Sectional chart distances east-west at different latitudes

[LongRoadBob]

I'm sure this is just a brain fart but...
I passed the exam. It's been a little while now since then, and I realized that latitude lines can be used to judge nautical mile distances when flying relatively north-south, but I am stuck (in a northern latitude, 60 deg is my home base) with the thought that the longitudinal lines cannot really reflect distances since the distance is less as one goes more northern.

So though I know one degree of latitude is 60 nm, how can the longitudinal lines be used to judge distance? Say one is flying NE, there has to be a (shorter?) variance to actual distance, or is the great circle playing into this somehow?

A distance at 60 deg north can not be the same at 71 deg. North. On the chart. How does this work again?

 

[dmspilot]

That's because you can't. A nautical mile is ~ 1/60th of a degree of latitude, not longitude. Lines of longitude are very close together at the poles while latitude lines are parallel and spaced equally.

 

[Cap'n Jack]

What sort of chart are you using? A sea chart uses a Mercator projection and latitude gets distorted.

As this is a aviation forum, the sectionals used are Lambert Conic conformal projections. The choice of "standard parallels" reduces the distortion fairly close to those latitudes

 

[Van Johnston]

A very close approximation is that the length of a minute of longitude varies with the cosine of the latitude. So at the equator they are the same, at 45 deg latitude about .7x. Looks like you are about 60 deg N, so about .5x.

 

[Capt. Geoffrey Thorpe]

Thumb and one finger on the two points, then move your hand to a line of longitude or the scale on the edge. Digital Calipers.

 

[Jeff Oslick]

Digital Calipers.

That method only works if you aren't in the habit of telling fish stories.

 

[Eric Gleason]

Like Cap'n'Jack says, the type of projection used on aviation charts is designed so that there is little distortion in the area of the map. Any direction you place your plotter should result in accurate distance and heading measurements.

At least that's how it is on NOAA charts for the US. I'd assume it's the same for NavCanada charts.

 

[Hank S]

Like Cap'n'Jack says, the type of projection used on aviation charts is designed so that there is little distortion in the area of the map. Any direction you place your plotter should result in accurate distance and heading measurements.

At least that's how it is on NOAA charts for the US. I'd assume it's the same for NavCanada charts.

But Bob is at the end of a longer road--he's in Northern Europe . . . . Lord only knows what they use there.

My guess is simply that the sectionals are drawn to the same scale, and the longitude lines get closer together. If your flight is much longer than your plotter, measure it with a yard stick (meter stick?) and divide by the number of plotter lengths.

I only use Lat & Long to line up when plotting across more than one sectional. My longest trip used 5 sectionals.

 

[luvflyin]

But Bob is at the end of a longer road--he's in Northern Europe . . . . Lord only knows what they use there.

My guess is simply that the sectionals are drawn to the same scale, and the longitude lines get closer together. If your flight is much longer than your plotter, measure it with a yard stick (meter stick?) and divide by the number of plotter lengths.

I only use Lat & Long to line up when plotting across more than one sectional. My longest trip used 5 sectionals.

RIP WAC Charts. I miss those things

 

[LongRoadBob]

That's because you can't. A nautical mile is ~ 1/60th of a degree of latitude, not longitude. Lines of longitude are very close together at the poles while latitude lines are parallel and spaced equally.

Yeah, I knew this but sometimes mix the two, since one refers to the other...

What sort of chart are you using? A sea chart uses a Mercator projection and latitude gets distorted.

As this is a aviation forum, the sectionals used are Lambert Conic conformal projections. The choice of "standard parallels" reduces the distortion fairly close to those latitudes

But Bob is at the end of a longer road--he's in Northern Europe . . . . Lord only knows what they use there.

My guess is simply that the sectionals are drawn to the same scale, and the longitude lines get closer together. If your flight is much longer than your plotter, measure it with a yard stick (meter stick?) and divide by the number of plotter lengths.

I only use Lat & Long to line up when plotting across more than one sectional. My longest trip used 5 sectionals.

Here we also use Lambert "conformal conic" and as an example if I measure longitude line distance at 60 degrees it is 11.1 cm.

When I move north to say 63 deg north, now the distance is 10.4.

So mainly here I was just thinking about, if one plans a trip east to west or diagonal part of the distance is going to be off per chart scale and trying to figure out how significant. Or check too that I'm correct about this.

I mean taking it further, if I'm right near the North Pole, using one of these, I'd be thinking I could fly at a couple thousand knots...east or west.

 

[kath]

I mean taking it further, if I'm right near the North Pole, using one of these, I'd be thinking I could fly at a couple thousand knots...east or west.

I'm confused about what the question is exactly...

Even if you're flying above the Arctic Circle, you'd be using a chart which has been projected in such a way so that "1 nautical mile" is the same scale in all directions (north or east or whatever), and is the same as what your brethren at low latitudes are using. So you can take the little "sectional ruler" with NM on it, or the "chart scale" printed in the corner, and use it to measure distances in NM, no problem.

Now, that chart is going to have longitude lines that look tightly spaced together, with fewer NM between them. But that's just fine; the definition of a NM has nothing to do with longitude lines at all.

If you left your little ruler at home, or just wanted to use the lat/long lines by themselves to estimate distance for some reason, you could use the spacing between LATITUDE lines as a proxy for 60 miles, but you'll have to use trigonometry before doing the equivalent on the Longitude lines, since they get closer together according to the cosine of the latitude.

 

[chemgeek]

If you look at a mid-latitude sectional chart closely, you will discover that the half-degree lat-long grids are not square but rectangular, with the width narrower than the height, i.e. longitude lines are closer together then the latitude lines as you go from the equator to the pole. The map projection, plus the scale as well, makes the longitude lines appear parallel.

 

[LongRoadBob]

I'm confused about what the question is exactly...

Even if you're flying above the Arctic Circle, you'd be using a chart which has been projected in such a way so that "1 nautical mile" is the same scale in all directions (north or east or whatever), and is the same as what your brethren at low latitudes are using. So you can take the little "sectional ruler" with NM on it, or the "chart scale" printed in the corner, and use it to measure distances in NM, no problem.

Now, that chart is going to have longitude lines that look tightly spaced together, with fewer NM between them. But that's just fine; the definition of a NM has nothing to do with longitude lines at all.

If you left your little ruler at home, or just wanted to use the lat/long lines by themselves to estimate distance for some reason, you could use the spacing between LATITUDE lines as a proxy for 60 miles, but you'll have to use trigonometry before doing the equivalent on the Longitude lines, since they get closer together according to the cosine of the latitude.

Thanks. Even if you didn't understand my question, you answered it and I see where my reasoning was faulty.
So, within reason no matter the longitude, the lambert is accurate in distance. Because the long. Lines are not meant to be a specific distance on the ground.

So I can trust, no matter which direction that measuring in scale is correct. I just got confused there suddenly, because lots of ground school books mention using the span of thumb to little finger (I checked and it is roughly 60 nm for me, so average hands) stretched but just comfortable. Tip of thumb to first line is around 9 nm.

And even though they mentioned it in NAV course, that each degree Lat. is 1nm. and I understood, it only dawned on me later that using only N-S track one could accurately read off NM. So I got to thinking about the longitude and started a faulty logic train of thought.

Thanks all, got my mind set right now...

 

[azure]

Thanks. Even if you didn't understand my question, you answered it and I see where my reasoning was faulty.
So, within reason no matter the longitude, the lambert is accurate in distance. Because the long. Lines are not meant to be a specific distance on the ground.

If I remember correctly, it's only approximately accurate in distance. The Lambert projection has two reference latitudes, and between them the map scale is roughly constant, but not exactly so. Good enough for government work, though! (This is from memory, and my memory is a bit foggy, but I think that's basically correct.)

And even though they mentioned it in NAV course, that each degree Lat. is 1nm. and I understood, it only dawned on me later that using only N-S track one could accurately read off NM. So I got to thinking about the longitude and started a faulty logic train of thought.

I think you meant to say that each minute of latitude is 1 nm. Each degree of latitude is 60 nm. (I think this is approximate as well. In fact it has to be, since the Earth is not a perfect sphere.)

 

[Matthew]

https://en.m.wikipedia.org/wiki/Lambert_conformal_conic_projection

It helps me to visualize how the chart is drawn.

It allows a straight line drawn on the chart to approximate a great circle.

 

[LongRoadBob]

If I remember correctly, it's only approximately accurate in distance. The Lambert projection has two reference latitudes, and between them the map scale is roughly constant, but not exactly so. Good enough for government work, though! (This is from memory, and my memory is a bit foggy, but I think that's basically correct.)


I think you meant to say that each minute of latitude is 1 nm. Each degree of latitude is 60 nm. (I think this is approximate as well. In fact it has to be, since the Earth is not a perfect sphere.)

Yes, I did mean that. I usually am pretty precise, but have been sloppy here with the terminology. Like I say, I know it is minute, and use it correctly, but no excuse.

 

[Cap'n Jack]

Yeah, I knew this but sometimes mix the two, since one refers to the other...





Here we also use Lambert "conformal conic" and as an example if I measure longitude line distance at 60 degrees it is 11.1 cm.

When I move north to say 63 deg north, now the distance is 10.4.

So mainly here I was just thinking about, if one plans a trip east to west or diagonal part of the distance is going to be off per chart scale and trying to figure out how significant. Or check too that I'm correct about this.

I mean taking it further, if I'm right near the North Pole, using one of these, I'd be thinking I could fly at a couple thousand knots...east or west.

@kath explained it better than I, but she said what I was trying to explain. I'm glad you understand now.

 

[BillTIZ]

I always used speed dividers and measured against the north south lines, 1 arc minute = 1 nm.

Speed dividers, set the apex to your groundspeed, Long side of dividers measure distance, short side of dividers measure time in minutes against the latitude lines.