Mark Kolber wrote in
:

When we look at a map, our minds generally expect it to be to scale.
If the final approach course is 10 NM long, we will tend to (actually
should) use that segment as a "ruler" for situational awareness. That
2000' AGL tower 5 NM left of the FAC "looks" 5 NM away. Unfortunately,
the similar tower on the missed that takes us 10 NM away from the
airport might be closer than it appears on the chart.

Thanks Mark, that makes sense.

The circle is =visually= telling us the limits. A simple statement
that "Not to scale beyond 10 miles" may be enough for you, but some
people want a picture. Ever notice that DPs and STARS are done both in
text and graphically?

Here's where I get confused, and maybe the problem is that I'm thinking
about it too much. Every NACO chart I've looked at is to scale, even
outside the reference circle. The exceptions to that are when the wiggly
lines are shown on the route segments, or the conentric dashed circles
indicating enroute & feeder facilities. Does anyone know of a NACO chart
that is not to scale and does not show either the wiggly lines or the
concentric dashed circles?

Your point about obstructions outside the reference circle is good.
That's what got me measuring distances on the charts and discovering
that things even outside the reference circle are actually to scale. By
the way, NACO charts are the same scale as a sectional (when the
reference circle is 10 NM).

By way of example, take a look at the approaches to Boise, ID (BOI).
Most of them show a 7310' obstacle 12.5 NM NNE of the airport. That
obstacle is outside the reference circle, but it's to scale on every
chart. Additionally all the feeder routes are to scale unless they have
the wiggly lines on them (or there are the concentric dashed circles).

Compare the Boise ILS 10R approach to the RNAV (GPS) 10L. Both show
feeder routes from the RENOL intersection, and RENOL is not shown to
scale on either plate as indicated by the wiggly line. I find the GPS
plate easier to read since it doesn't have the reference circle
distracting me. If the whole point of the reference cirlce is to show me
the boundary of where the "to scale" part of the map ends, then the
wiggly lines from RENOL are redundent on the ILS plate.

Notice on the ILS 10R approach, there's a feeder route from SALLA
without wiggly lines on the route. SALLA is drawn to scale at 14.4 NM
out (4.4 NM outside the reference circle).

So my contention is the reference circle doesn't really show anything,
and I find it distracting. I think I can safely ignore it. Please
correct me if you feel that is unsafe to just ignore the reference
circle.

Phil
www.pfactor.com