2
$\begingroup$

At Nome Alaska-- KOME-- the E2 airspace (dashed magenta line) is listed as extending 13.2 (nautical) miles east of the airport-- according to this source page E-9 -- https://www.faa.gov/documentLibrary/media/Order/JO_7400.11C.pdf --

"Within a 4.1-mile radius of the Nome Airport and within 3.4 miles each side of the Nome Airport 106° bearing extending from the 4.1-mile radius to 13.2 miles east of the airport..." 'AMENDMENTS 07/07/05 70 FR 22590 (Revised) Corr: 70 FR 33349""

But when I measure on the online sectional chart http://vfrmap.com/?type=vfrc&lat=64.513&lon=-165.444&zoom=10, using the tic marks on the lines of longitude (the lines running north and south) as a scale equal to nautical miles, the far edge seems to measure more like 15.5 nautical miles from the middle of the airport symbol (as shown by the little blue square) -- now why would that be? That's about 20% further -- about 2.3 miles different -- not insignificant.

What is going on here?

Is the scale wrong, or is the dashed magenta line shown in the wrong place, or is the airport shown in the wrong place?

I assume "east" in the above usage means "generally east, measuring along the 106-degree radial".

It's not a one-time thing, I'm getting a similar discrepancy with some other similar measurements of other similar regions of Class-E-to-surface airspace, always biased in the same direction.

Improve this question
$\endgroup$
4
  • $\begingroup$ I just tried it and came as close to 13.2 as I could expect to get. I even tried going to the furthest NE corner due East and don't get close to 15.5. I can get shorter than that by going to the little blue square centered on Nome City, but I can't get further than that. The only way I can stretch it to 15 is if I go to the edge of the blue compass card around the VOR. So, I'm not sure what you are doing, but it appears correct scale to me... $\endgroup$
    – Michael Hall
    May 6, 2019 at 23:18
  • $\begingroup$ Measuring from the airport on the left, the one with the big paved runways? I tried it again too and got no change. $\endgroup$
    – quiet flyer
    May 6, 2019 at 23:23
  • $\begingroup$ Clearly they are using True bearings/ radials not magnetic but that's not the cause of the discrepancy here $\endgroup$
    – quiet flyer
    May 7, 2019 at 4:52
  • $\begingroup$ @MichaelHall - I get over 17 miles (tick marks) if I measure from the airport, generally toward the ESE, perpendicular to the far E border of the dashed magenta area, but extend the measurement all the way to the blue compass rose that is centered on the VOR. I'm truly baffled about what is going on here. $\endgroup$
    – quiet flyer
    May 7, 2019 at 4:55

2 Answers 2

Reset to default
2
$\begingroup$

All the clues were there; another member figured it out (see comments). My computer screen was stretching out the image in the left-right sense, so when I took my nautical miles scale off the tick marks on the north-south line of longitude, and then made an east-west measurement, the measurement ran significantly high. The question noted I had seen this elsewhere and the fact that that was also on an east-west measurement, but at much lower latitude than Alaska (neither of which were noted in the question), should have been a clue to me as to what was going on. Nothing to do with high latitudes and nothing to do with errors in the way the airspace or the map grid were charted.

I looked at the same url on a different computer screen and the measurement came out fine.

Improve this answer
$\endgroup$
3
  • $\begingroup$ @MichaelHall good job in figuring this out; sorry for the waste of time! $\endgroup$
    – quiet flyer
    May 7, 2019 at 5:30
  • $\begingroup$ No worries, I welcome a challenge! Glad you got it sorted out, that was a puzzling one. It hit me this morning that a good way to verify any distortion of the aspect ratio would be to measure a perfect circle, (like the VOR ring) in the X and Y axis to check that they are equal North and South. $\endgroup$
    – Michael Hall
    May 7, 2019 at 15:40
  • $\begingroup$ Now that you mention it I've noticed before when using a MapSource program for downloading GPS traces that circles did not look round; I thought it was an artifact of that program $\endgroup$
    – quiet flyer
    May 7, 2019 at 15:47
-1
$\begingroup$

The sectional map “ticks” and 30 mile grid squares do correspond to nautical miles. The sectional represents a 3D object (the earth) which is roughly a sphere in two dimensions. This is the reason you will find that the left and right edges of the sectional are actually not straight, but curved, and in the US (northern hemisphere) The bottom (South) part is bigger than the top (North) part because the earth at lower latitudes is larger in diameter than it is at higher latitudes. Measure the sectional with plotting tools, like an E6B with a sectional scale, and you will also find that vertical measurements that go straight north and south (measuring across latitudes) are accurate (due to my above explanation, and the fact that degrees latitude are equidistant from each other at all such points) and that east/west measurements vary drastically as you go up in latitude (further north) because lines of longitude “scrunch together” as you approach the poles. You will find the same with WAC charts in the Southern Hemisphere, except when approaching the South Pole at lower latitudes... the same effect but on the other side of the equator. The error rather nulls out at the equator.

You can even see this curvature in the overview of sectional charts included on the front of each one: sectional overview Compare this to the higher latitude Alaska charts: enter image description here

You can also see the difference in the curvature/shape of the Houston sectional at ~30 degrees north latitude vs the Green Bay sectional just 15 degrees of latitude north of Houston. This is why 2D direct measurements that do not take a 3D great circle into account are always wrong when taken east-west.

enter image description here enter image description here

I found this when plotting flight plans several years ago and it is an interesting point to realize that when measuring with a scale, it will give you inaccurate distances unless the route of flight is straight north/south. Basically, we cannot measure a segment of a great circle while projecting the 3D earth onto a 2D map with a 2D measuring instrument that doesn’t consider scale variance between latitude and longitude in such a projection.

Improve this answer
$\endgroup$
14
  • $\begingroup$ Thanks but-- surely that can't account for the kind of distortion I'm reporting can it? Certainly I understand why the ticks on the east-west lines no longer equate to nautical miles when we are far away from the equator, but the ticks on the north-south lines still are good. But on any given chart, a straight line measured with a ruler is surely a reasonable approximation of a true great circle route, at least on scales of 20 miles or less, isn't it? Is the deal really just that Nome is so far north that these errors are exaggerated this much? $\endgroup$
    – quiet flyer
    May 7, 2019 at 1:53
  • $\begingroup$ @quietflyer this error is extremely exaggerated in northern latitudes. The closer you get to the North or South Pole the worse it gets. It sounds pretty reasonable to me from what I’ve seen. This is something I teach my students pilots when training them how to plan cross country flights. One other thing: the sectional is not legal, it is an aid to navigation... the FAA order is. Sometimes things are not perfectly to scale and this happens sometimes. I can’t speak to the website, or as to how they compute measurements, but without using great circle (3D) measurement, the error is unavoidable. $\endgroup$
    – Pugz
    May 7, 2019 at 2:00
  • $\begingroup$ Well-- Nome is pretty far north, but not so far north that the lines of latitude on the sectional don't APPEAR to be straight lines of you lay down a straight edge and check a length on the order of 20 miles, with the projection that they are using. Lots of useful points in your answer but I'm not convinced that's the whole story here. My best guess is that it may simply be an error in charting the airspace, maybe they want to err on the conservative side? $\endgroup$
    – quiet flyer
    May 7, 2019 at 2:11
  • $\begingroup$ @quietflyer for what it’s worth, I just did a great circle measurement on the sectional and it comes out to right at 13.2 nautical miles. Even on short flights (50-60 miles) in lower latitudes, the error is regularly several nautical miles when measuring with a straight edge/scale - and not due to measurement error, but due to the curvature problem. Again, this error at that latitude seems totally reasonable from my experience (I also have a background in GIS and mapping systems :-) $\endgroup$
    – Pugz
    May 7, 2019 at 2:16
  • 1
    $\begingroup$ Down voted as this answer is wrong. The tic marks on the lines of longitude (the lines running north and south) ARE equal to 1 nautical mile. This holds true at any latitude. There is also negligible error on a short route segment like the example given. $\endgroup$
    – Mike Sowsun
    May 7, 2019 at 13:22

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .