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Old September 28th 16, 08:40 AM posted to sci.geo.satellite-nav
Terry Pinnell[_3_]
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Default Puzzling change in trackpoint rate

Alan Browne wrote:

On 2016-09-26 13:47, Terry Pinnell wrote:
Many thanks Alan, much appreciate that comprehensive explanation.

I'm fairly satisfied with my distance measurements (after editing out
any serious errors by visual inspection). It's those gross ascent
descent estimates that vex me!

Walks (including holidays with my wife) are sometimes led, sometimes
self-led. The former often include a briefing beforehand from the
leader along the lines of "I'll be leading the higher of the three
walks tomorrow, which will be 10 miles, 1500 ft of gross ascent and
1750 ft of gross descent." Leaving aside the obvious question as to

Hmm - aren't you arriving where you departed?

Not on linear walks! Some of these involve buses with various drop-off
and pick-up points.

the source of his/her estimates, they do at least give us a rough idea
of expected difficulty based on past experience. And for our own
planned self-led walks I like to prepare my own estimates.

BTW, my walks never get repeated exactly. When they're close I do try
to make some comparisons.

Do you think that estimates based on digital elevation models, like
the one that Google Earth uses, or the UK's LIDAR model with 2 m
are more likely to give a more realistic GA/GD than recorded GPS data?

Sure - but not sure how you'll integrate all that - though a coarse
estimate of longest ascending/descending paths should be close.

(BTW, no altimeter on the iPhone.)

Yes, there definitely is an altimeter on the iPhone 6S+. And various
apps read it. And the Health App should be able to estimate your climbing.

You're right, of course, what I meant to say was no altimeter used by
the iPhone GPS apps I use, Memory Map and Pocket Earth.

I like the hat idea, although I suspect I'd quickly lose my iPhone!
Used to have my ancient Garmin strapped to my rucksack, so maybe I'll
try that again.

As high up as possible.

FWIW, here's a screenshot of the GPS record of a short walk yesterday,
opened in GE. My iPhone was in a shoulder pocket. I'm guessing that
the great variation in the density of the trackpoints depends on how
open or wooded it is. There were no fewer than 2,166 points recorded
in this 4.3 mile walk.

Terry, East Grinstead, UK