diff --git a/MAGFit/index.html b/MAGFit/index.html
index 21598ae6..4da50d67 100644
--- a/MAGFit/index.html
+++ b/MAGFit/index.html
@@ -76,6 +76,20 @@ <h1 style="text-align:center"><a href="" style="color: #000000; text-decoration:
             </fieldset>
         </td>
         <td style="width: 10px;"></td>
+        <td>
+            <fieldset style="width:200px;height:80px">
+                <legend>Location
+                    <img id="TT" src="../images/question-circle.svg" style="width: 1em; vertical-align: bottom" 
+                    data-tippy-content='Location used to get the expected earth field from the World Magnetic Model. Filled in from the EKF origin or last position in the log, or from the AHRS_ORIGIN_LAT and AHRS_ORIGIN_LON parameters. If the log has no location, for example when flying without GPS, enter it here. The Calculate button must be clicked after any change of location.'
+                    data-tippy-maxWidth='750px'/>
+                </legend>
+                <label for="Lat">Latitude</label>
+                <input id="Lat" name="Lat" type="number" min="-90" max="90" step="any" onchange="set_need_calc(true)" style="width:90px"/><br><br>
+                <label for="Lng">Longitude</label>
+                <input id="Lng" name="Lng" type="number" min="-180" max="180" step="any" onchange="set_need_calc(true)" style="width:90px"/>
+            </fieldset>
+        </td>
+        <td style="width: 10px;"></td>
         <td>
             <fieldset style="width:200px;height:80px" id="ATTITUDE">
                 <legend>Attitude source
@@ -90,7 +104,7 @@ <h1 style="text-align:center"><a href="" style="color: #000000; text-decoration:
             <input id="fileItem" type="file" accept=".bin" onchange="readFile(this)"><br><br><br>
             <input type="button" id="calculate" value="Calculate" onclick="loading_call(() => { calculate(); })">
         </td>
-        <td style="width: 300px;"></td>
+        <td style="width: 90px;"></td>
         <td style="text-align:right; vertical-align:top">
             <input id="OpenIn" type="button" value="Open In" disabled><br><br><br>
             <img id="TT" src="../images/question-circle.svg" width="30px" 
diff --git a/MAGFit/magfit.js b/MAGFit/magfit.js
index 6fc31719..7f312942 100644
--- a/MAGFit/magfit.js
+++ b/MAGFit/magfit.js
@@ -784,8 +784,41 @@ function find_end_index(time) {
 }
 
 // Run all calculation steps
+// Update the expected earth field from the location inputs, return false if there is no valid location
+function update_earth_field() {
+    const Lat = parseFloat(document.getElementById("Lat").value)
+    const Lng = parseFloat(document.getElementById("Lng").value)
+    if (isNaN(Lat) || isNaN(Lng)) {
+        return false
+    }
+    if ((earth_field != null) && (earth_field.Lat === Lat) && (earth_field.Lng === Lng)) {
+        // No change
+        return true
+    }
+
+    const field = expected_earth_field_lat_lon(Lat, Lng)
+    if (field == null) {
+        return false
+    }
+    console.log("EF: " + field.vector[0] + ", " + field.vector[1] + ", " + field.vector[2] + " at Lat: " + Lat + " Lng: " + Lng)
+
+    earth_field = field
+    earth_field.Lat = Lat
+    earth_field.Lng = Lng
+
+    // Expected body frame field must be re-calculated
+    source = null
+
+    return true
+}
+
 function calculate() {
 
+    if (!update_earth_field()) {
+        alert("Enter a valid latitude and longitude in the Location box")
+        return
+    }
+
     select_body_frame_attitude()
 
     calculate_bins()
@@ -1652,7 +1685,7 @@ function calc_error(A, B) {
     return ret
 }
 
-function extractLatLon(log) {
+function extractLatLon(log, get_param) {
   var Lat, Lng
   if (("ORGN" in log.messageTypes) && ("instances" in log.messageTypes.ORGN) && (0 in log.messageTypes.ORGN.instances)) {
     const ORGN = log.get_instance("ORGN", 0)
@@ -1667,6 +1700,12 @@ function extractLatLon(log) {
     Lng = POS.Lng[POS.Lng.length-1] * 10**-7
     return [Lat, Lng]
   }
+  // Recorded origin params, zero if not set
+  const origin_lat = get_param("AHRS_ORIGIN_LAT")
+  const origin_lng = get_param("AHRS_ORIGIN_LON")
+  if ((origin_lat != null) && (origin_lng != null) && ((origin_lat != 0) || (origin_lng != 0))) {
+    return [origin_lat, origin_lng]
+  }
   return [Lat, Lng]
 }
 
@@ -1948,13 +1987,11 @@ async function load(log_file) {
     // Assume constant earth field
     // Use origin msg
     // Use last EKF origin for earth field
-    var [Lat, Lng] = extractLatLon(log)
-    earth_field = expected_earth_field_lat_lon(Lat, Lng)
-    if (earth_field == null) {
-        alert("Could not get earth field for Lat: " + Lat + " Lng: " + Lng)
-        return
-    }
-    console.log("EF: " + earth_field.vector[0] + ", " + earth_field.vector[1] + ", " + earth_field.vector[2] + " at Lat: " + Lat + " Lng: " + Lng)
+    // If there is no location in the log it can be entered manually
+    var [Lat, Lng] = extractLatLon(log, get_param)
+    earth_field = null
+    document.getElementById("Lat").value = (Lat != null) ? Lat : ""
+    document.getElementById("Lng").value = (Lng != null) ? Lng : ""
 
     // Workout which attitude source to use, Note that this is not clever enough to deal with primary changing in flight
     const EKF_TYPE = get_param("AHRS_EKF_TYPE")
@@ -2223,6 +2260,12 @@ async function load(log_file) {
         }
     }
 
+    if (!update_earth_field()) {
+        alert("No location found in log, enter latitude and longitude in the Location box then click Calculate")
+        set_need_calc(true)
+        return
+    }
+
     calculate()
 
 }