Right, I've been asked for a method of transmitting data(4 areas with a single transmitter in each) to a base just out side of the woodland. The whole area is around 50,000 square feet.
I was thinking of maybe using Xbees in each area transmitting to a central tranceiver that is sat above the trees, the only problem is the 4 other transmitters need to compete with dense woodland. Will any transmitters make it through woodland or will I be best running antennas up above the tree tops for all of the nodes(with this option I could do away with the need for a central node to just handle the transmitting)?
Could FM radio be an option? Each node only has to transmit 1 or 2 bits of information.
Maybe look on the libelium website. They have done some stuff in woodland. If you weren't in the UK you could get away with 900Mhz XBee modules which can do up to 13 miles LOS. In the UK these are illegal. They are also pretty power hungry though.
You might get away with a low frequency RF system. I haven't seen any long distance ones though. I would take a look at what frequencies you can use as an ameteur (ofcom has a list). Some of these cannot be used for data though.
I will be interested on what ideas other people have. I might have a need for some long distance, low data transmission interfaces soon.
The problem with woo land is the trees. they act a long thin conductors that can block signals. Best to use horizontal polarisation if you can.
Also here zigbee would be better as you could then establish a network of nodes that you could configure to cover the whole area. That way you are not limited to simple point to point communications.