious how these two devices record distance. I figure there are 2 ways to do this. 1) by compareing latitude and longitudes of track points. 2) by computing speed X time to get distance between points.

 

Active track logs are more accurate than saved track logs. Saved track logs whittle down the number of track points and draw straight lines between them and then calculate distances. Here is a graphic example of the same track, one active and one saved by Tom Chester. The track points went from 1400 to 125:

 

 

The best way to save tracks is on the SD card for 60cx owners. This keeps all the track points.

 

I will try "Distance" compare that to "time" compare that to "Auto -most" compare that to the "odometer" and try and figure out which is most accurate over all.

 

Input?

[rwsmith123]

I think you are right that the odometer uses speed multiplied by the time interval to calculate distance. I also think this is the more accurate mesurement.

 

For example, take your gps and mark a waypoint at your current position. While standing at the same spot, navigate to the waypoint. You'll see that the distance to the destination is going to vary, one second you'll be 1 foot away, the next you'll be 4 feet away, the next you'll be 2 feet away, etc. A change of position of 3 feet per second is equal to 2.046 mph. However if you look at your current speed it would have read zero or very close to it. So standing in the same spot over a period of time the distance recorded by the tracklog is going to increase. The odometer will not increase (or increase much slower than the tracklog). The gps uses doppler shift of the gps signals to calculate speed and is accurate to .1 mph. So calculating distance by speed * time will be more accurate than calculating distance by comparing two coordinates taken 1 second apart.

 

As to why your odometer showed a higher reading than your tracklog, I'm not sure.

[PerryB2]

I’m curious what you found on the odometer vs. active track question.

 

I got the 60CSx about a month ago to replace an older GPS. While using it on a couple of recent hikes I noticed that the trip odometer distance traveled does not match what the track log shows for distance when uploaded to MapSource on my PC.

 

Here are the data from a couple of recent hikes:

Trip odometer 8.0 mi.; track log 7.4 mi.

Trip odometer 6.6 mi.; track log 6.1 mi.

 

At the start of the hikes I cleared the track log and set Track Record Method to Distance, 0.01 miles. I also reset the Trip Computer data.

 

Perry

[+EraSeek]

I'm still playing with it and trying to figure it out. If you save it the gps and display it in Mapsource it edits out some of the track points and does not show times and such. Save it to the card or display an unsaved active track and it will show this and I take it not edit track points.

 

I noticed today that the trip computer showed 77.00 miles, the odometer showed 77.04 miles (hmm!) and the gps showed 77.12 when trying to save a track. I had switched from auto to track record method "Time 00 hrs, 00 mins, 01 seconds."

[PerryB2]

I’ve done some testing to try to figure out why the trip odometer was reading more than the active track log on recent hikes. I think poor reception is a large part of the problem; not bad enough that the GPS will report that it lost reception; but bad enough that it will cause the odometer and track log to record movement that did not really happen.

 

The specifications in the manual for the 60CSx say that accuracy is within 33 ft 95% of the time. The GPS is calculating a new ‘location’ once each second. Even with good reception and standing still there will be some distance from one calculated ‘location’ to the next.

 

If I’m reading the GPSInformation web site’s report on the 76Cx correctly it shows 95% of errors are less than 14.5 ft, average error is 6.6 ft and drift is 10 ft / minute. This is for a “moderate tree cover” situation, which I interpret to mean not great but OK reception.

 

If this ‘false movement’ was added to the odometer or the track log, it would really add up; so the GPS has to try to filter out the ‘false movement’ from the real movement.

 

I found that standing in one spot with fairly good reception, both the active track log (Record Method set to Distance: 0.01 mi) and odometer would show no movement. It took only about 3 feet of real movement for the GPS to record the movement to the odometer.

 

However, when reception is poor (generally 4 satellites showing bars ranging from ¼ to ½ strength), then the accuracy goes down and the ‘false movement’ will be large enough that the odometer and track log will both record it as real movement. For example, in one test I did with poor reception and the GPS in one spot; the odometer recorded 0.39 miles of travel and the active track log showed 0.2 miles of travel (Record Method set to Distance: 0.01 mi) in less than 6 minutes. When hiking, that could add up to a significant odometer error in several hours, if reception is poor. Trouble is: how do you know when reception is poor enough to mess up the odometer or track log, but not bad enough that the GPS will tell you it lost reception!

 

Why the difference between the odometer and track log? The odometer has one method to filter out the ‘false movement’. The track log will use one of many different methods to filter out ‘false movement’ depending on how track Record Method is set…Auto, Time, Distance and all the choices under those.

 

Perry