Not tested coordinate rotation module

coordinates.erl

-module(coordinates).

-export([
        to_cartesian/2,
        to_polar/2,
        rotate_against/5
]).

to_cartesian(Angle, R) ->
    {R*math:cos(Angle), R*math:sin(Angle)}.

to_polar(X, Y) ->
    {trimaths:rad2deg(math:atan2(Y,X)), math:sqrt(X*X + Y*Y)}.

rotate_against(X, Y, AgainstX, AgainstY, Angle) ->
    {ShiftX, ShiftY} = {X-AgainstX, Y-AgainstY},                           %% Shift x,y to 0,0
    {ShiftAngle, ShiftR} = to_polar(ShiftX, ShiftY),                       %% transform it to polar
    {ShiftRotateX, ShiftRotateY} = to_cartesian(ShiftAngle+Angle, ShiftR}  %% rotate shifted vector
    {ShiftRotateX+AgainstX, ShiftRotateY+AgainstY}.                        %% return result