[To follow up on TANSTAAFL’s letter,] I have worked for several engineering firms as a GIS technician, then manager. Counties will advertise when they will be re-flying parts or all of the county. Most county engineers, auditor, or Property Valuation Administrator (PVA) offices will tell you what the schedule for mapping is out a couple of years (usually the department in charge of tax assessments). A give away that it is happening is when you see large X’s painted in intersections with a metal spike sunk in the middle of the X (these are control points), with survey trucks with GPS receivers sitting in intersections or other open ground. Most orthophotography is done in late winter or late fall, when the leaves are off the trees and there is no snow on the ground. Evergreen trees are good for masking what lies on the surface. Not much you can do to hide any earth work that changes contours. There is another means of gathering contour information, LIDAR. Basically a laser that oscillates and paints the ground. Even trees won’t fully obscure it.
On a side note, the old USGS quadrangle maps are now almost supplanted by FEMA‘s flood insurance rate maps, which are all digital. There is all kinds of info available through those maps for interested parties.
On the non-government side, Google Earth is getting better resolution all the time, farther and farther away from population centers.
Your best bet to avoid attention is anonymity. That is true for all sorts of things. – School Dude