Very few of us appreciate that the horizon, on level ground, is only just over 5kms away.
It is therefore understandable that we are always uncertain as to how far things are away from us particularly over water and if the viewer is elevated. More so when you are fielding a call from someone who has sighted drama out to sea but has no idea how to calculate the distance. Obviously height above sea level plays a major role.
All that you need do to calculate how far the horizon is, that is how far you can see, is to insert your height above sea level, in metres, into the green block on this spreadsheet, hit enter and the calculation is done for you. No need to tamper with any other fields.
You may need to approximate, say your caller is on the 6th floor of a sea facing block of flats and the casualty is half way to the horizon – the callers height would be say 26m above sea level, the horizon then being 18,2kms away. You would dispatch your rescue boat to an area 9km off the coast on the opposite bearing to the caller.
Download the spreadsheet by clicking here.