How often to pick up trackpoints?

[bobmaloney@compuserve.com]

I have a couple of questions not about TOPOFUSION but about a topic you probably know a lot about..

    But first, the problem. I use a Magellan GPS most other people I know have Garmin GPS receivers.

    Typically when I hike with them , we get a difference of about 10% in the length of the hike.

    I have investigated the reason for the difference and believe I know what causes it. On my Magellan SPORTRAK, I pick up trackpoints every 0.01 miles. Looking at the trackpoints they are between 50 and 56 feet apart which makes sense. I looked at trackpoints picked up by a GARMIN recently and they were from 2 to 15 feet apart!

    The 0.01 mile setting on the Magellan is the lowest you can set. I am not sure how low the GARMIN receivers can be set. I do have the impression that GARMIN permits you to set the trackpoint frequency based on both distance and/or time.

    It is clear that the hike will be reported longer the more frequently trackpoints are picked up. The questions I have is:
    How often should track points be picked up to get reasonable accuracy?
    If you can answer that question, do you have a physical or mathematical basis for your answer?

    All info is appreciated.


                BobM

[Krein]

You are correct, less track points (more distance between points) will result in a shorter track.  Just use TopoFusion to simplify a track and see the distance drop.  However, when TF simplifies it removes the least significant points, so the effect will not be as pronounced as comparing a Magellan tracklog.

What you've noticed is one of the main reasons I don't like Magellan units.  It's all about the tracklog for me, and I want as high resolution as possible.

There is a 'mathematical' explanation yes, but it also just makes sense.  You could read up on sampling theory, but basically, the less samples you have, the less detail you are recording.  The more sinuous your path (switchbacking trail) the more distance you will lose by only taking points at a set distance.

The Garmin units have an "auto" mode which is what I prefer to collect data at.  It drops points when turns are made and also when speed changes.  This is best for recording the path and also playing the track back.

Hope that helped.

[bobmaloney@compuserve.com]

Thanks for quick reply.

 I understand why picking up trackpoints at a smaller interval will increase track length (fractals, etc).

 Using both the Magellan 0.01 mi setting and the Garmin high precision setting overstate guide book mileages on a hike which is also understandable, (e.g. guide book distances set with chain).

 I know nothing about the Garmin "automatic" setting but Magellan has one too. On our next hike, I will set my Magellan to "automatic" and ask my friend to set his Garmin to automatic. We can then check to see how close the results are then.

 Thanks again.

                          BobM