Noisy complaints of "undercount" from the cities of the East have drowned out the truly important message of the 1980 census. The message is that the country is growing, and growing much more rapidly than anticipated. i

The flow of growth, whatever its exact size, is plainly away from the big cities of the East and toward the more open spaces of the South and West. While agonizing racial an urban problems result, they must be met within the context of underlying change -- not by trying to mask reality.

The big number thrown up by the census was a total population of 226.5 million persons. That is 4.5 million, or 2 percent, more than anticipated in all previous estimates. It not only means that the U.S. population grew much faster than expected in the 1970s, it also means -- because the population base and the rate of increase are higher than before -- that the growth in the next decade will also be bigger than anybody figured.

The exceeding of expectations for the nation as a whole tends to weaken the claim of undercount by New York, Detroit and other cities. For it is intrinsically unlikely that the census would miss masses of people in the East while catching every last living soul in the South and West. Especially since those most likely not to be counted -- illegal immigrants -- are concentrated in the South and West.

Even assuming some tilt against the East, the evidence of a steady flow of people away from the great cities of this part of the country is unambiguous. Of the 30 largest cities in the country, 20 lost population between 1970 and 1980. Fourteen of those 20 also lost population in the previous decade.

Of the 10 that grew in the past decade, nine (Dallas, El Paso, Houston, Jacksonville, Memphis, Phoenix, San Antonio, San Diego and San Jose) are in the South and West. The only growing big city in the East is one that boasts the rare combination of a state capital and a great university -- Columbus, Ohio.

State-by-state figures tell the same story. Eleven states grew enough in population to pick up more congressional representation. All of them (Arizona, California, Colorado, Florida, Nevada, New Mexico, Oregon, Tennessee, Texas, Utah and Washington) are in the South and West. Ten other states lost population to the point of a cut in congressional representation. Eight of them (Illinois, Indiana, Massachusetts, Michigan, New Jersey, New York, Ohio and Pennsylvania) are eastern industrial states. Given the sweeping nature of those shifts, there is no chance of turning the tide. The supposed renaissance of big eastern cities is a mirage. All of them are in decline. Far from trying to resist the change, it makes sense -- as a panel of the President's Commission on the 1980s has concluded -- to go with the flow. Insofar as people can find better lives in the growing areas, they ought to be encouraged to move.

Unfortunately, however, not every person and not every problem can pick up and head South or West. Large communities -- especially blacks -- have established themselves and the whole apparatus of their social and cultural being in the eastern cities. They would not be welcome elsewhere, nor could they easily uproot themselves. A quick, mass migration from Harlem to San Diego is just not in the cards.

So America as a whole has an obligation to help those trapped in the declining cities to help themselves. That means chiefly opening employment opportunities. But making jobs available in the declining cities is necessarily a work of fine detail. It cannot be done by saving outmoded industries -- like basic steel -- or propping up falling companies, like Chrysler. The trick is to target industries of the future, notably energy and specialty manufacturing, in the urban ghettos. In that connection, the tax-free "zone of enterprise" idea advocated by the incoming Reagan administration sounds interesting.

A second inescapable burden is fiscal crisis. The eastern cities have become repositories of national poverty. Inevitably the tax base shrinks, while the costs for welfare, education, health and safety mount. Though federal payments already make up about 40 percent of their budgets, most of the big eastern cities continue to experience financial difficulty. It is hard to see how the country can remain livable without substantial fiscal subsidies from the federal Treasury to the declining cities.

But if problems exist, nothing is served by taking potshots at the census. Harlem can't be moved to San Diego, but neither should anybody pretend that it is San Diego.