Basic advice

My unit stores its last position when shut down and I would assume all others do also. The device probably uses this position to speed up satellite acquisition at start-up and it also becomes the first point in the track log at start-up.

Once your unit starts showing your current location, you should wait ca. 10-15 minutes if you want better data at the beginning. GPS satellites broadcast two sets of data for calculating satellite position. The almanac data provides a coarse position for all satellites. The ephemeris data will give a more precise satellite location, but only for one satellite. Your initial fix will use the almanac data and will improve as the ephemeris data for each satellite is acquired.

You can clear your track data at the beginning as SUSSAMB suggests. Another option is to just delete some of the data at the start of the track log once it is downloaded to BaseCamp.

Hi Thanks.

Successfully started at the right point and deleted the hour t the end which was in the car - so both sets of advice used.
The mileage at the end of the walk on the trip computer (on the Garmin450) said 25.25miles. When I download the track is says 23.6m on Basecamp (almost the same as the route I uploaded). Any ideas why the difference?? Is the 23.6m in 2D and so doesn't take ac of the 4750feet of climb (which i got from uploading into Google Earth because I couldn't find it reported in Basecamp for either the track or the route I created from the actual track)? I then used the track to create a Garmin adventure which says 24.7miles and 4864ft of ascent...bit confused as to which is correct....

cheers
Andy

As far as I know, all Garmin distances are 2D. The trip computer probably updates the odometer at the frequency the position updates on the unit, while the track logs data less frequently. More points will give longer distances due to position error (a logged position isn’t the true position so you will see an accumulation of errors).

I would have to see the BaseCamp gpx of your track and your adventure to make a guess why those distances differ. FALAGAR may be able to answer that part of your question.

As far as I know, all Garmin distances are 2D.

That is my understanding as well. That works in relatively flat terrain but in areas where the roads/trails are steeper there can be errors.

Now basic math (geometry) tells you that the hypotenuse of a triangle is longer than the base (what is measured by GPS). Now for longer distances it's more complex because of the arc of the earth which is slight longer than a straight line.

Now from my testing, years ago, Garmin software (and units) properly calculate distances using the arc of the earth (e.g. for two points on opposite sides of the planet). However for routes and tracks distances are calculated from point-to-point and totaled so the difference between a straight line and arc are very small.

Thx all.
In summary:

Trip odometer showed we'd done 25.25 miles.
The planned route was 23.4 miles
The actual track was 24.7miles; which when converted to a Route became 23.7m and when converted to an Adventure stays at 24.7m

Interesting.

Andy