APRS RANGE SCALES: 6 Jan 2003
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WB4APR
The absence of a common reference to "Range Scales" opens up one of my
"APRS standardization" issues that prevents us all from speaking a common
language. That is zooms and range scales. APRS was designed with the
concept of Range Scale. Thus unambiguously, anyone can refer to a map
Range scale and everyone else will all know what he can see. There are
two things about many APRS implementations that I dont like:
1) Absence of any MAP SCALE legend at all
2) Totally arbitrary references to ZOOM scales (X-Zooms)
What could be simpler than the concept of Range Scale?
If you are on a 64 mile "range Scale" that means that everything within
64 miles of the center is visible. If you zoom to the 8 mile range scale,
then anyone else that is on an "8 mile range scale" (no matter what
version of APRS he is running) he will see the same "area". It is only
one line of code to convert from an arbitrary "zoom factor" or "map size"
to a meaningful "range scale". And APRS would be so much the better
for it...
Remember, the Range scale does not need to be "an exact measurement"!
It simply means that if I am looking at a 64 mile range scale in APRSdos,
then "anything within 64 miles of the center of my screen IS VISIBLE to
me. The fact that the screen is actually wider and the top and bottom of
my screen show out to about 90 miles or so don't matter. The Bottom
line is that a circle of that radius will fit on my screen. The range
scales are defined as powers of two. That is 1,2,4,8,16, etc... and of
course down to 1/2, 1/4, 1/8, 1/16th, 1/32, etc miles.... (or Km scale)
The ability of anyone with any platform of APRS to say "go to the 32 mile
range scale centered on Washington DC, and then BOTH of us will be
looking at the same area. This is fundamental to human comnmunication
with APRS!
BACKGROUND:
Maps (like radar scopes) have TWO variables that completely define the
field of view. 1) MAP CENTER. and 2) RANGE.
APRSdos defines "RANGE SCALE" for every map view. THis is only an
"approximate" referece to the radius of the largest circle of radius that
will fit on the map view and uses powers of 2. (thus it doesnt care if the
view is round or square or a 3/4 aspect ratio. This "RANGE SCALE" is a
universal reference used by all military, and civilian enteties involved
in RADAR screens, tactical plots and now MAP displays.
If I am looking at a MAP and you are looking at a map, and we are
communicating by VOICE, APRS message, or whatever means is available
to us, we can see TOTALLY different things. (I may be zoomed into the
2 mile scale and you may be looking at the 128 mile scale.) but we are
BOTH looking at the same town (or so we think)...
Thus, NO map view on any system is "described" unless you also know the
"RANGE SCALE". I find that this concept of "RANGE SCALE" must be
pervasive throughout APRS if we are to communicate between users.
APRSdos can zooom from any range scale from 8182 miles down to .0625 miles
in powers of 2. ALthough I would like to see powers of two, it is not
required. As long as I am on an 8 mile scale and say you are on a 10 mile
scale, then we should see about the same thing.
I communicate 'RANGE SCALE" to my users in 3 ways:
1) UPPER left corner of all maps shows "RANGE = N miles"
2) Bottom of map has a "distance LEGEND bar" that goes from the
bottom center of the map and to the right N miles. There are
no tick marks, just a gentle reminder that that bar is N miles
long and represents the Range Scale.
3) Whenever the user presses the R key or does any other function that
involves distnance of objects from OWN station, it overlays a
"radar" style bull's-eye who's outer ring has a radius of N miles.
4) Note, all of these can be displayed in equivalent METRIC units also
But in any case, looking at any map, anwyere, there is no ambiguity of
scale. I do NOT like many APRS implementations' maps. Many do not
display ANY concept of range or scale. Some maps do show a small
1/2 inch long "1" mile mark, but I find it impossible on large maps
to "infer" from that any meaningful feeling of how big a map I am
looking at without stoping to think... And I hear it on the
voice nets all the time. Two guys absolutely not agreeing to what the
other is seeing but they BOTH swear they are looking at the same "map".
But without a range scale, who can tell?
ZOOM scales are similarly important.
Its crazy to use zoom scales of 1X, 2X, 4X, 8X, which have no reference
to anything consistent. One one map, the 1X scale might be 5 miles.
on another, the 1X scale might be 50 miles. This is so easy to fix
when a single algebraic equation could trivially convert that to the
standard APRS RANGE-SCALE in miles (or Km). Defining "RANGE SCALE"
as a means of describing map views was fundamental to the definition
of APRS so that people with different platforms could communicate
verbally about what they were "seeing"...
Bob Bruninga
WB4APR