When you watch football on television - often they will automatically
add the line for 10 yards. It shows up right there along the line,
atop the field, but it still looks like the players walk on it and
other lines on the field are above it. Essentially it replaces the
green. It is done well - the camera pans and zooms in, and the line
pans and zooms with it - really looks like they actually painted the
green, though they turn it off and on.

I would like to know - how do they know how to put the line? There
must be camera-only emitters like flashing ultraviolet lights or
something giving the cameras some sort of cue and orientation of the
field to map the field when it pans and zooms - can't be GPS sensors
because that wouldn't affect panning and zooming.  But still the line
is very accurate - if there are ultraviolet beacons as cues for the
cameras, the whole border must be full of them...

You'll find an excellent explanation of how this is done on the "How
Stuff Works" site. Here's a brief excerpt (for details, click the link
below and read the full 3-page article):

"Television viewers have always been at a disadvantage in knowing
where the first-down line is in reference to where the offense is. A
small arrow located below the end pole isn't usually visible on the
television screen. But if you've watched any football games in the
last few years, you probably noticed the fluorescent yellow or orange
line extending from one side of the field to the other -- seemingly
painted on the field. In fact, the line is computer generated,
representing exactly the spot that the offense must get to for a first
down.

Sportvision, a company based in New York City, debuted its '1st and
Ten' system on September 27, 1998, and football fans everywhere
rejoiced!... The idea to paint a first-down line across the field on
people's TV screens sounds so simple. As it turns out, implementing
this is incredibly complex. It takes a tractor-trailer rig of
equipment, including eight computers and at least four people, to
accomplish this task!"

How Stuff Works: How the First-Down Line Works
http://www.howstuffworks.com/first-down-line.htm

Additional info may be found here:

Wikipedia: 1st & Ten (graphics system)
http://en.wikipedia.org/wiki/1st_&_Ten_(graphics_system)

G4TV: TV Sports Score With Tech
http://www.g4tv.com/techtvvault/features/37250/TV_Sports_Score_With_Tech.html

Time: How to Score on the Small Screen
http://jcgi.pathfinder.com/time/insidebiz/article/0,9171,1009652,00.html

My Google search strategy:

Google Web Search: tv OR television OR televised football "first down line"
://www.google.com/search?hl=en&q=tv+OR+television+OR+televised+football+%22first+down+line%22

I hope this is precisely what you need. If anything is unclear or
incomplete, or if a link doesn't work for you, please request
clarification; I'll gladly offer further assistance before you rate my
answer.

Best regards,
pinkfreud

great - links, a brief answer, great job.