This is a practical question, so if you find something missing, please assume a real life scenario.

The question is related to GPS and the Earth.

`lat1`: my latitude, GPS position I'm standing at (degrees)  

`lon1`: my longitude, GPS position I'm standing at (degrees)  

`dir1`: my direction I'm facing (from north, 0-360°, positive clockwise)  

`lat2`: latitude of object  

`lon2`: longitude of object  

`view`: my viewport

I'm a person with position lat1, lon1, facing dir1. My viewport is 180°. Object is located at lat2, lon2. 

How do I calculate that object is within my viewport?

Edit: viewport is assumed to be 180° and has a 1km radius.