MILE-MARK BASIC CODE                                        30 DEC 2003
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This pseudo BASIC code is my routine for reading the MM.DAT file while 
looking for the road that matches the given interstate name. When found 
it draws 1 mile ambiguity circles for every Nth mile mark. 

'This routine overlays Mile-Mark circles every Nth mile along the
'highway named HWY$. The names are of the form SS## where SS is the
'state abbreviation and ##... is the interstate number.
   
 INPUT "State-Interstate (SSNN)", HWY$: HWY$=UCASE$(HWY$) 
 OPEN "MM-DATA.DAT" FOR INPUT AS #3
 LINE INPUT #3, a$    'skip the first line comment
 Nth = 2                   'sets every  2nd MM below  2 mile Range Scale
 IF RS >  2 THEN Nth =  5  'sets every  5th MM below 16 mile Range Scale
 IF RS > 16 THEN Nth = 10  'sets every 10th MM below 32 mile range scale
 IF RS > 32 THEN Nth = 20  'sets every 20th MM above 32 mile range scale
 DO UNTIL Id1$ = "END"                  'read data ID's till the end
    INPUT #3, Id1$, K1, LA1, LO1        'read the next data line
    IF HWY$ = Id1$ THEN                 'If Highway is found then do it
       INPUT #3, Id2$, K2, LA2, LO2     'Get starting point (or next one)
       FOR Mk = 0 TO 900 STEP Nth       'For up to 900 MM's do every Nth 
           IF Mk > K2 THEN              'Get next line if needed
              Id1$ = Id2$: K1 = K2           'and save the values
              LA1 = LA2: LO1 = LO2           'before reading in the next
              INPUT #3, Id2$, K2, LA2, LO2
           END IF
           IF HWY$ <> Id2$ THEN EXIT FOR     'exit at end of highway
           LA = LA2 - (K2-Mk) * (LA2-LA1) / (K2-K1) 'interpolate LAT
           LO = LO2 - (K2-Mk) * (LO2-LO1) / (K2-K1) 'interpolate LON
           y = Yval(LA)       'convert LAT to my Y map coordinates
           x = Xval(LO)       'convert LON to my X map coordinates
           r = 175 / RS: IF r < 6 THEN r = 6 'scale 1 mile circle with
           CIRCLE (x, y), r, 15              'map Range Scale (RS)
           CALL DoLabels(x, y, STR$(Mk))     'Add mile mark label 
       NEXT
    END IF
 LOOP
 CLOSE #3