How can I connect a field of pressure plates to a single output?

Solution 1:

Step 1:

Place Redstone under each block that will have a pressure plate on it.

The Blue Wool represent ground level. That's where the pressure plates will go. I didn't make mine 70 blocks long because I got lazy and the design works the same for any length. I'll cover width limitations at the end.

Step 2:

Cover the area with Pressure Plates. If mob detection is your goal, use Stone Pressure Plates. If you want to detect any entity, use Wooden Pressure Plates.

Step 3:

Place repeaters along both long edges of your area. After each Repeater, place another Redstone.

As you can see, the Creeper activates the Redstone under the Pressure Plates causing the Repeaters within 15 blocks to activate. Were he to go too far from these repeaters, the repeaters on the opposite side would activate.

Step 4:

To ensure your output signal can make it to wherever you want it, you're going to need a few more repeaters. To be safe, I recommend putting an additional repeater every 15 blocks as shown below.

This will ensure that there is always a repeater within 15 blocks of the original signal. These repeaters are where you want to take your output. Wire them together as you see fit, keeping in mind that distances over 15 blocks will require more repeaters.

Limitations

Width: Since Redstone signal can travel 15 blocks from a power source, 30 blocks wide is exactly the limit on this design. If you want wider than that, your going to have to deal with deadzones.

Solution 2:

For a 30×N area, this is just barely possible. Place redstone under the entire field of pressure plates. On the long edges (perpendicular to the 30 dimension) place repeaters facing outward. Then, use any wiring you want to combine the outputs of all the repeaters; there's lots of flexibility there.

Since redstone signals travel 15 blocks, any plate's signal will reach at least one of the repeaters.

I don't see a way to accomplish this with no dead-zones for wider areas.

Solution 3:

What you can do is the following setup, where (P = pressure plate, B = Block, R = Redstone)

P P P P P P P P
B B B B B B B B
R R R R R R R R R R R R R R ---> output

As you may have guessed, eventually you will go past the 15 block redstone limit. To solve this place a repeater somewhere on the going towards the output, and with nothing else near it. This will cause a deadzone of approximately 3x2 for every 15 blocks.

Related: How do I power Redstone through a block?