You can call GeographicLib functions from Excel.  Thanks to

  Thomas Warner <warnerta@gmail.com>

for showing me how.  This prescription has only been tested for Excel
running on a Windows machine.  Please let me know if you figure out how
to get this working on MacOS versions of Excel.

Here's the overview

(A) Write and compile little interface routines to invoke the
functionality you want.

(B) Copy the resulting DLLs to where Excel can find them.

(C) Write an interface script in Visual Basic.  This tells Visual Basic
about your interfrace routines and it includes definitions of the actual
functions you will see exposed in Excel.

Here are the step-by-step instructions for compiling and using the
sample routines given here (which solve the direct and inverse geodesic
problems and the corresponding rhumb line problems):

(1) Install binary distribution for GeographicLib (either 64-bit or
32-bit to match your version of Excel).

(2) Install a recent version of cmake.

(3) Start a command prompt window and run

  mkdir BUILD
  cd BUILD
  cmake -G "Visual Studio 16" -A x64 ..

This configures your build.  Any of Visual Studio 14, 15, or 16
(corresponding the VS 2015, 2017, 2019) will work.  If your Excel is
32-bit, change "-A x64" to "-A win32".  If necessary include -D
CMAKE_PREFIX_PATH=DIR to specify where GeographicLib is installed
(specified when you ran the GeographicLib installer).  Compile the
interface with

  cmake --build . --config Release

(4) Copy

  Release\cgeodesic.dll   # the interface routines
  Release\Geographic.dll  # the main GeographicLib library

to directory where the Excel executable lives.  You can find this
directory by launching Excel, launching Task Manager, right-clicking on
Excel within Task Manager and selecting Open file location.  It's
probably something like

  C:\Program Files\Microsoft Office\root\Office16

and you will probably need administrator privileges to do the copy.
If it's in "Program Files (x86)", then you have a 32-bit version of
Excel and you need to compile your interface routines in 32-bit by
specitying "-A win32" when you first run cmake.

(5) Open the Excel workbook within which you would like to use the
geodesic and rhumb routines.

Type Alt-F11 to open Excel's Visual Basic editor.

In the left sidebar, right-click on VBAProject (%yourworksheetname%) and
select Import File

Browse to Geodesic.bas, select it and click Open

Save your Workbook as Excel Macro-Enabled Workbook (*.xlsm)

(6) You will now have 10 new functions available:

Solve the direct geodesic problem for
  lat2: geodesic_direct_lat2(lat1, lon1, azi1, s12)
  lon2: geodesic_direct_lon2(lat1, lon1, azi1, s12)
  azi2: geodesic_direct_azi2(lat1, lon1, azi1, s12)

Solve the inverse geodesic problem for
  s12: geodesic_inverse_s12(lat1, lon1, lat2, lon2)
  azi1: geodesic_inverse_azi1(lat1, lon1, lat2, lon2)
  azi2: geodesic_inverse_azi2(lat1, lon1, lat2, lon2)

Solve the direct rhumb problem for
  lat2: rhumb_direct_lat2(lat1, lon1, azi12, s12)
  lon2: rhumb_direct_lon2(lat1, lon1, azi12, s12)

Solve the inverse rhumb problem for
  s12: rhumb_inverse_s12(lat1, lon1, lat2, lon2)
  azi12: rhumb_inverse_azi12(lat1, lon1, lat2, lon2)

Latitudes, longitudes, and azimuths are in degrees.  Distances are in
meters.
