APRS RANGE SCALES: 6 Jan 2003 ------------------------------------------------------------------------- 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