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Parallax On Planes?


halhal
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A scenario: I'm flying east in a plane on the left side of the plane which means I'm looking to the north from my seat. I look down and see land but no major highway. On my GPSr, I see my position as being south of, say, Interstate 80. Given my postion on the GPSr, I should be looking down and seeing the highway. I can't see the highway even looking as straight down as I can.

 

I've chalked this up to the fact the few satellites I'm receiving are all on my side of the plane and their angle pointing down at me would position me south of my real position. However, the GPS experts I have asked say that even those few satellites I'm receiving signals should accurately indicate where I am regardless of the fact that I'm seven miles up in the air and traveling over 500 miles per hour.

 

I have a Garmen emap. Has anyone else had similar experiences? Could there be a parallax issue with less expensive/smaller GPSr's?

 

Hal

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Simply put, there is no parallax issue with GPS. It doesn't work that way.

What Prime Suspect said. The closer the satelites are (either because of their current configuration, or because you only have a narrow view of the sky (window), the greater the potential error in the position:

 

Here are more details:

 

Trimble

 

This page specifically descibes the geometric dilution of precision issue:

 

GDOP

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Do you mind my asking what parallax is?  I remember hearing about it in science class, but I've forgotten already...

I found this example on the web:

What is parallax?

Hold out a pencil at arm's length so that it covers your view of a more distant object. Now close each eye in turn. The pencil seems to move relative to the distant object when a different eye is closed. Each eye looks at the pencil from a slightly different direction. With both eyes open you get more visual clues as to how far away any object is. The following diagram shows the situation:

p2d2.gif

Your brain instinctively determines the object's distance from the slight change in direction (measured by the angle - P). This method of measuring distances is called PARALLAX.

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Oh yeah, now I remember where I heard it, it's how scientists determine how far a star is away from the earth. They look at it once, and then they wait six months and look at it again and see how much and to where it moved.

 

What does that have to do with GPS at all though?

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Chances are you just can't look down enough. It would be a parallax issue but not of the GPS. If you are 5 feet off the ground and the road is 5 feet to the north you would be looking at a 45 degree angle from the window. At 10 feet from the ground you would be looking at a 63 degree angle. At 20 feet you would be looking at a 73 degree angle. At 500 feet you would be looking at a 89.8 degree angle. Anything above 20 feet and you would start to have your view obstructed by the window. As you can see you would never see the road above 500 feet. The math works out so that the road you want to see would need to be about as far north of you as you are high. So you would need to be seven miles south of the road.

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Chances are you just can't look down enough. It would be a parallax issue but not of the GPS. If you are 5 feet off the ground and the road is 5 feet to the north you would be looking at a 45 degree angle from the window. At 10 feet from the ground you would be looking at a 63 degree angle. At 20 feet you would be looking at a 73 degree angle. At 500 feet you would be looking at a 89.8 degree angle. Anything above 20 feet and you would start to have your view obstructed by the window. As you can see you would never see the road above 500 feet. The math works out so that the road you want to see would need to be about as far north of you as you are high. So you would need to be seven miles south of the road.

That's what I needed to know. My mind was thinking I could see "enough straight down" when that isn't the case. I appreciate the comments.

 

Hal

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In skydiving, when you want find the correct spot (exit location) for your jump you have to get you head out the door and look straight down to see exactly where you are. Looking out the window does not work.

 

I suspect spotting with gps is becoming more common now. I haven't mad a jump in at least 4 years.

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