I think most ardent football fans would agree on this - NECESSITY OF GOAL LINE TECHNOLOGY.We have seen a lot of so called GHOST goals like the one in   Chelsea's 2–1 victory over Tottenham in 2011, a goal which was off the mark and prompted officials to think about it.Time and time again the issue of GLT comes up.
             Even in 2000 the the issue had raised up. Victor Ikpeba's penalty for Nigeria against Cameroon was deemed by the referee to not have crossed the line after deflecting off the crossbar.Cameroon won the African Cup of Nations.
             In 2005 in a match between Manchester United and Tottenham Hotspurs,Pedro Mendes hit a shot from 55 yards.Roy Carroll dropped it well within a yard of the goal but nobody noticed it. 
In order to avoid even the marginal errors the technology should be applied.The current debate over goal-line technology cannot be adequately resolved by a simple answer of either yes or no.
There are two types of technologies FIFA is currently looking at for making goal-line decisions-


1. Hawk-Eye

This is similar to the ones used in Tennis and Cricket. These have an accuracy of 3.6 mm. It is capable of simulating the ball's actual position in less than a second using various images made available from cameras installed in different locations

Pros -

1. Correct Decisions
2. Visual pleasure and confirmation for the spectators

Cons -

1. The installation of six cameras on each side of the goal will mean the installation costs will be high. Consider the costs if this were to be ensured for all football matches around the globe.
2. Although, the system is capable of simulating the ball's trajectory within 1 second, it would actually eat up a lot of time when shown on replays.
3. It requires the ball to be at least 25% visible, or else we have to rely on the referee's judgement like old times.

2. GoalRef

This requires electo-magnetic antennas to be placed around the goal posts and the cross bar to create a low-power magnetic field and the ball to be fitted with electronic probes in between inner and the leather out-lining. The moment the ball crosses the line, a signal can be sent to the referee's microphone.

Pros -

1. Correct decisions
2. Really fast and no need for the play to stop

Cons -

1. Probably wouldn't be as great from a spectator's point of view
2. All balls would have to be manufactured with this chip

Overall, the most important negative about both these technologies is that they are not easily duplicable at all levels of football. But, considering the stakes at modern day football, this is one sacrifice that FIFA will have to make.
Also, the fans/managers would no more be able to blame the referee for their frustration.

I think the GoalRef technology makes more sense, although it would be fun to have a look at free-kick trajectories and a lot more amazing stuff with the Hawk-Eye technology. No matter how much some people might protest, these technologies are inevitable in the future.

Advantages
1.Reduce Errors: The first obvious advantage would be that It will reduce the burden of the referees.I mean one can imagine what would happen if a "GHOST GOAL" would go in the El Classico.The refs are under tremendous stress for a game.Often a ref takes flak if they make a wrong decision
For eg:Lampard's goal which had clearly crossed the line by a few distance was disallowed.The game would have been different had GLT been used.

2.Technology helps Human Judgement :GLT should be used when a team doubts about the referee's decision on a goal score, like the challenges used in tennis where each player has a limited number of challenges they can use through the whole game.

3.Technology is available :If anything that helps the refs is available why not use it

4.Benefits Outweigh The Costs:If the game is of most importance,then GLT should not matter.

5.Accuracy:i The results are more accurate


Disadvantages 

1.Reliability:It amy not be a 100% reliable

2.Renders Referee Redundant:The role of a ref is reduced considerably.

3.Cost:The cost of applying such technology is high and not feasible for all the teams to apply.