Convert between linear referenced M values and Station Values given conversion locations as an array.

station-convert.js

var toStation = function(m,calib,fmt){
    var retValue;
    var r = null;
    if(m < calib[0][0] || m > calib[calib.length-1][0]){
        return 'm is out of bounds, valid range is ' + calib[0][0] + ' to ' + calib[calib.length-1][0];
    }
    for(var i = 0; i < calib.length; i++){
        if(m >= calib[i][0] && m < calib[i+1][0]){
            r = ((m - calib[i][0]) / (calib[i+1][0] - calib[i][0]) * (calib[i+1][1] - calib[i][1])) + calib[i][1];
            break;
        }
    }
    if(fmt===true){
        retValue = parseInt(r).toString();
        if(retValue.length>=3){
            retValue = retValue.substr(0,retValue.length-2) + '+' + retValue.substr(-2);
        }else{
            retValue = retValue.length < 2 ? '0+0' + retValue : '0+' + retValue;
        }
    }else{
        retValue = r;
    }
    return retValue;    
}
 
var toM = function(station,calib){
  var numStation = Number(station.toString().replace('+',''));
  var r = null;
  if(numStation < calib[0][1] || numStation > calib[calib.length-1][1]){
    return 'station is out of bounds, valid range is ' + calib[0][1] + ' to ' + calib[calib.length-1][1];
  }
  for(var i = 0; i < calib.length; i++){
  	if(numStation >= calib[i][1] && numStation < calib[i+1][1]){
  	  r = ((numStation - calib[i][1]) / (calib[i+1][1] - calib[i][1]) * (calib[i+1][0] - calib[i][0])) + calib[i][0];
  	  break;
  	}
  }
  return r;
}
 
console.log(toStation(27,[[0,0],[200,250],[750,900]],true));
 
console.log(toStation(352,[[0,0],[200,250],[750,900]],true));
 
console.log(toStation(800,[[0,0],[200,250],[750,900]],true));
 
console.log(toM('4+45',[[0,0],[200,250],[750,900]]));
 
console.log(toM(203,[[0,0],[200,250],[750,900]]));
 
console.log(toM('-1+32',[[0,0],[200,250],[750,900]]));

console.log() statements are included for running in firebug or other JS run-time for testing.