September 18, 2006

Good Questions About GPS Devices And Compasses

GPS Says NoI got a good question about the image to the right. I’m going to break it down into two questions. The image is confusing partly because of the way the GPS works and partly because I was not careful in the way I arranged it.

Q1: Is the GPS really telling us which direction is north in this picture.

A1: In a way “no” and in a way “yes.”

(Part a) “No” in the sense that this GPS does not have a compass feature in it, even though one of the display screens looks like a compass. I cannot turn it on, put it down, and have it tell me which direction is north, like you can with a compass.

Here’s what it does do. If I am traveling more than 2 miles per hour, it uses my change in location to tell me where north is with respect to my direction of travel. The angle I hold the GPS does not matter, only my direction of travel matters. The GPS assumes that I’m holding it with the top away from me, and that that is my direction of travel. So, if I walk east at 3 MPH, and hold the GPS flat in front of me, the display of the GPS will show “EAST”.

If I were to stop and suddenly walk backwards quickly, the display will suddenly reverse and now show I am walking west, even though I am still facing east. It’s the walking that matters.

This becomes obvious if you walk north while looking at the display and then stop completely. The display says “NORTH.” Turn in place in a circle. The whole time it will continue to say “NORTH” even as you face every compass point.

This causes all sorts of problems for beginner geocachers, and it’s no wonder because it’s just not obvious.

(Part b) “Yes” this GPS is trying to tell me something about north even while I’m not moving. The little sun icon is telling me that if I line it up with the Sun, even if I am not moving, the “N” on the display’s dial should point north. That fact, combined with the shadows (which line up with the bottom and top edges of the image) indicate that the “N” on the GPS dial is basically indicating north, but only because I have placed the GPS down semi-carefully.

The problem is, scrutinizing this picture I can see that I wasn’t that careful. A line through the sun and the center of the display is not exactly horizontal. But it’s close. If it’s off, you can blame the way I placed it.

Q2: But wait a minute! The needle on the compass doesn’t match the needle on the GPS. What gives? Is it a problem with magnetic north?

A2: You’re right, the needles don’t line up. In this case, that’s because of another feature of the GPS that happens to be activated. The “needle” on the GPS display isn’t trying to point north. It’s trying to point me to a nearby geocache which happens to be at a bearing of about 204 degrees (that’s south-southwest) and 19 feet in that direction. It’s the fake dial on the GPS display that’s trying to give us our compass directions (as described above).

Of course, the actual compass on the right has a needle that moves freely to indicate north, and a dial below that can be spun arbitrarily for different purposes. In this picture I’ve basically tried to line the dial up with the body, and line them both up with the needle, but I wasn’t being careful about it. So, ignore the green dial.

BTW - the GPS is set to use magnetic north because that makes it easier for me to use it with my compass.

Bonus Q3: Hey, don’t some GPS receivers have a built-in electronic compass?

A3: Yes, some do. Since I carry a few pretty responsive magnetic compasses anyhow (they don’t need batteries and you shouldn’t be out and about without a backup) I don’t feel the need for the electronic compass. Keeping them separate allows me to look at a different display of the GPS while looking at the compass.

Posted by James at September 18, 2006 7:23 PM
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