Although there are parts of Northern Ireland that fall within the GB Grid, Northern Ireland Grid References should relate one of the Irish Grid reference systems.

Public Shared Function IsValidOs(EN As EastNorth) As Boolean
 If EN.East <= 2 OrElse EN.East >= 699999 OrElse EN.North < 2 OrElse EN.North >= 1249999 Then Return False
 ' technically, the grid is 0-700000 by 0-1250000 (inclusive) however Northern Ireland should use Irish Grid
 ' and there is no other land on extreme edges of grid. Values within 2m of edges can cause problems in some programs.
 Return True
End Function
  

Private Function Square_IsValid(sq As String) As Boolean
 ' Square references should have 2 chars & 10 digits
 ' If you encounter less than 10 digits, it may be an approximate reference
 ' squares never contain the letter I
 Dim ea, no As Integer
 Dim s1, s2 As Char
 sq = sq.ToUpper.Replace(" ", String.Empty)
 If sq.Length <> 12 Then Return False
 If sq.Contains("."c) Then Return False
 If Not Integer.TryParse(Mid(sq, 3, 5), ea) Then Return False
 If Not Integer.TryParse(Mid(sq, 8, 5), no) Then Return False
 s1 = CChar(Mid(sq, 1, 1))
 s2 = CChar(Mid(sq, 2, 1))
 Select Case s1
  Case "H"
   If Not "LMNOPQRSTUVWXYZ".Contains(s2) Then Return False
   If no > 500000 Then Return False
  Case "J"
   If Not "LMQRVW".Contains(s2) Then Return False
   If no > 500000 Then Return False
  Case "N", "S"
   If Not "ABCDEFGHJKLMNOPQRSTUVWXYZ".Contains(s2) Then Return False
  Case "O", "T"
   If Not "ABFGLMQRVW".Contains(s2) Then Return False
  Case Else
   Return False
 End Select
 Return True
End Function
  

Private Function EastNorth_To_12Figure(EN As EastNorth) As String
 ' Although this type of grid reference is called 12 figure, North Scotland locations may require a 13th digit
 ' Leading zeros (if present) MUST be included in 12 figure grid references
 ' Always round down to integer when converting from easting/northing to 12 figure (bottom left od square containing reference)
 ' When converting 12 figure to easting/northing add 0.5 toshow middle of square containing reference
 ' Eastings and Northings can be accurate to 1mm, however, 12 figure refrences are only accutate to 1m
 ' The space is optional but recommended when reference is being used in a spreadsheet. (Space discourages spreadesheet removal of leading zeros.)
 Dim ea, no As Integer
 ea = Math.Floor(EN.East)
 no = Math.Floor(EN.North)
 Return ea.ToString("000000") & " " & no.ToString("#000000").Trim
End Function
  

Private Function Figure12_To_EastNorth(fig12 As String) As EastNorth
 Dim ret As EastNorth
 Dim eInteger, nInteger As Integer
 Dim eString, nString As String
 ret.East = 0 : ret.North = 0 ' default value indicating invalid fig12 value
 fig12 = fig12.Replace(" "c, String.Empty)
 If fig12.Length < 12 OrElse fig12.Length > 13 Then Return ret
 eString = Mid(fig12, 1, 6)
 nString = Mid(fig12, 7)
 If Not Integer.TryParse(eString, eInteger) Then Return ret
 If Not Integer.TryParse(nString, nInteger) Then Return ret
 If eInteger <= 0 OrElse eInteger >= 700 Then Return ret
 If nInteger <= 0 OrElse nInteger >= 1250 Then Return ret
 ret.East = eInteger + 0.5
 ret.North = nInteger + 0.5
 Return ret
End Function
  

Private Function EastNorth_To_Square(en As EastNorth) As String
 ' Square references (with 1m accuracy) are always two letters followed by 10 digits
 ' All digits (including leading zeros must be included
 ' Optional spaces allowed before first digit and between 5th/6th digit
 ' when the easting or northing is invalid,function will return an empty string
 Dim eInteger As Integer = Math.Floor(en.East)
 Dim nInteger As Integer = Math.Floor(en.North)
 If eInteger <= 0 OrElse eInteger >= 700000 Then Return String.Empty
 If nInteger <= 0 OrElse nInteger >= 1250000 Then Return String.Empty
 Dim eSquare As Integer = eInteger \ 100000
 eInteger = eInteger Mod 100000
 Dim nSquare As Integer = nInteger \ 100000
 nInteger = nInteger Mod 100000
 Dim sq1, sq2 As Char
 If eSquare < 5 Then
  Select Case nSquare
   Case < 5 : sq1 = "S"c
   Case >= 5 < 10 : sq1 = "N"c
   Case Else : sq1 = "H"c
  End Select
 Else
  Select Case nSquare
   Case < 5 : sq1 = "T"c
   Case >= 5 < 10 : sq1 = "O"c
   Case Else : sq1 = "J"c
  End Select
 End If
 Dim eSqMod5 As Integer = eSquare Mod 5
 Dim nSqMod5 As Integer = nSquare Mod 5
 Dim alpha As String = "VWXYZQRSTULMNOPFGHJKABCDE"
 sq2 = CChar(Mid(alpha, eSqMod5 + (nSqMod5 * 5) + 1, 1))
 Return sq1 & sq2 & " " & eInteger.ToString("00000") & " " & nInteger.ToString("00000")
End Function
  

Private Function Square_To_EastNorth(sq As String) As EastNorth
 ' Will return East=0,North=0 for an Invalid square reference
 Dim ea, no As Integer
 Dim err, ret As EastNorth : err.East = 0 : err.North = 0
 sq = sq.ToUpper.Replace(" "c, String.Empty)
 If sq.Length <> 12 Then Return err
 If Not Integer.TryParse(Mid(sq, 3, 5), ea) Then Return err
 If Not Integer.TryParse(Mid(sq, 8, 5), no) Then Return err
 Select Case Mid(sq, 1, 1)
  Case "S" : ea += 0 : no += 0
  Case "N" : ea += 0 : no += 500000
  Case "H" : ea += 0 : no += 1000000
  Case "T" : ea += 500000 : no += 0
  Case "O" : ea += 500000 : no += 500000
  Case "J" : ea += 500000 : no += 1000000
  Case Else : Return err
 End Select
 Select Case Mid(sq, 2, 1)
  Case "V", "Q", "L", "F", "A" : ea += 0
  Case "W", "R", "M", "G", "B" : ea += 100000
  Case "X", "S", "N", "H", "C" : ea += 200000
  Case "Y", "T", "O", "J", "D" : ea += 300000
  Case "Z", "U", "P", "K", "E" : ea += 400000
  Case Else : Return err
 End Select
 Select Case Mid(sq, 2, 1)
  Case "V", "W", "X", "Y", "Z" : no += 0
  Case "Q", "R", "S", "T", "U" : no += 100000
  Case "L", "M", "N", "O", "P" : no += 200000
  Case "F", "G", "H", "J", "K" : no += 300000
  Case "A", "B", "C", "D", "E" : no += 400000
  Case Else : Return err
 End Select
 ret.East = ea + 0.5
 ret.North = no + 0.5
 Return ret
End Function
  

Private Function ShortSquare_To_Square(sSq As String) As String
 ' Short square references should be avoided because they are only approximations
 ' 2 letters 10 digits = accuracy 1m = normal square reference
 ' 2 letters 8 digits = accuracy 10m
 ' 2 letters 6 digits = accuracy 100m
 ' 2 letters 4 digits = accuracy 1km
 ' 2 letters 2 digits = accuracy 10km
 ' 2 letters 0 digits = accuracy 100km
 ' Function will return string.empty when ShortSquare reference is invalid
 ' When using reduced accuracy, try to point to middle of posible region
 Dim ret As String
 Dim err As String = String.Empty
 sSq = sSq.ToUpper.Replace(" ", String.Empty)
 Dim ea, no As Integer
 If sSq.Contains("."c) Then Return err
 Select Case sSq.Length
  Case 12
   If Not Integer.TryParse(Mid(sSq, 3, 5), ea) Then Return err
   If Not Integer.TryParse(Mid(sSq, 8, 5), no) Then Return err
  Case 10
   If Not Integer.TryParse(Mid(sSq, 3, 4), ea) Then Return err
   If Not Integer.TryParse(Mid(sSq, 7, 4), no) Then Return err
  Case 8
   If Not Integer.TryParse(Mid(sSq, 3, 3), ea) Then Return err
   If Not Integer.TryParse(Mid(sSq, 6, 3), no) Then Return err
  Case 6
   If Not Integer.TryParse(Mid(sSq, 3, 2), ea) Then Return err
   If Not Integer.TryParse(Mid(sSq, 5, 2), no) Then Return err
  Case 4
   If Not Integer.TryParse(Mid(sSq, 3, 1), ea) Then Return err
   If Not Integer.TryParse(Mid(sSq, 4, 1), no) Then Return err
  Case 2
   ea = 0
   no = 0
  Case Else
   Return err
 End Select
 ret = Mid(sSq, 1, 2) & " " & ea.ToString("00000") & " " & no.ToString("00000")
 If Not Square_IsValid(ret) Then Return err
 Return ret
End Function
  

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