In January 2016, The Guardian’s ‘The Fiver’ featured a letter from Jim Butler asking:
‘Why is conceding a goal at the near post deemed to be an inexcusable crime, compared to being beaten anywhere else? Surely, if a keeper is positioned correctly, the striker will have equal-sized target to aim at on either side of the keeper?’
Well Jim, this is why: ANGLES, motherflipper. I’ll demonstrate very roughly, using Subbuteo figures and some relatively straightforward geometry.
Imagine slicing the face of goal in two: left and right. Next, think of the shape of a goal from head on – rectangular, obviously. Finally, picture how it appears from a 45 degree angle. It tapers towards the far post, correct?
Now let’s compare the surface area of the two halves of the face of goal from those two different angles:
The half of the goal nearest the striker presents a much larger area to shoot at successfully than the far half, and so her margin for error for a shot to the near post is much wider – it’s nearly twice as easy to get a shot on target by aiming for that half of the goal.
Here is a more geometrically robust expression of the same idea, this time viewed from above. A striker aiming for the centre of the goal from this position can afford to skew the shot 14 degrees to the right and still find the net at the near post – but could only shank it 8 degrees left of centre and still score (for simplicity’s sake, we’ll ignore the posts here). In other words, even with an open goal, the striker can afford to get things nearly twice as wrong at the near post.
The upshot is that if you’re a goalkeeper in a 2D world, you give the striker nearly half as much to aim at if you protect the near post side of goal and force them to aim for the far post.
I concede these next few examples are drastically over-simplified as they assume goalkeepers can only dive at 90 degrees in relation to the striker and they do not cover lobs or curling shots; plus, you know, it uses Subbuteo figures and a bit of quick-and-nasty photoshopping… but nonetheless, hopefully they’re illustrative of the difference good positioning can make.
Let’s start with a shot from face-on to the goal, such as you would find at a penalty kick.
The red circular area indicates the area that the goalkeeper can reach (the radius of these circles is equal to the height of the goalkeeper from tip of gloves to bottom of base). Here, assuming the striker shoots on target, she has a 51% chance of putting the ball beyond the goalkeeper’s reach.
Now let’s look at a similar chart for an angular shot from near the corner of the six-yard box.
Here, the goalkeeper stands right next to the post. This allows her to dive and tip the ball away 71% of the time, with the striker able to aim at just 29% of the goal and guarantee finding the net. The goalkeeper has a definite advantage if a shot comes in at the near post.
But you can see from the large reachable area outside that post that the goalkeeper’s positioning is far from perfect here: she could get better coverage by taking a step away from the post, like this:
In this scenario the goalkeeper is covering 84% of the goal, and the striker would have to put the ball either straight into the near top corner, or hit a good, accurate shot towards the far post. In other words, unless it’s an unbelievable finish, the goalkeeper really shouldn’t be getting beaten at the near post here.
For a final comparison, let’s say the goalkeeper is scrambling after parrying a shot straight to our striker here, and finds herself closer to the far post when her opponent lets fly.
This actually allows a better scoring chance for the striker than a penalty situation. (In case you’re wondering why the circle is cut off square towards the near post, it’s because if the keeper dives in that direction to her fullest extent, the ball would already be over the line by the time she touched it).
So, if a one-on-one situation ends with a goal going in at the near post from a wide angle, chances are that either the goalkeeper’s positioning was wrong, or they have – through some blunder or other – let in a very preventable goal.
Steven Chicken – go and follow him on Twitter