Roy Borgstede, a computer mapping specialist with the Census Bureau, sat down with his machines and some mathematical formulas recently and plotted the latest leg in the march of the United States' center of population. During the 1970s, he confirmed, this imaginary point crossed the Mississippi River and came to rest in a tract of wooded land near a small pond 45 miles south of St. Louis.

This marked not only a historic crossing of the traditional border between East and West, reflecting massive shifts of people to the Sun Belt during the 1970s, but it may also help explain why Americans have felt more and more alienated from Washington.

In 1970 the stretch of swamps and cow pastures along the Potomac, picked out ast the site of the capital by George Washington as part of a compromise between notherners and southerners, was sensibly near the center of the fledgling nation. But in the 191 years since, as Borgstede's map illustrates, the country has runn off and left its capital.

The population center was 23 miles east of Baltimore in 1790, when the first census was taken. It inched westward through Maryland, northern Virginia, West Virginia, Ohio, Indiana and Illinois. The 1970 census found it in a cornfield near Mascoutah, Ill.

The value of this symbol is unclear. The center is now on a 50-acre parcel of land owned by Missouri election official Henry Koch, 58, who three months ago put the property up for sale with a price tag of $300,000. Koch considers it a likely splot for a shopping center and had hoped to develop it himself, but, he says, inflation and sky-rocketing interest rates killed that dream. Now, he may just peel off 10 acres that include the pond and the population center and turn it into a little park for area residents and passing motorists, in honor of his late father and mother.

The creeping population bullseye, which is calculated every 10 years, is defined as the point where the country would balance perfectly if it were a flat surface and every person on it had equal weight.

Jefferson County, where Koch's woods are, is a fitting locus, because it is typical of rural areas all over the country that are swelling with the people and money pouring out of the cities. This reverses a trend that had continued since the early 1800s, in which people flowed toward the cities and away from these areas.