Northeast corner of Colorado
According to the Congressional definition of the boundary of Colorado, this point is theoretically where the 41st parallel north latitude intersects 25 degrees west longitude as measured from the Washington Meridian*. However, this monument was first set by Oliver Chaffee during his 1869 survey - and due to the technological limitations of his day, it's not exactly right (although his survey does define the official boundary, regardless of its accuracy).
* Many American surveys between 1850-1884 were based on this meridian, which was officially abolished by Congress in favor of the Greenwich Prime Meridian in 1912. "25 degrees west of Washington" is a few miles away from the 102nd degree of longitude west of Greenwich (more info on this page).
NOTE: The book "Colorado Mapology" (Erl H. Ellis, 1983, Jende-Hagan Book Corporation) is an excellent resource, from which I obtained much of the information presented on these pages.
Chaffee's original limestone marker is broken; the photo below shows the condition of the monument in 1969:
That was looking eastward (photo by Erl Ellis, published in the reference above). Mr. Ellis was instrumental in prompting improvements to this site - plans for which began in 1972, and were finally realized in 1979. What's left of the base is now preserved in the middle of a concrete apron and a fenced enclosure, shown in the photo below:
me, Fall 1993
In that shot, the camera is in Colorado. The view is northeast into Nebraska; that's the South Platte River valley behind me. I am standing a few feet into Nebraska, pointing down the north and east statelines of Colorado. Below is a close-up, shot from the same angle:
Jeff Morrison, May 2009