Multiline Object Format

WX Home

Example:
SPCSVR>APRS:;SPCS1528z*262100z3500.00NS07730.00WWSvr TStormWatch #174 }e0]FgcBS6:W{QFSAA
"FgcBS6:W"  is the string from the example which gives the 4 corners of a polygon

The multiline weather object packet is a standard aprs object up to the "}" which starts the multiline portion.

Thereafter-
  1. Single character denoting type of line.
    Line Type
    Character Color Type of Line Use in NWS Messages
    a red solid Tornado Warning
    b red dashed Tornado Watch
    c red double dashed N/A
    d yellow solid Severe Thunderstorm Warning
    e yellow dashed Severe Thunderstorm Watch (Example Packet)
    f yellow double dashed N/A
    g blue solid Test Warning
    h blue dashed Test Watch
    i blue double dashed N/A
    j green solid N/A
    k green dashed Mesoscale Discussion Areas
    l green double dashed N/A

  2. Single character denoting type of object
    Shape Type
    CharacterShape Type
    0Closed Polygon
    1Line Segment
    ?TBD

  3. Single Scale Character
    Scale character (ascii value) = (integer(log to base 10 of the scale factor relative to .0001 degree) X 20 ) + 33

    Example:
    tenths of a degree = scale factor 1000 ( .0001 X 1000 = .1)
    log to the base 10 of 1000 = 3
    3 X 20 = 60
    60 + 33 = 93
    ascii 93 = "]"

    other decimal degree scale factors are:
    .0001 = "!"
    .001 = "5"
    .01 = "I" (Used by NWS for Areas Of Maximum concern for tornados and Thunderstorms)
    .1 = "]" (Used by NWS for Watch Boxes)
    1 = "q"

  4. 2 character offset (can be up to 23 sets of these before the sting gets too long for aprs)
    First Character represents latitude offset relative to the position of the object in the standard portion of the packet
    Second Character represents longitude offset.

    Offset Character = (integer( offset in degrees / scale factor)) + 78

    Range is limited to 44 scale factors plus and 45 minus from the reference.
    Upper case "N" = ascii 78 is zero offset
    Lower case "z'   = ascii 122 or +44 scale factors ( 122-78 = 44)
    Exclamation "!" = ascii 33 or -45 scale factors (33 - 78 = -45)
Programming Notes:

Wxsvr always places a space before the multiline sequence - and shares the "{" character with the sequence number at the end of the packet which is always 5 alphanumeric characters in length.  This can be used to filter false polygons generated by random garbage.  The multiline starts with a "<space>}" then printable characters with no spaces up to 46 characters then "{" and 5 alphanumerics at the end.

This spec is western hemisphereic centric in that North and West are assumed to be positive.  This is the way the NWS deals with it.  For other coordinate systems, the offsets may need to be reversed- i.e. where West longitute is defined as negative.

This protocol is versatile enough to allow a small "map" to be transmitted on the fly.

Copyright © 2014 - Peter Loveall AE5PL