Re: [IRCA] [dxld] World is not flat

[Glenn Hauser <wghauser@xxxxxxxxx>]

Glenn, Jim's Great Circle Map (Pizza) has both rectangular and polar map
projections menus. The rectangular map has the curved lines. The polar map has
straight lines, but both are great circle bearings. I assume the map he is
using in this exercise is the polar projection. The menu is slightly different
than his freeware version. Perhaps he can clear this up. Come to think of it,
polar projection may only give straight lines for one point of origin, and not
all points on the map. 73, (Gil NN4CW Stacy, IRCA via DXLD)

I think that only an azimuthal equidistant projection (and therefore centered
on only one location) would give straight lines to any reporter's location.
Polar - per se - wouldn't do the trick. So maybe Jim took a reasonable guess as
to that location, did an az equi and any resulting errors are minimal. Dunno.
I'm just taking an educated guess (Chuck Hutton, ibid.)

Glenn, There's a map (I can't recall the name) which is a projection of the
earth from the standpoint of a globe where the bearings can be drawn as
straight lines. The map is distorted as the map would be if it were a picture
of the earth. This map is used by the military for DFing. I don't know if it's
available for civilians or if it's unclassified (Chuck Bolland, FL, dxldyg via
DX LISTENING DIGEST)

I guess that`s the azimuthal equidistant projection as the previous Chuck just
mentioned. But straight lines as Great Circles apply only to radiating from the
central point of the map. Nothing classified about them; hams commonly have
them custom-made to know where to point their beams (Glenn Hauser, DX LISTENING
DIGEST)

Gents: First, someone wrote to tell me the lines I drew on the 1610 map were
not correct. The map, I replied, was really a great-circle projection and was
correct. Ha! The joke was on me. The map was indeed correct; the LINES were off
by varying amounts. It will take some work to correct the situation. End of
confession!

As of 10:30 this Saturday morning we have three pretty good bits of data on the
1020 signal location. Each of the data has been entered into the "system" and
the results drawn without error (this time). To do this within a reasonable
time frame required some manipulation on
my part and I had to draw the lines by hand and scan the resultant. It is now
posted here: http://tonnesoftware.com/1020.gif

As can be seen, a couple of listeners with decent accuracy in getting the
signal direction from points in central or eastern New York or Pennsylvania
will nail it.

Send your name, city/state, latitude, longitude and the apparent source azimuth
and I'll add it to the list. So far the data seems pretty good (JimTonne, Dec
2, radio-insight.com DX board via DXLD)

Have not yet seen Jim`s direct reply to me, held up in some digest, I suppose.
Latest verson of above map shows the lines are still straight. I don`t see any
way to change the projexion; the buttons on the edge don`t work (Glenn Hauser,
DX LISTENING DIGEST)

Here`s another pertinent post:

We need to refine our direction finding capabilities. My I suggest that instead
of using compass angles from your location use the deviation angle to a known
station transmitter site instead. Transmitter site coordinates are very
accurate. Get a null on a big power house and then retune and adjust antenna
for a null on the unknown signal. Anyone with a Quantum loop or equivalent or a
good quality portable radio can easily do this. For example the test tone on
1020, report your bearing relative to KDKA, KYW, WBAL or other easily heard
powerhouse in the vicinity. From my location the signal from the tone tester on
1020 is about ten degrees south of bearing to KDKA. The station on 1610 was a
few degrees south of bearing to WTAM on 1100. The map created by Jim Tonne
looks good and is a great idea, lets give him more accurate information to use
for a better fix on the tone testers. Anyone with ideas or suggestions to
improve on this are welcome (Tom Jasinski, Shorewood, IL, Dec 2, IRCA via DXLD)

>   From: Glenn Hauser 

>   Jim,
> 
>   Looking at your 1020 map as well as 1610 map, it seems that all the lines
> are
>   straight. Since a bearing is really a great circle, shouldn`t they be
> curved on
>   that map projection? Or is it possible for your base map to be based on
> great
>   circles even tho we don`t really know the center, since that is what we are
>   trying to locate? As someone else mentioned a while ago, NOT using great
>   circles, but drawing straight lines on a flat map can cause large errors at
>   several hundred miles distance, defeating the purpose of this exercise.
> 
>   This is a separate issue, of course, from making correxions for magnetic
>   declination, which is also essential if using a compass.
> 
>   73, Glenn Hauser



 
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