How to read an array one time only, and use it in a UDF evaluated in multiple cells?

This code I'm developing (my first VBA script) essentially works, but given real-world data attached to it, will be inefficient and likely slow.
There is a reference sheet called "markers" containing 25000 lines (example attached only has 20) that need to be cross-referenced to about 30000 data points. Currently my code would need to read the 25000 lines at each of the 30000 data points getting cross-referenced. But, I'm sure there's a better way.

Can you show me how to read the reference arrays in worksheet "markers" just one time (from what I've read I think this would need to be done outside of the UDF or with Public scope - not clear to me), while my UDF is called repeatedly and makes use of those arrays to make evaluations in worksheets "A" and "B" (see range A7:A16 in both those workseets).

Thanks!

Option Base 1
Function WName(ByVal XCP As Range, ByVal YCP As Range, ByVal SD As Range) As String

Dim i As Integer, n As Integer, Nmin As Integer, Nmax As Integer, MinIndex As Integer
Dim Dist As Single, MinDist As Single
Dim XMKR(6 To 25) As Single     'Ideally the array size would be defined by variables Nmin and Nmax but I get a compiling error saying the "needs to be constant".
Dim YMKR(6 To 25) As Single     'I can live without Nmin and Nmax passed as variables to define the arrays if there is a way to read them once only, but refer to them repeatedly.
Dim WMKR(6 To 25) As String     'My thinking to use variables to define the array dimensions repeatedly was to read from pre-defined appropriate portions of "markers" instead of the whole list.

Select Case SD
   Case Is = "TsdA"
        Nmin = Range("markers!$B$2")
        Nmax = Range("markers!$B$3")
        Debug.Print "comment case A "; SD; Nmin; Nmax
   Case Is = "TsdB"
        Nmin = Range("markers!$C$2")
        Nmax = Range("markers!$C$3")
        Debug.Print "comment case B "; SD; Nmin; Nmax
End Select

'Debug.Print "got this far. Nmin = "; Nmin   'First time through, code evaluates this line BEFORE the Select Case block above ?!!
For i = Nmin To Nmax
    Debug.Print "Reading arrays from markers worksheet.  i = "; i  'How can I read the markers worksheet arrays just once (outside of this procedure?) and use that memory block repeatedly?
    XMKR(i) = Range("markers!$E" & i)
    YMKR(i) = Range("markers!$F" & i)
    WMKR(i) = Range("markers!$C" & i)
Next i

MinDist = Sqr((XCP - XMKR(Nmin)) ^ 2 + (YCP - YMKR(Nmin)) ^ 2)
MinIndex = Nmin

For n = Nmin + 1 To Nmax
    'Debug.Print "n = "; n
    Dist = Sqr((XCP - XMKR(n)) ^ 2 + (YCP - YMKR(n)) ^ 2)
    If Dist < MinDist Then
        MinDist = Dist
        MinIndex = n
    End If
    If MinDist = 0 Then
        Exit For
    End If
Next n

If MinDist > 3 Then
    WMKR(MinIndex) = "DISTerr>3m"
End If

Debug.Print WMKR(MinIndex); MinDist; MinIndex; n
WName = WMKR(MinIndex)
'ActiveCell.Offset(0, 4).Value = MinDist   'Nice to have: Output the MinDist value four columns to the right of the WName function call.  This not working.  Need to update ActiveCell selection somehow first.

End Function

protonLeah,

Thanks once again for replying to my post. The coding update you provided nicely illustrated how to use public functions, enabling a one-time-only read of a large array for any number of function calls that use the array. This indeed saved a lot of time when scaling up and running the code on my real dataset. Greatly appreciated.

There were some index mismatching issues lurking in v3(vba), which are resolved in v4 and posted here for completeness. [You may not have caught it because your sheet "B" function-calls reference the Select Case SD = "TsdA", but should reference SD = "TsdB". When the functions are given the intended SD argument, the result blows up]. What I learned was that the array read (at least as implemented here) always results in an index starting with "1" (note we elected to use Option Base 1 for this function). I thought that this code would (as I intended) put data from worksheet "markers" row X into array-element-row X. To resolve this I started doing some more complicated index adjustments, but then realized it was just simpler to let the array read .Range("E1:E25"). instead of starting at E6. So, the first few elements of the array capture whatever garbage I have in the first five header rows, but the array-element-index is now aligned with the "marker" worksheet row numbers, making QC much easier when dealing with all the data.

I need help with at least one last enhancement to this project, and will post that on another thread.

This issue SOLVED.

Cheers!
G