Converting constantly updated units in textboxes on command

Question

I have three textblocks whose text are bound to three different properties.

<TextBlock Style="{StaticResource textBlockStyle2}"
           Text="{Binding Path=TWD,
                          Mode=TwoWay,
                          UpdateSourceTrigger=PropertyChanged},
                          StringFormat={}{0:F1} M}" />
<TextBlock Style="{StaticResource textBlockStyle2}"
           Text="{Binding Path=Alt,
                          Mode=TwoWay,
                          UpdateSourceTrigger=PropertyChanged},
                          StringFormat={}{0:F1} M}" />
<TextBlock Style="{StaticResource textBlockStyle2}"
           Text="{Binding Path=Dep,
                          Mode=TwoWay,
                          UpdateSourceTrigger=PropertyChanged},
                          StringFormat={}{0:F1} M}" />

These are the properties in the viewmodel:

private double _TWD;
public double TWD
{
    get { return _TWD; }
    set { _TWD = value; OnPropertyChanged("TWD"); }
}

private double _Alt;
public double Alt
{
    get { return _Alt; }
    set { _Alt = value; OnPropertyChanged("Alt"); }
}

private double _Dep;
public double Dep
{
    get { return _Dep; }
    set { _Dep = value; OnPropertyChanged("Dep"); }
}

Now, these are in meters, which is what the 'M' is for in the StringFormat property of the textblocks. What I'm trying to do is, when I 'click' (via command) on a separate textblock (this will be inside of a button using a control template), I want to convert the values in the above textblocks to feet and add an 'F' after the value. Another click will convert it back to meters, and so on.

I was thinking about adding a command that just converted the values based on a bool isMeters. However, the values in the textblock are constantly updated (every second) and I don't want to have to call the function every time the values change. Is there a simpler way I'm not thinking of to achieve this functionality?

Note:

1 meter = 3.2808 ft
1ft = 0.3048 meter

Answer 1

I got it working using a converter.

public class MetersToFeetConverter : IValueConverter
{
    /// <summary>
    /// Converts meters to feet.
    /// </summary>
    public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        value = (double)value * 3.2808;
        return (value.ToString() + " F"); 
    }

    public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        return Binding.DoNothing;
    }
}

Then in the viewmodel:

private bool _isMeters = true;
public bool IsMeters
{
    get { return _isMeters; }
    set { _isMeters = value; OnPropertyChanged("IsMeters"); }
}

//called when I click the button to convert
public void ConvertData(object parameter)
{
    if (_isMeters == false)
    {
        IsMeters = true;
    }
    else
    {
        IsMeters = false;
    }
}

Then the xaml binding for the textboxes are like this, using a datatrigger:

<Style.Triggers>
    <DataTrigger Binding="{Binding IsMeters, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Value="False">
        <Setter Property="Text" Value="{Binding Path=TWD, Converter={StaticResource metersToFeetConverter}}" />
    </DataTrigger>
</Style.Triggers>

Note: @haindl answer also worked, just didn't see it till after I did it this way.