[CelticNHBlue]

Your position is dictated by both the preceding action and development of the play.

If you are working alone with runners on base, you may be coming from the center of the diamond and reacting to the throw, in which case you may be inside the diamond.

In a two umpire system you may have a responsibility at third and the play may continue to the plate at which point you can only hope to get to your third base extended, 90 degree calling position.

However, if you have the luxury of being at the plate when the play develops (which I believe is the intent of your question) then:

You should start half way between the first and third base extended positions recognizing the position of the catcher (for obstruction purposes), reading the throw and the tendancy/direction of the runner. If the runner is coming straight down the line, and you see that the tag will be in front of the plate (along the third base line) rotate to first base extended to get your 90 degree angle as the runner comes into the plate. If the runner angles away from the line for a sweep slide, your best position to see the tag of the runner and contact with home is third base extened with the runner coming toward you. This allows you to clearly see if the tag is applied or missed and relative to the player reaching home.

In all cases you want to be about 5 - 8 feet from the play in a set position at the time of the tag. In the case of the third base extended position, or a sweep tag when you can't get to this position you may need to be closer to look over the top of the play to get all the information you need, though this is less desirable.

In the picture you provided, I think the umpire is in the proper calling position, though I would like to see him closer to the play, like the back corner of the left handed batters box.