[Talk-ca] GPS inaccuracy

Connors, Bernie (SNB) Bernie.Connors at snb.ca
Mon Nov 19 16:14:49 GMT 2012


Different models of GPSr have different types of antennas.  The two most common types that I am aware of are:

1. Patch antenna (e.g. Garmin eTrex models).  The patch antenna in the eTrex models gets its best reception when the LCD screen is parallel to the ground.

2. Quadrifilar antenna or Quad Helix antenna (e.g. Garmin 62 models).  The Quadrifilar antenna in the 62 models gets its best reception when the LCD screen is perpendicular to the ground.

If you have a GPSr you can easily test which type of antenna you have.  Go outside and turn on your device and then display the satellite page that shows the bar graph of the signal strength for each satellite that is being tracked.  Wait until you are tracking several satellites.  Now move your GPSr from a horizontal position to a vertical position and watch what happens to the bar graph.  In testing I did between 2003 and 2007 it was quite easy to see the bars shrink or grow depending on which way you held your GPSr.

In a smart phone the GPSr may share an antenna with the cellular radio or Bluetooth radio.  The antenna design is most likely NOT optimized for GPS reception.  In other words you should not expect the GPSr in a smartphone to perform as well as a dedicated GPSr device.

Bernie.
--
Bernie Connors, P.Eng
Land Information Infrastructure Unit, SNB
bernie.connors at snb.ca



-----Original Message-----
From: Harald Kliems [mailto:kliems at gmail.com] 
Sent: Monday, 2012-11-19 11:44
To: Tom Taylor
Cc: Connors, Bernie (SNB); talk-ca at openstreetmap.org
Subject: Re: [Talk-ca] GPS inaccuracy

Other things you could do: check if the Canvec data has been moved after being imported. As I've said, Bing imagery can be improperly aligned, but not everybody is aware of that. So it's possible that a well-meaning mapper has moved the Canvec data to match the imagery.
Did you check your GPS devices accuracy reading? While they shouldn't be taken too literally, they do give some indication of what's going on (IIRC Garmin devices unfortunately do not write the accuracy reading into the GPX file). You could also try to improve result by making sure the GPS is in an ideal position: This usually means:
device parallel to the ground, with as little obstruction of the upward line of sight. Putting it on top of a backpack or your handlebar bag if you're riding a bike works best.

 Harald (who tends to get pretty good data with his Etrex Vista Hcx but not so much with his smartphone)

On Mon, Nov 19, 2012 at 10:29 AM, Tom Taylor <tom.taylor.stds at gmail.com> wrote:
> I will do a resurvey in the way you suggest. It was really 
> disappointing to find I couldn't trust my GPS at all (well, I did some 
> mental adjustment of waypoints to place building entrances), and being 
> off by a constant amount for 4 km certainly makes one suspicious.
>
> A bit to the north, my GPS survey of some footpaths behind a school 
> matched Bing nicely. It would be really interesting to find some sort 
> of disconnect in between.
>
> Je manquais de la courtoisie envers les francophones qui suivent cette 
> liste. En sommaire, j'ai tracé une piste de longueur totale environ 4 
> km, mais l'a trouvé toujours environ 10 metres à l'ouest des entités 
> déja presents sur la carte et les images Bing. Je vais faire un autre 
> sondage pour mieux comprendre la situation.
>
> Tom Taylor
>
> On 19/11/2012 8:37 AM, Connors, Bernie (SNB) wrote:
>>
>> Tom,
>>
>> The transmission lines would not affect your GPS accuracy.  I agree 
>> with Pierre that you could repeat the track and see if the two tracks 
>> are similar.  Another thing you could do is identify several 
>> identifiable points in the Bing Imagery such as the intersection of 
>> two sidewalks, the corner of a sports field, etc.  Go to those points 
>> and use your GPS to record a waypoint and use position averaging with 
>> about 3 minutes of recording to get a more accurate location and then 
>> compare those waypoints to the Bing imagery.  Position the Bing 
>> imagery so they match up with your waypoints and then look at your 
>> GPS tracks top see how they line up with the Bing Imagery.
>>
>> Bernie. -- Bernie Connors, P.Eng Land Information Infrastructure 
>> Unit, SNB bernie.connors at snb.ca
>>
> ...
>
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> Talk-ca mailing list
> Talk-ca at openstreetmap.org
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