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Re: Coordinates from iPhone

Subject: Re: Coordinates from iPhone
From: "Keith Smith" smith9e499
Date: Tue Jun 25, 2013 9:38 pm ((PDT))
Hi Peter,
I went through a bit of this several years ago, shooting panoramas of the
eastern faces of the Rocky Mountains west of Calgary. I was using a Garmin
60CSx and a 1" optical theodolite to triangulate and identify very distant
peaks, sometimes 120km away behind other peaks 40 to 60km away. I'm no
mountain climber, so finding good viewing points in the foothills with
views in great depth required a lot of hunting and then understanding what
I was seeing (again in great depth)  required many hours spent with top
maps on the computer.

My constant enemy was 'station eccentricity' which is the inability to
reliably establish the position of the instrument and therefore having to
consider that as somewhere within a circle of radius x. Even with fairly
reliable backsights, the results were often less than convincing. By mid
2009 the financial crash reduced my income to the point where I was put out
of my misery by the price of fuel.

The 60CSx does offer some 'averaging' to increase the odds of a good fix.
At the time, I was discussing rentals of pro grade 'static gps' units from
my survey supplier who was interested in the project and prepared to make
the rate very attractive. These units need to collect data for an hour or
more and then need significant post processing to get to anything closer
than a metre, but can get down much finer than that with more data. To do
much better you need to tie into a terrestrial network (which seems to
presuppose that you're part of the surveying community).

Before I packed it up, I was looking at the Ashcroft Mobile Mapper, sold by
Magellan along with their post-processing software. It looked promising,
but at 1600 CAD it was a bigger bite than I could afford. It does promise 1
metre, though.
My Galaxy S3 seems pretty good (with Trimble Outdoors), but I haven't
compared it seriously with the Garmin. Personally, I hate these telephone
user interfaces and only use them to track an occasional hike or find my
way to an address somewhere. I'm not sure if the repeatability is better
'at home' where it can find a few cell towers. I haven't found it so
impressive in 'fringe areas'. Heh, when you're out *there*, then it can't
download the bloody maps anyway, and the ones you have cached are always
missing in a useful scale.

Software is another challenge. My fav's are ExpertGPS and OziExplorer. I
use both with geotiff maps which can be downloaded free in most countries
these days, I think. I was able to provoke the EGPS developer to show 0.1"
but it didn't solve my problems, as indicated above. The Garmin software is
great for loading maps into the handset, but they do remove a fair bit of
data (often structures etc.) to get the file sizes down.

Having run a machine shop for a decade with another spent in steel
fabrication, I think I can say with some confidence that consumer GPS is
the best example of 'Precision Without Accuracy' that I have ever seen.
They're wonderful devices and I love them but they won't thread a needle
er, well, they can't do it twice!


Keith Smith - Freelance Guitarist & Location Recording

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