Zoomer,
There is no set distance between markers on the US-CA border. It really depends on topography and cultural structures. The rule as far as I understand it is the markers are placed at least within the distance required to see the next marker down the line with surveying equipment. In other words, within visual distance of one another. In the case of the International Peace Garden (North Dakota-Manitoba), the IPG Inc. got permission from the International Boundary Commission to build the Peace Chapel right on the borderline. The one requirment by the IBC was that a border marker needed to be placed on the roof of the Chapel so that surveyors would have a continuous surveying line.
Dallen