Convergence Factor

[MJR]

Hi folks

I have to do a presentation on map reading and in studying the ICAO 1:500,000 chart I note that it has a convergence factor of
0.78829. If this is a Lambert conformal Conic projection does this assume a mean lattitude of 52 degrees?

And secondly, so what! Why is it important to know the convergence factor and what does it really mean?

cheers

MJR

[oxford blue]

The convergence factor tells you what the inclination of the meridians to each other is (or the rate at which the track angle of a straight line on the map changes - which is the same thing). In this case it is telling you that it is .78829 degrees of convergence per degree of change of longitude everywhere on the chart, whereas on the Earth this occurs only at 52 degrees N (or S).

The main reason for knowing the convergence factor is that it is the rate at which a straight line on the chart is changing its direction with respect to True North. In most cases, and certainly near the parallel of origin of the chart (52N), this will be very close to the rate at which a Great Circle track changes.

If you fly by gyro (as opposed to flying by magnetic compass) you will fly a Great Circle track.

If you need more detail than this for your presentation, read a good textbook on navigation.

Alll the best,

Oxford Blue

[dorosenco]

MJR,

to be more accurate 0.78829 is called the "Constant of the Cone" for a Lambert Conformal chart.

The exact equation is 0.788 = sin (52). Where 52N is called the "Parallel of origin" and is given for any Lambert Conformal projections.

The Convergence Angle = Change of Longitude x Constant of the Cone.

[reynoldsno1]

Lamberts charts can have more than one standard parallel BTW, depending on the cone angle. These should also be marked on the chart somewhere.

[mbcxharm]

This is in danger of getting confusing! I think Oxford Blue pretty much summed things up very well but to summarise the properties of a Lambert's Chart, is has:

1 (and only one) Parallel of Origin, where the convergency on the chart (i.e. the angle with which the meridians of longitude come together as you go North in the Northern Hemisphere) is equal to the Earth's convergency. Elsewhere on the chart this will not be the case.

2 Standard Parallels (either side of the Parallel of Origin, although the P of O is not exactly halfway between) where the scale on the chart is exactly what it says on the tin. This is where the cone used for the projection intersects the reduced Earth. The scale contracts between the standard parallels and expands outside of them.

I'm not aware of any chart that you could get hold of where there is only 1 standard parallel, which would have to be the same as the parallel of origin. I'm willing to be corrected though...:ouch: