OK, the finish of a race is when the nose of the dragon crosses the finish line. But what is the beginning, especially from a held start? Is the beginning of the, say, 500-meter distance measured from the tail of the boat?

Measured from the nose of the boat as well.

The race is measured from the start line to the finish line. The time it takes to complete a race is from the starters signal until the nose of the boat crosses the finish line.
The relationship between the boat and the start line at the beginning varies from boat to boat and race to race (especially in a non held start and windy conditions). The starter doesn't start (is not supposed to)the race until they are satisfied that all the boats are reasonably lined up and preferably behind the line.

Even on a held start, the nose of the boat could be over the line, on the line or behind the line, the advantage is that all the boats should be the same

Nose lined up at the start, nose over the line at the finish. IDBF rules.

Occasionally local conditions will require a different point for lining up, but the same location on the boat used for the start line should also be used for the finish line (for example, lining on the number boards or lining on the drummer's seat).

"Nose lined up at the start, nose over the line at the finish. IDBF rules."

Right, but is a 200 meter race really just a 190 meter race? That is, are the noses at the start of a 200 meter races actually 200 meters from the finish line? Or is the tail 200 meters from the finish line?

The nose of the dragon boat starts 200m from the finish line in a 200m race. This isn't hard here people.

"Occasionally local conditions will require a different point for lining up, but the same location on the boat used for the start line should also be used for the finish line (for example, lining on the number boards or lining on the drummer's seat)."

It doesn't matter if the same location on the boat is used for the start and finish. What matters is that the same location for lining up the boats at the start and judging the finish is used on every boat. So if you decide to use the seat to line up the start, you can still use the nose to judge the finish, each boat goes the same distance.

"Right, but is a 200 meter race really just a 190 meter race? That is, are the noses at the start of a 200 meter races actually 200 meters from the finish line? Or is the tail 200 meters from the finish line?"

Unless you are racing on well established courses with some permanent infrastructure (Ottawa maybe, Olympic Basin, Lake Banook definitely), then the distances are going to be nominal at best. A 200m race at one festival might actually be 203m and 195m at another (or worse). I saw one suggestion that Waterloo make each lane a different length to account for lane bias.

In general it is easier to line up boats with the nose on the start line because it is easier to see a boat sticking out into clear water, then it is to see the back of a boat being ahead when the view is blocked by the rest of the boats.

Are you people joking? This isn't rocket science

Why does it matter how a race is measured? A boat crosses the line, you get their time....(btw - most festivals watch dragonheads).

Can we have this thread removed on the basis of sheer stupidity? Please?
Who the **** in this world doesn't understand how the finish of a race is judged?

Auto race,
Boat race,
Dog race,
Horse race,
PEOPLE race!

You really don't get how the winner is decided? Jesus Christ...

"The nose of the dragon boat starts 200m from the finish line in a 200m race. This isn't hard here people."

Not so fast. The OP may have a point. Maybe I'm mistaken, but weren't the people who hold the tails in Montreal standing on platforms that were at the 500-meter mark? If that were true, then the tails of the boats were at the 500-meter start.

I think you've got something there. Unless of course they back the holding docks up by approximately a dragon boat length so that the noses are actually on the 500m line.

Not a big deal, but good point.

This is just another one of the variables that prevent you from comparing times from one event to another. Even if they set the holding docks on the 500m point, they would still call it a 500m race, simply because it is easier than calling it the 490.372m race.