Standard Parallels used on ICAO 500k charts ?

[hugel]

The ICAO charts use a Lambert conformal conic projection which is only completely scale-accurate along the two standard parallels, with the distortion increasing as you move away from these lines.

My question is: I am looking at a single ICAO chart (northern Germany) which may be one of a large set (Europe presumably). Is the projection generated using standard parallels that best display the complete area of Europe and then chopped up into individual charts, or is the projection performed so that the parallels fall within a single chart ? If it is done on a single chart basis, do the individual charts fit together ?

Thanks for any help
hugel

[hugel]

OK I am digging a bit deeper and note from :

the “one-sixth rule” places the first standard parallel at one-sixth the range above the southern boundary and the second standard parallel minus one-sixth the range below the northern limit (figure below). There are other possible approaches.

also:

The standard parallels are usually chosen at 1/6 and 5/6 of the latitude range of the map.

As regards the standard parallels used by aero charts, this seems to answer the question from the jeppesen website (my bold):

In order to support the European wide standard, a Lambert Conformal Conic Projection with Standard Parallels at N37° and N65° is used for production of all VFR+GPS charts. This enables the seamless combination of all charts and the use for continuous flight planning. A scale distortion is a consequence of the projection and varies with the distance from the Standard Parallels. The true scale is depicted on every VFR+GPS chart on the Cover panel (e.g. EB/EH: True Scale at N51° - 1:515.000) and near the Scale Bar. In addition, a factor is given to multiply measured distances with every parallel (e.g. 1.03).

Is this also the case for the ICAO charts (I cannot find a similar scale multiplier on the chart at which I am looking)...?

Does anyone have any idea of the worst case scale accuracy for such a 1/6, 5/6 projection ?

hugel

[Tim Zukas]

I was too lazy to figure out the formulas for the spheroid, but if you only need a rough idea we can use the sphere-- if we do, it turns out 1:515500 is the scale at lat 51.8-51.9, with standard parallels 37 and 65. That's the minimum scale between 37 and 65.

[bookworm]

Interesting question.

ICAO Annex 4 Ch 16 requires that WAC projections are Lambert Conformal Conic in separate band for each tier of charts. The standard parallels for each 4 degree band are 40 minutes from the north and south extremes of the tier. There is a recommendation in Ch 17 that half-mil charts do the same.

This suggests that the projection of an ICAO half-mil is likely to be on a single chart basis and, unlike the Jepp charts, you should not expect them to fit together.