Inconsistent runway numbering

[Minsky]

While working on my aerodrome tables, I noticed that, while most of the runways on the Channel map are numbered based on true headings, two of them have their numbers based on what appears to be magnetic headings of the WWII era.

 

The violators are:

 

Headcorn 10-29 - another error here, should be 10-28 magnetic or 09-27 true
TRUE: 092-272
MAG: 104-284

 

High Halden 11-29
TRUE: 102-282
MAG: 113-293

 

Measured in 1940s (according to DCS, the average magvar for this period is +11)

 

Two random airfields from this map for comparison:

 

Lympne 13-31
TRUE: 133-313
MAG: 145-325

 

Merville Calonne 14-32
TRUE: 139-319
MAG: 149-329

Edited April 27, 2021 by Minsky

[Minsky]

I haven't included Biggin Hill in the list above, because this one is kinda tricky.

 

While its primary runway 05-23 has true headings, the other two are numbered based on magnetic headings:

 

 

05-23 TRUE: 048-228 MAG: 059-239 = true headings

04-22 TRUE: 028-208 MAG: 039-219 = magnetic headings

12-30 TRUE: 108-288 MAG: 119-299 = magnetic headings

 

Now, here's the photo dated 1960. Note the runway numbers: 05-23, 03-21 and 11-29:

 

 

And here's the same place in 1940:

 

 

Runway "05-23" didn't have any numbers back then, but if it did, it would probably be 06-24. And this will give us a perfect match with its magnetic headings (059-239). The same match that two other runways have.

 

 

[Frederf]

There are more headings than just true and magnetic in DCS. It stands to reason that if ruler is considered true heading (and it is for much of DCS) then measurement of lines of longitude and latitude would always be 360-090-180-270 but they aren't. Going north and ruler 360 aren't the same thing. What you are seeing in DCS is a globe Earth protected onto a 2D grid. This necessarily involves distortion. Generally when a terrain is made for DCS the orientation of things like runways is traced from a map projected onto the grid.

 

If the ruler heading of a longitude which passes through a runway is 359 and the ruler alignment of the runway is 001 then that runway is actually rotated 2 degrees clockwise from the line of longitude or 002 true. The farther toward the edges of the terrain the larger the difference between longitudes and ruler. To find the real true heading of a runway the ruler direction of both the runway and the nearest longitude line should be measured. The heading is the number of degrees rotation clockwise from longitude north to runway. Then apply the known magnetic variation to this and you should find good agreement with the historical name of the runway (numbers painted in photos, number in documents, etc.).

[Minsky]

 

1 hour ago, Frederf said:

To find the real true heading of a runway the ruler direction of both the runway and the nearest longitude line should be measured. The heading is the number of degrees rotation clockwise from longitude north to runway. Then apply the known magnetic variation to this and you should find good agreement with the historical name of the runway (numbers painted in photos, number in documents, etc.).

 

I'm not gonna pretend I understood everything you wrote, and I can't apply this to The Channel map (it doesn't have the geographical grid overlay --yet?), but here's the Caucasus example. Can you explain how to calculate the real true heading here?

 

 

[Frederf]

That would be starting along the longitude rotating CW 5 degrees rotation to get "page upward" orientation plus another 331 CW to align with the runway for a true heading of 5+331 = 336. Take 7 degrees off for variation and that's 329 MH. Gudauta is less useful as an example because it's a military airbase and the layout is semi-fictional plus its true heading wouldn't be in any public document to compare. See picture below. The actual TH of the runway is about 331 and you can see the ~5 degree twist in the photo.

 

We can check is Tbilisi UGTB 31L because it has public documentation as a civil airport plus I think it's rendered without drastic artistic changes like Gudauta. By AIP 31L has THDG 316.56°. DCS ruler shows 308 (308.0007 using XZ grid math, 307.998628 using the ILS localizer lua) and longitude 352 (352.0113 using XZ grid). (360-352.0113)+308.0007 = 315.9894° and that's close but not exactly real 316.56. Take off E6.5 for default ME date and that's 310.1 MH which agrees with 31L naming.

 

Compare that to taking 308 ruler minus 6.5 variation to get 301.5 and wondering why runway is called 31L and not 30L. This happens a lot when the difference between longitude and map up is so large (8° Tblisi). It happens less when difference is small (5° Gudauta) and rarely closer to the central area (3° Anapa). At the special meridian (33°E) it's impossible.

 

You should be able to find where the longitude direction is in Channel by using Ctrl-Y to bring up L/L display and fly different headings until you find the one that doesn't change your longitude value.