projections in long range plotting charts

[compressor stall]

As an academic exercise I am trying to create (in ARCGIS 9.1) some ETOPS plotting charts covering aerodromes from mid to high latitudes. The extremities of two aerodromes are at latitudes 40/42 and the other pair of aerodromes at latitudes 67/78.

I am using a Lambert's Conformal Conic projection, standard parallels of 55 and 65.

The trouble is that the 90 minute range rings between the lat 67 and lat 78 aerodromes are not the correct distance apart on the map due to the projection. ie. I have told it to project a 609nm ring around both airports. The airports are 1144nm apart, but the two 609nm rings do not overlap - they miss by about 30nm.

Any suggestions, other than reduce the extent of the map to try and eliminate this error?

What are real etops charts like in their coverage, and what projections, extents and std parallels are used?

Are there any ETOPS charts that cover this vertical (n/s) extent? Or are ETOPS charts for say LAX-LHR broken down into more bitesize chunks, thus eliminating this problem.

Thanks in advance

[Thridle Op Des]

Interesting problem CS, The North Atlantic Plotting Chart we use is a LCCP as well with SP's of 5140N and 7820N. The Chart coverage is from 3500N to 7500N. The size of the circles will depend on the latitude, but for the life of me I cannot remember the calculation for the scale change. I'm sure there is someone out there who does. I use AutoCAD Map incidentally, but try to avoid circles like the plague for the reasons stated above - ie I'm lazy!

Regards

TOD

[compressor stall]

Thanks for that initial feedback.

I can't believe that I would be the first to come across the problem, so thought this would be a good place to start!

My company has them, but for the exercise, I thought I would try to make some as an academic exercise and learn how to use ARCGIS better.

[BAMRA wake up]

Hmm, I'm not sure if I know the answer but I know a man who does - I'll ask on Monday. However, sounds like your standard parallels are close together relative to the extreme aerodromes (from an old text book, limit the projection to: 4/6 of the latitude range between std parallels and max 1/6 of the range above and below std parallels. Distance scale 1.0 at std pars, 1.01 at 1/6 above and below and 0.99 at mid lat).

[Old Smokey]

The problem is not insurmountable, but there's a lot of work involved. My suggestion for such a mathematically complex shape is to calculate the Lat and Long of 360 Radial / Distances (609 nm) from the contingency airport, and join them. It works well in other plotting applications where the shape is complex, and the calculation of the series of Lat and Longs are 'straight' spherical trigonometry.

It probably has a name, but for the moment, in your honour, I've labeled it the "Compressorstalloidodrome.:ok:

Lots of luck,

Old Smokey

[Brian Abraham]

My solution would be to use this program "COMPSYS 21 Geodetic Calculations" (very simple to use) to calculate points around the arc and plot on map. Cumbersome, but I'm a simple guy. Program free at http://avn.faa.gov/index.asp?xml=naco/online/compsys

[compressor stall]

Thanks for the replies and PMs. Flat out at the moment with a few things, will get back to you in a few days.

[LH2]

I do not have access to the tools in question at the moment, but it cannot be a projection issue unless there's a bug in the software.

My best guess is you are not using the same ellipsoid or datum as your reference chart. Also keep in mind that 30nm is only a 2.5% error (assuming they are supposed to be tangent at exactly one point) so a lot of factors could be involved, and not just with your chart.

If you can pass me the projection, datum and ellipsoid data you have used, and the coordinates of the origin point for the circles as well as the coordinates of a point in the circumference on the, for example, 360, 090, 180, 270 radials, I'll have a check and see if they come up to what they're supposed to be.

[alphacentauri]

Are you using the BUFFER command in ARCGIS9.1??

If so, make sure that you go into the advanced settings and set the environment variables up to apply the buffers to a projection. ARCGIS by default will do all its GIS commands in what is known as 'geographicals' which equals no projection. As as example buffers on a UTM projection should appear as ovals. But if you let ARCGIS set the default they will appear as circles and the area of coverage will be wrong.

I suspect this is whats happening when you say;

The trouble is that the 90 minute range rings between the lat 67 and lat 78 aerodromes are not the correct distance apart on the map due to the projection. ie. I have told it to project a 609nm ring around both airports. The airports are 1144nm apart, but the two 609nm rings do not overlap - they miss by about 30nm.

A good way to check is as Brian Abraham recommends and that is to calculate some points on the circumference and then cross check them against what the buffer command gives you.

Good luck mate

[Tim Zukas]

Dunno exactly what your academic exercise is-- but if you're trying to find the lat-lon that's X nm from each of two given lat-lons, why wrestle with map projections? Just calculate it, using successive approximation with the geodetic formulas. My calculator program does it in maybe 30 seconds; it'll take longer with online calculators, but still not prohibitive.