Nice Reporting as usual! I'm planning on geocaching around here this summer.
The "Crosses" you saw are aerial photography control panel points set by surveyors and used in photogrammetric mapping. GPS coordinates with "Survey Grade Accuracy" (+/- a couple centimeters) are established on the actual point in the center of the panel
which is then used by the photogrammetrist to orient the aerial photo's to the regionally accepted horizontal / vertical coordinate system. Same application for those seen by travelers on paved routes that are painted on the asphalt along the sides of highways/streets, etc. A great deal more technical info to this brief explanation, but that becomes very boring in a real hurry.