var half = 0.5
   var one  = 1.0
   var two  = 2.0

   var pie  = 3.1415926536
   var miles_to_feet = 5280.0

   var alpha // radian measure
   var beta  // elevation/radius

   var standard_Earth_radius = 3959.0 // in statute miles

   var max_input_distance  = (pie/two)*standard_Earth_radius
   var max_input_elevation =  100.0*miles_to_feet*standard_Earth_radius // about 2 billion feet

   var distance
   var elevation

   var secant
   var tangent

   var elev_depression
   var horizon_distance

function get_Set_ObserverObjectDistance() {
   distance = window.document.elevation_depression.dist_input.value

   if (!checkDistance())
        return
   
   compute_Display_elevationDepression(distance);
}

function get_Set_ObserverElevation() {
   elevation = window.document.horizon_distance.elev_input.value

   if (!checkElevation())
        return
   
   compute_Display_horizonDistance(elevation);
}

function checkDistance() {

   if (distance < 0.0) {
       alert("Distance must be non-negative - reenter value.")
       return false
   }

   if (distance > max_input_distance) {
       alert("Distance must be less than 1/4 of Earth's circumference - reenter value.")
       return false
   }
 
   return true

} // end CheckDistance

function checkElevation() {

   if (elevation < 0.0) {
       alert("Elevation must be non-negative - reenter value.")
       return false
   }

   if (elevation > max_input_elevation) {
       alert("Please enter a sane elevation (in feet).")
       return false
   }
 
   return true

} // end CheckElevation

function compute_Display_elevationDepression(distance) {
   
   alpha = distance / standard_Earth_radius

   if (alpha < 0.1)
       secant = computeSecant(alpha)
   else 
       secant =  one / Math.cos(alpha)

   elev_depression = (secant - one) * standard_Earth_radius
   elev_depression *= miles_to_feet

 //elev_depression = 0.6668 * distance * distance

   window.document.elevation_depression.depression.value = roundA(elev_depression)
}

function compute_Display_horizonDistance(elevation) { 
   
   elevation /= miles_to_feet 
   beta = elevation / standard_Earth_radius 
   
   tangent = Math.sqrt(beta * (two + beta)); // tangent of distance along ground

   alpha = Math.atan(tangent)

   horizon_distance = alpha * standard_Earth_radius

   window.document.horizon_distance.distance.value = roundB(horizon_distance)
}

function computeSecant(alpha) {

  var const4 = 5.0/24.0
  var const6 = 61.0/720.0
  var const8 = 277.0/8064.0

  var a2 = alpha*alpha
  var a4 = a2*a2
  var a6 = a4*a2
  var a8 = a6*a2

  return secant = one + half*a2 + const4*a4 + const6*a6 + const8*a8 
}

function roundA(elev_depr) {

   var inv_precision

   if (elev_depr < 0.000001)
       inv_precision = 1000000000.0

   else if (elev_depr < 0.00001)
       inv_precision = 100000000.0

   else if (elev_depr < 0.0001)
       inv_precision = 10000000.0

   else if (elev_depr < 0.001)
       inv_precision = 1000000.0

   else if (elev_depr < 0.01)
       inv_precision = 100000.0

   else if (elev_depr < 0.1)
       inv_precision = 10000.0

   else if (elev_depr < 1.0)
       inv_precision = 1000.0

   else if (elev_depr < 10.0)
       inv_precision = 100.0

   else if (elev_depr < 100.0)
       inv_precision = 10.0

   else if (elev_depr < 1000.0)
       inv_precision = 1.0

   else if (elev_depr < 10000.0)
       inv_precision = 0.1

   else if (elev_depr < 100000.0)
       inv_precision = 0.01

   else if (elev_depr < 1000000.0)
       inv_precision = 0.001

   else 
       inv_precision = 0.0001

   var rounded  = Math.round(inv_precision*elev_depr)
       rounded /= inv_precision

   return rounded
}

function roundB(h_dist) { // CONSTRUCT THIS METHOD

   var inv_precision

   if (h_dist < 0.0001)
       inv_precision = 10000000.0

   else if (h_dist < 0.001)
       inv_precision = 1000000.0

   else if (h_dist < 0.01)
       inv_precision = 100000.0

   else if (h_dist < 0.1)
       inv_precision = 10000.0

   else if (h_dist < 1.0)
       inv_precision = 1000.0

   else if (h_dist < 10.0)
       inv_precision = 100.0

   else if (h_dist < 100.0)
       inv_precision = 10.0

   else if (h_dist < 1000.0)
       inv_precision = 1.0

   else 
       inv_precision = 0.1

   var rounded  = Math.round(inv_precision*h_dist)
       rounded /= inv_precision

   return rounded
}

function resetElevationCorrectionValues() {
   window.document.elevation_depression.dist_input.value = ""
   window.document.elevation_depression.depression.value = ""
}

function resetHorizonDistanceValues() {
   window.document.horizon_distance.elev_input.value = ""
   window.document.horizon_distance.distance.value = ""
}