A few slight corrections:
Private land is owned by individuals and by corporations. Individuals may be more likely to give permission to collect -- assuming you can find the land owner (Tax offices can help). Corporations are another matter entirely. They have lawyers looking out for their "interests" and those interests include not being sued if you get bitten by a rattlesnake or chop off a finger while you are collecting on their property. Permission is difficult to get.
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just a clarification of the clarification- as this is what i deal with every day. corporations are just another way for certain individuals or groups of individuals to hold title to real property or other assets. not all corporations are large businesses with impenetrable walls of bureacracy. a corporation can have as few as two people behind it- a mom and pop style situation. in fact, of the hundreds of corporate documents i review yearly, a comfortable majority fall into a small business framework where only a few are managing the enterprise. this also holds true for LLCs, partnerships, etc. these are all just titles for groups of people who have chosen to do business a certain way.
i post this only to say that just because a piece of land is vested in a corp or some such business entity, doesn't necesarily mean you will have a lot of trouble getting persmission. you might of course, as there certainly are large corporations as pointed out above, but not always.
as for discovering who holds title to property, i would recommend a title company if your local assessor office can't help. try for a locally owned company, as they usually have more incentive to help and a customer service desk that can look up ownership fairly quickly. you do need to have some identifying information about the parcel though.
just my two cents. well, maybe only one in this case.