procedure Measure(dataName : Str255; x1, y1, x2, y2 : longint;showDialog : boolean);

Measures the specified line and returns the following:

longintArray[1] = delta X 
longintArray[2] = delta Y 
realArray[1] = slope 
realArray[2] = ArcTan2(deltaY, deltaX) 
realArray[3] := calibrated distance 

Also see:  GetLIndex  GetRIndex