Garmin GPS - Urban Interference Question

Author

Urban Interference Question

emanon

2006-02-08, 5:49 pm

I'm not sure if this is a problem with my GPS or something I'm doing wrong.
I just got my first GPS, a Garmin Legend. Since I have no previous
experience, I decide to do a little learning in a controlled environment.
Since I work in the city, I decided on an urban hike where I can't get lost
and know both ends of my route.

There's a coffee shop that's a routine stop for me in the morning. Standing
outside the shop, near the front door but still in the open air, I marked a
waypoint as directed in the manual. When I arrived at work, about a half
mile away, I marked another spot as per the directions. At lunchtime, I
turned the two waypoints into a route and proceeded out the door.

It took a few seconds for the needle to orient itself, not unusual I'm told,
and was soon pointing in the approximate direction I needed to go. I decided
to simulate coming to a fork in the trail by making a Right turn earlier
than I needed to. The needle corrected and showed the direction to the
building.

My surprise was when I approached the corner where the spot I'd marked was.
About 20 feet before the corner there is a small gate to the Left and if I
followed the sidewalk through that gate for another 20 feet or so, I'd be at
the front door I'd marked earlier that day. Instead, the direction needle
was telling me I was to go across the intersection and go to the corner on
the Right diagonally from me!

OK, I know old-fashioned compasses can be effected by large pieces of metal
or electrical fields nearby. Are GPS units influenced by any of these? I ask
because the coffee shop is one that has a Wi-Fi Hot Spot as does the hotel
the coffee shop is in. Could this cause some interference with the reading?
If I bring the waypoint for the coffee shop up in Map View, it looks
reasonably located, but I only have the base map, no detail overlay maps.

Seagull

2006-02-08, 5:49 pm

emanon <emanon@erehwon.com> wrote:
>
> OK, I know old-fashioned compasses can be effected by large pieces of metal
> or electrical fields nearby. Are GPS units influenced by any of these? I ask
> because the coffee shop is one that has a Wi-Fi Hot Spot as does the hotel
> the coffee shop is in. Could this cause some interference with the reading?

GPS will tell you your position to within about 20 to 30 feet. With
WAAS augmentation, you can get that down to about 10 feet.

Now: that assumes good satellite reception and a clear view of the sky.
In the middle of a city, you generally have neither. That means some
satellites are blocked by buildings, and some signals are reflected off
of buildings rather than going straight to your unit. These translate
to positional errors, and decrease the "accuracy" of your reported
position.

Cheers,
-+JLS

--
\ carpe cavy!
seagull @ aracnet.com \
http://www.aracnet.com/~seagull/ \ (seize the guinea pig!)

emanon

2006-02-09, 11:48 pm

"Seagull" <seagull@aracnet.com> wrote in message
news:dsddjc02n1v@ene
ws3.newsguy.com...
> emanon <emanon@erehwon.com> wrote:
metal[color=darkred]

ask[color=darkred]
hotel[color=darkred]

reading?[color=darkred]
>
> GPS will tell you your position to within about 20 to 30 feet. With
> WAAS augmentation, you can get that down to about 10 feet.
>
> Now: that assumes good satellite reception and a clear view of the sky.
> In the middle of a city, you generally have neither. That means some
> satellites are blocked by buildings, and some signals are reflected off
> of buildings rather than going straight to your unit. These translate
> to positional errors, and decrease the "accuracy" of your reported
> position.
>
>
> Cheers,
> -+JLS
>
> --
> \ carpe cavy!
> seagull @ aracnet.com \
> http://www.aracnet.com/~seagull/ \ (seize the guinea pig!)

Thanks for the help! That could also explain the anomalies I've noticed
while driving (quick peeks, I'm not really trying to drive by GPS).

Guess I'm going to have to wait until the weekend, pending weather of
course, to try this outside in the open.

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