Question about ForeFlight Map Ruler feature?

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Eric Reyes

Filing Flight Plan
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So when you place two fingers at a distance apart on the Maps page, a ruler pops up and the tape on the top and bottom side of the ruler lists magnetic course, distance, and the estimate time in route between the two points on each end of the ruler.

If the two points on each end of the ruler are relatively close, I get the exact same information on the top and bottom side of the ruler however if you zoom out the map view that show the entire US and then touch two points that are 1,000 to 2,000 miles away, then numbers on the top side and bottom side of the ruler will be different.

For example I touched near New York and SanFran. The top side of the ruler shows 56 degrees magnetic course from San Fran to New York , 2,181 nm, 20 hours 2 minutes, 109 knots TAS, and 300 magnetic course from New York to San Fran.

However the numbers on the bottom side of the rules say: 6 degree magnetic course from San Fran to New York, 2,172 nm, 20 hours and 2 minutes, and 295 magnetic course from New York to San Fran.

The ruler uses the performance characteristics for the plane I have selected.

So why are the magnetic courses and distances different on the top and bottom sides of the ruler whenever the two points are far away from each other however the estimated travel time is always the same?

ForeFlight tech support didn’t know and said they would get back to me.
 
Because one to going one way, the other is going the other way

Like if you put it north, one side will say 360, the other 180
 
I wonder if it is doing some averaging along the route. If you use the flight plan tool to go from SFO to JFK VORs the NavLog says that the heading is 70°M but if you look at the SFO VOR, the route starts at around the 53° radial. Reverse the route and the heading is 303°M but it hits the JFK VOR at slightly less than the 300° radial. And like your example they don’t add up to 360°.
 
JScarry said:
I wonder if it is doing some averaging along the route. If you use the flight plan tool to go from SFO to JFK VORs the NavLog says that the heading is 70°M but if you look at the SFO VOR, the route starts at around the 53° radial. Reverse the route and the heading is 303°M but it hits the JFK VOR at slightly less than the 300° radial. And like your example they don’t add up to 360°.
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The radial doesn't really tell us much of anything. Not only do you have the station declination error (you can find some very short Victor airway segments here the two ends are different) but I would not be surprised if on a direct route that long, the software calculates it in great circle terms.
 
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Eric Reyes said:
So when you place two fingers at a distance apart on the Maps page, a ruler pops up and the tape on the top and bottom side of the ruler lists magnetic course, distance, and the estimate time in route between the two points on each end of the ruler.

If the two points on each end of the ruler are relatively close, I get the exact same information on the top and bottom side of the ruler however if you zoom out the map view that show the entire US and then touch two points that are 1,000 to 2,000 miles away, then numbers on the top side and bottom side of the ruler will be different.

For example I touched near New York and SanFran. The top side of the ruler shows 56 degrees magnetic course from San Fran to New York , 2,181 nm, 20 hours 2 minutes, 109 knots TAS, and 300 magnetic course from New York to San Fran.

However the numbers on the bottom side of the rules say: 6 degree magnetic course from San Fran to New York, 2,172 nm, 20 hours and 2 minutes, and 295 magnetic course from New York to San Fran.

The ruler uses the performance characteristics for the plane I have selected.

So why are the magnetic courses and distances different on the top and bottom sides of the ruler whenever the two points are far away from each other however the estimated travel time is always the same?

ForeFlight tech support didn’t know and said they would get back to me.
Click to expand...

I got 057 degrees M, 2144 miles and 296 degrees magnetic going the other way on the top edge. On the bottom edge it was 292, 2137 and 061. I'm guessing it's trying to deal with variation somehow which changes from about 14E to 14W on that route. Where the 7 mile difference comes from I can't guess. Let us know what they say
 
F780B38F-B4FD-4596-A272-E29087A3BDD5.png Because the shortest distance is a great circle, not a straight line since the surface of the earth is curved. The shortest route between the two actually looks like this.
 
Wheels said:
View attachment 60917 Because the shortest distance is a great circle, not a straight line since the surface of the earth is curved. The shortest route between the two actually looks like this.
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That.
And the rep CSR on the phone should know that. He needs more training in the app features, they should be able to answer such simple questions.
 
I don't think the question is "why are the bearings not reciprocals of each other?" - that is due to great circle routes.

I think the question is that in Foreflight, the information on top and bottom of the finger ruler is duplicated, so that you can read it in two orientations, whichever is easiest. But in the OPs example, the bearings and even distances don't match. The bearing issue could be related to averaging or something, but the distance should be exactly the same either direction.

I just did a test with the same results as the OP. I wonder - since it draws a rectangle, is it actually measuring distance and bearings along the sides of the rectangle, instead of along the centerline of it? This would result in slightly different answers. Look closely and you'll see what I mean.
 
Wheels said:
View attachment 60917 Because the shortest distance is a great circle, not a straight line since the surface of the earth is curved. The shortest route between the two actually looks like this.
Click to expand...

The charts/maps in ForeFlight aren't flat maps like on your example. You're correct, in the mercator projection map of your example the shortest line between two points is a great circle, but on ForeFlight the shortest distance is a straight line. If you look, the flight path is the same as your great circle line. Someone from ForeFlight please chime in. SFO-JFK.jpg
 
The paper charts are like that too. A straight line is approximately a great circle route. The lines of latitude "sag". They distort the map. They have to. There is no way to draw an absolutely correct map of a sphere (planet earth (and its not really a sphere)) on a flat piece of paper.

You dont need a ruler. The tick marks on the lines of longitude are 1nm apart. This is because one minute of latitude equals one nautical mile. 360 x 60 = 21,600. The earth's circumference is 21,600 nm. So just take a piece of paper, mark the distance and put it on the line of longitude on the map and count the tick marks.
 
RussR said:
I don't think the question is "why are the bearings not reciprocals of each other?" - that is due to great circle routes.

I think the question is that in Foreflight, the information on top and bottom of the finger ruler is duplicated, so that you can read it in two orientations, whichever is easiest. But in the OPs example, the bearings and even distances don't match. The bearing issue could be related to averaging or something, but the distance should be exactly the same either direction.

I just did a test with the same results as the OP. I wonder - since it draws a rectangle, is it actually measuring distance and bearings along the sides of the rectangle, instead of along the centerline of it? This would result in slightly different answers. Look closely and you'll see what I mean.
Click to expand...
Yeah. That would account for the slight distance differences. Seems odd they'd 'program' it to read the edge of the ruler rather than the chosen route centerline.
 
SkyDog58 said:
What does ForeFlight say?
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Now a line in that description has me curious.

“When using the ruler in flight, the current groundspeed will be used to show the travel time between the points being measured. When not in flight, the TAS from your current route or default aircraft will be used instead.”

So the backward route side of the ruler will show a negative number in flight? I’m going the wrong way! :)

Hahahahaha. I have to go test this while moving now. I wanna see how they did what they claimed they do up there on the bottom of the ruler. :) :) :)
 
Does it consider the flight profile as well? For instance if I come over the mountains, my descent rate and speed will be greater than when it’s time to go back where I’ll have an anemic climb and lower speed to build enough altitude to cross.
 
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