A common economic marketplace that embraces the Baltimore and Washington metropolitan areas has been evolving with particular strength in the past decade, a leading economist said yesterday.

Moreover, Robert R. Nathan told reporters at a symbolically situated Baltimore-Washington International Airport briefing, between the two cities, growing urbanization will fill much of the vast open areas in the region, as has happened in the New York, Los Angeles and Chicago areas.

For now, Nathan added, the existence of diverse and extensive property for all types of development represents one of the premiere attractions of the combined Baltimore-Washington "common market," in terms of population the fourth-largest urban concentrattion in America.

Noting, for example, that Prince George's County still has 41 percent of its land undeveloped, Nathan said the region's land "is not confined to narrow corridors or obstructed by natural barriers . . . is distrubited widely throughout the region" to meet such needs as access to the sea, air or rail transport, industrial-business service concentration, research and technology skills, consumer markets and government centers.

And the land is not merely open space but acreage served by water, disposal service, elecric power and natural gas.

Other factors cited in the study as major attractions of this region for outside employers include an unusually mobile, large and diverse labor force -- skilled professionals and blue collar workers -- and central city rehabilitation of both Baltimore and Washington, the two points of greatest density.

Nathan's consulting firm in Washington, Robert R. Nathan Associates Inc., reached these conclusions in the first detailed economic analysis of the common market.

The latest study was commissioned by the Washington/Baltimore Regional Association, an organization of business leaders in the two metropolitan areas that is seeking to promote economic development and new jobs throughout the broad market which covers some 6,300 square miles in 15 counties and 22 political jurisdictions of Maryland, Virginia and D.C.

"Two metropolitan areas so large and so close in proximty cannot help but interact on each other," the report stated. "It is mutually advantageous that the economies of Baltimore and Washington are to a large extent complementary rather than duplicative or competitive. This may be the most compelling basis for the linkage of the areas in meaningful economic terms. The resources of one area reinforce and enhance the economy of the other."

The marketplace which Nathan surveyed has a population of more than 5 million persons, ranking Baltimore-Washington behind only New York, Los Angeles and Chicago. But in many other measures, the common market surpasses competing regions. In terms of affluence, for example, spendable income is growing more rapidly than in any urban area and median household incomes are the highest. The total value of all commercial construction exceeds all other areas and there are more scientists and engneers per 10,000 population (107,7 vs 72.8 in the second-ranked area. San Francisco- /Oakland).

The paramount question still unanswered is whether the natural develoment of the regional economic market, encompassing such diverse political jurisdictions, will move forward haphazardly or under guidance of cooperative government and private-sector planning. To date, individual jurisdictions appear to be going their own way and many business people in Washington and Baltimore are not convinced that there is anything to gain by joining forces with another community, 37 miles up or down the parkway.

Nathan pointed out that untold amounts of sewage and wastes are being dumped in Chesapeake Bay, a vast regional resource and a process that must be reversed. "Only joint efforts and a regional approach can be effective in coping with environmental concerns . . . water supply and sewage, land use and development, transportation, utilities and education must be addressed in coordinated manner in order to maximize the benefits and minimize the disadvantages that are the consequences of density and growth," he stated.

What is clear, Nathan emphasized, is that even without planning, economic interdependance of this region's 5 million residents has developed measurably and forces underlying regional business development will be "even stronger in the years to come."