There are two basic requirements before property can be taken under Eminent Domain.
1) The taking has to be for the public good.
2) The property owner must be given fair market value for the property.
Both of these leave too much room for interpretation.
FIRST, get local real estate agents to give you a WRITTEN CMA (Comparative Market Analysis) of the value of the property. Real Estate agents normally do this for free. I suggest at least 3, because they will come up with different values. If they are not reasonably close, get more CMA's.
IF you are not being offered a fair price, contact the agency taking the property, and ask them why they are not giving you fair market value for the property, and bring in copies of the CMA's from the real estate agents. You may be able to get an adjustment without having to bring in an attorney.
As stated above you may need an attorney, but they can get very expensive. If you can not get satisfaction from the authority, a few letters to the editor may help. Also contact the others involved, there truly is strength in numbers.
If you have ANY political connections, NOW is the time to use them.
You may not have much time, so act quickly. After they get the property it will be much more difficult to negotiate.
Real Estate Agent
Real Estate Instructor