GPS - Pinpoint precision?

When I talk about GPS, many people think I am a technophobe. Let it be clear: I love technology, and I couldn't live without my toys. I use them for my job, obviously, but I also enjoy them in my free time. However, I do not enjoy toys because they are cool. I enjoy toys because of what they do for me.

GPS is fantastic. I have a Garmin GPSMAP 76CS and I use it all the time. I am very happy with it. But, it is not perfect, very far from it.

First of all, GPS is based on satellite reception. That means that the receiver needs to be able to see those satellites. That works fine on a boat deck, in a field, even on a highway. It works a lot less well in the centre of a city. Downtown Toronto I lose satellite contact all the time. This is a smaller problem for people who drive cars, because they are more in the middle of the road, but for pedestrians like me, it is somewhat dangerous to walk down the middle of Toronto's Bay Street as if we were cars.

There are of course other possibilities to lose satellite contact besides high buildings. Walking in a forest or bushes with dense foliage is another surefire way. In my experience, smog and fog are enough to lose contact. We have a lot of smog and humidity in Toronto right now, and it is perfectly possible for me to walk allover downtown, without a single satellite lock.

And then, if and when it works, the pinpoint precision is often laughable. Yes indeed, I have seen the GPS tell me it had a precision of +/- 2 meters. However, far more often, it tells me it has a precision of +/-115 meters up to more than +/- 200 meters.

The picture shows you a test I did one early morning. I followed the same track as well as I could, counting the tiles on the sidewalk so I would be within a few centimeters of the same track. I walked from my building West to Yonge Street, North on Yonge Street, East on McGill Street and then through our driveway back to the front of the building. I did this five times. You can see the result in the picture. The red line indicates where I walked according to the map.

I think the message is clear. GPS is great, it gives you a good general idea of where you were, where you are and where you are going, but it is by no means precise, least of all in a big city.


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