lolwut? if theres a place of interest shown on a map from say the 1800-1900's but is now on current private property.. its not open to anyone without permission of the property owner to be on said property.
if your saying if its on a map and its public.. well yes, its public knowledge.
parks are a different matter. here in Michigan, the state parks are for the most part closed to detectorist except a select few (like 3 or 4) which are fully open... but for the most part your only allowed to detect in the parkinglot and very few beaches iirc there.
my citys park is questionable because removal of sod/earth plants and such could be technically determined based on the officers opinion of what your doing.. ie digging a small plug carefully
and as for the neighboring county, the township parks there are primarily old farmland and fair game as there are no rules against detecting whatsoever.
Im not trying to create a argument... but always ask permission to hunt prior to walking out on privately owned property. check your parks rules/ordinances and always keep your digs clean and neat.. ie dont leave open holes and dirt everywhere + pick up any and all trash

HH