Truncate, don't round when calculating OS 6 figure references (fixes #1)

OSRef.php

@@ -38,11 +38,6 @@ class OSRef {
  * object from the six-figure grid reference TG514131, the easting would
  * be 651400 and the northing would be 313100.
  *
- * Grid references with accuracy greater than 1m can be represented
- * using floating point values for the easting and northing. For example,
- * a value representing an easting or northing accurate to 1mm would be
- * given as 651400.0001.
- *
  * @param int $aEasting the easting of the reference (with 1m accuracy)
  * @param int $aNorthing the northing of the reference (with 1m accuracy)
  */
@@ -67,41 +62,36 @@ public function __toString() {
  * @return string
  */
  public function toSixFigureString() {
- $hundredkmE = floor($this->easting / 100000);
- $hundredkmN = floor($this->northing / 100000);
- $firstLetter = "";
- if ($hundredkmN < 5) {
- if ($hundredkmE < 5) {
- $firstLetter = "S";
- }
- else {
- $firstLetter = "T";
- }
+
+ $easting = str_pad($this->easting, 6, 0, STR_PAD_LEFT);
+ $northing = str_pad($this->northing, 6, 0, STR_PAD_LEFT);
+
+ $hundredkmE = $easting[0];
+ $hundredkmN = $northing[0];
+
+ if ($hundredkmN < 5 && $hundredkmE < 5) {
+ $firstLetter = 'S';
  }
- else if ($hundredkmN < 10) {
- if ($hundredkmE < 5) {
- $firstLetter = "N";
- }
- else {
- $firstLetter = "O";
- }
+ else if ($hundredkmN < 5 && $hundredkmE >= 5) {
+ $firstLetter = 'T';
+ }
+ else if ($hundredkmN < 10 && $hundredkmE < 5) {
+ $firstLetter = 'N';
+ }
+ else if ($hundredkmN < 10 && $hundredkmE >= 5) {
+ $firstLetter = 'O';
  }
  else {
- $firstLetter = "H";
+ $firstLetter = 'H';
  }
 
- $secondLetter = "";
  $index = 65 + ((4 - ($hundredkmN % 5)) * 5) + ($hundredkmE % 5);
- $ti = $index;
- if ($index >= 73) {
+ if ($index >= 73) { //skip the letter I
  $index++;
  }
  $secondLetter = chr($index);
 
- $e = round(($this->easting - (100000 * $hundredkmE)) / 100);
- $n = round(($this->northing - (100000 * $hundredkmN)) / 100);
-
- return sprintf("%s%s%03d%03d", $firstLetter, $secondLetter, $e, $n);
+ return $firstLetter . $secondLetter . substr($easting, 1, 3) . substr($northing, 1, 3);
  }
 
 

README.md

@@ -28,7 +28,9 @@ Output of calculations have been changed from the original code in the following
  * Eastings and northings are rounded to 1m, and lat/long to 5dp (approx 1m) to avoid any
  misconceptions that precision is the same thing as accuracy
  * When calculating surface distances, a more accurate mean radius is now used rather than
- that derived from historical definitions of a nautical mile 
+ that derived from historical definitions of a nautical mile 
+ * Corrected calculation of OS 6-figure grid references (rounding instead of truncating meant the
+ grid square was sometimes off by 1) 
 
 Usage
 -----

tests/OSRefTest.php

@@ -38,11 +38,20 @@ public function testLatLngOSWorkedExample() {
  }
 
  public function testToSixFigureString() {
-
- $OSRef = new OSRef(530140, 184184);
-
- $expected = 'TQ301842';
-
+
+ $OSRef = new OSRef(530140, 184184);
+
+ $expected = 'TQ301841';
+
+ self::assertEquals($expected, $OSRef->toSixFigureString());
+ }
+
+ public function testToSixFigureString2() {
+
+ $OSRef = new OSRef(439145, 274187);
+
+ $expected = 'SP391741';
+
  self::assertEquals($expected, $OSRef->toSixFigureString());
  }