Are sectional paths correct across "long" distances?

No. Straight lines on Lambert Conformal maps are not great circles.

It's my understanding that the Lambert Conformal is better than any
other flat surface at representing the curved surface of the earth in
such a way that a straight line on the chart comes very close to being
a Great Circle.

Any straight line through the exact center of a chart, regardless of
direction, will be precisely a Great Circle. A line across a corner
of the chart will be the poorest representation of a Great Circle, but
still "good enough for government work." Probably as close as the
average GA pilot can hold a course, anyway.

vince norris

Awhile ago I pointed out in rec.aviation.piloting that one of my
tools will generate a map using stitched sectionals for a given
route.
http://groups.google.com/groups?hl=e....edu.au#link10
Ben Jackson mentioned that it didn't look correct to just draw a
straight line between two points so far away (across multiple
sectionals). I have looked into it a few times but I haven't
come up with a definitive answer.

So...anyone know the answer? Pilots are certainly accustomed to
drawing straight lines on a sectional to find the shortest path
between two points, and I've never been taught to do anything
other than align sectionals by sight to plan multi-sectional
flights. Does this not work over long distances?

One path I know fairly well is LAF-MER. The Great Circle path
happens to go right near Denver (where I usually stop). If that
path is plotted as a straight line on the sectionals
https://aviationtoolbox.org/Members/...selected.x=411
it appears to follow the path I'd expect.
https://aviationtoolbox.org/Members/...selected.x=427

Also, there's an easily-identified area on that path where Iowa,
Illinois, and Missouri meet. Take a look at the Great Circle
route.
http://gc.kls2.com/cgi-bin/gcmap?PAT....380N+120.568W
Again, this seems to match the area on the straight-line path
drawn on the sectional.
https://aviationtoolbox.org/members/...selected.y=324

Anyone know for sure whether or not this is an accurate way of
depicting Great Circle paths in the conUS?

Thank you.

--kyler

In a previous article, Kyler Laird said:
Anyone know for sure whether or not this is an accurate way of
depicting Great Circle paths in the conUS?

No. Straight lines on Lambert Conformal maps are not great circles. We
use it normally because within one section it doesn't make a huge
difference, but if you're crossing several, the errors add up.


--
Paul Tomblin http://xcski.com/blogs/pt/
"Pilots are reminded to ensure that all surly bonds are slipped before
attempting taxi or take-off"

vincent p. norris writes:

Any straight line through the exact center of a chart, regardless of
direction, will be precisely a Great Circle. A line across a corner
of the chart will be the poorest representation of a Great Circle, but
still "good enough for government work." Probably as close as the
average GA pilot can hold a course, anyway.

I decided to finally test this. I drew Great Circle segments on top of
the straight line path. The difference is small but significant.
https://aviationtoolbox.org/Members/...=1453666.76955
(The yellow line is straight. The red is made of ten GC segments.)

Time to start using GC calculations...

--kyler

"Kyler Laird" wrote in message news:h2sei1-

So...anyone know the answer? Pilots are certainly accustomed to
drawing straight lines on a sectional to find the shortest path

As a technical matter, the only chart projection on which a drawn straight
line is a great circle is a gnomonic. These are rarely used, particularly
over large areas, as they show about as much distortion as the standard
Mercator we all grew up with (remember thinking that Greenland was about
twice the size of the US?).

The Lambert Conformal projection, however, is made such that a straight
line, while not precisely a great circle, is so close that the differences
are inconsequential. Oceanic plotting charts used in aviation to monitor
navigation progress are Lamberts. The standard oceanic enroute chart is a
Mercator, but the plotting chart is Lambert.
Sectional charts are also Lamberts,iirc.

So, the short answer to your question is, just lay out the line, and go.
Note, though, that the straight line on your patched sectionals will require
you to alter heading periodically.

JG

"John Gaquin" wrote in message
...

"Kyler Laird" wrote in message news:h2sei1-

So...anyone know the answer? Pilots are certainly accustomed to
drawing straight lines on a sectional to find the shortest path

As a technical matter, the only chart projection on which a drawn straight
line is a great circle is a gnomonic. These are rarely used, particularly
over large areas, as they show about as much distortion as the standard
Mercator we all grew up with (remember thinking that Greenland was about
twice the size of the US?).

The Lambert Conformal projection, however, is made such that a straight
line, while not precisely a great circle, is so close that the differences
are inconsequential. Oceanic plotting charts used in aviation to monitor
navigation progress are Lamberts. The standard oceanic enroute chart is a
Mercator, but the plotting chart is Lambert.
Sectional charts are also Lamberts,iirc.

So, the short answer to your question is, just lay out the line, and go.
Note, though, that the straight line on your patched sectionals will
require
you to alter heading periodically.

JG

"the straight line on your patched sectionals will require you to alter
heading periodically" precisely because you will be flying close to a great
circle route, which requires a constantly changing heading.

Harvey

"John Gaquin" wrote in message
...

As a technical matter, the only chart projection on which a drawn straight
line is a great circle is a gnomonic.


A straight north-south line is a great circle on all the common chart
projections.

Any straight line through the exact center of a chart, regardless of
direction, will be precisely a Great Circle. A line across a corner
of the chart will be the poorest representation of a Great Circle, but
still "good enough for government work." Probably as close as the
average GA pilot can hold a course, anyway.

I decided to finally test this. I drew Great Circle segments on top of
the straight line path. The difference is small but significant.

That's a very interesting chart, Kyler.

I can't see the red GC line very well except against the dark brown of
the higher elevations; but it seems as if the two lines are only about
a line-width apart. I wouldn't consider that "significant," but of
course that's a personal judgment.

My reaction is the opposite of yours: I'm impressed by how well the
straight line follows a Great Circle.

Can you tell me how many nautical miles separate the two lines, at the
point of widest divergence?

Thanks. vince norris

"Steven P. McNicoll" wrote in message

A straight north-south line is a great circle on all the common chart
projections.



Correct. Those are the two [possibly rare] exceptions to my post -- if you
happen to be flying a course of true north or south anywhere, or a course of
true east or west on the equator, then your course will layout as a straight
line and will be a great circle on any chart projection. I probably should
have mentioned it, lest someone get lost and run out of fuel.

In article ,
(Paul Tomblin) wrote:

In a previous article, Kyler Laird said:
Anyone know for sure whether or not this is an accurate way of
depicting Great Circle paths in the conUS?

No. Straight lines on Lambert Conformal maps are not great circles. We
use it normally because within one section it doesn't make a huge
difference, but if you're crossing several, the errors add up.

You've got to go pretty big distances before GC errors start to become
significant. For example, to go from 38N/77W to 38N/122W (roughly
Washington, DC to San Francisco, CA), the rhumbline is 270 and the GC is
284. 14 degrees on a coast to coast trip. If you're flying it nonstop
in a jet, it makes sense to take that into account. For most of us
flying spam cans, we just can't fly long enough legs for it to become
significant.

I just tried another one. From 38N/77W to 38N/100W is just under 1100
nm, or about the limit for the longest legged GA airplane I know of.
Again, a rhumbline of 270, CG of 277 (7 degrees correction).

For most of us, CG routes are just not something to worry about.