BLACKTON, Ark. (AP) _ To some, western civilization in the United States started in the middle of an Arkansas swamp _ the reference point from which lands west of the Mississippi River were first measured.

After Thomas Jefferson bought Louisiana and Lewis and Clark explored it, the nation's land office sent a lesser known but just as important duo _ Robbins and Brown _ into southern Arkansas to begin surveying the 830,000-square-mile purchase.

Their first measurements established land grants for soldiers from the War of 1812. Eventually, plots throughout the Louisiana Purchase could be traced to a pair of gum trees between the Arkansas and St. Francis rivers.

``Without that, settlers moving west would have no assurance that they could occupy land and buy it,'' said S. Charles Bolton, a history professor at the University of Arkansas at Little Rock.

Few soldiers accepted their military bounties _ and their land was later seized by the state for taxes due _ but the survey's importance is that the West came to be settled in a relatively orderly fashion, as Jefferson had desired. In New England, groups of people moved in after receiving land grants from legislatures and, in the South, settlers occupied land before later-arriving survey crews took note of their property.

``Here, the settlement of the American west began,'' proclaims a sign at the Louisiana Purchase Historic State Park, built around a concrete marker atop the site where survey teams led by Prospect K. Robbins and Joseph C. Brown met Nov. 10, 1815.

``It's hard to believe how they had to walk through a swamp, keep a chain tight, fend off cottonmouths and then accomplish what they did,'' said Josh Epperson, an interpreter for the Arkansas State Parks system.

Surveys break down land into a grid so each property can be identified by how far it is from a starting point. The federal land office had already determined that Louisiana lands _ bought 200 years ago this year _ would be measured from a spot between the Arkansas and St. Francis rivers, but needed surveyors to establish that point.

Robbins worked north from the mouth of the Arkansas and Brown worked west from the mouth of the St. Francis. They met in a cypress and tupelo gum swamp and marked a pair of gum trees to establish their base line and meridian.

From here, in general, the rest of the new American frontier was laid out in six-mile squares known as townships.

America's first meridian helped define land in Ohio; the Arkansas site marks the first meridian established west of the Mississippi River and is the primary reference point for land as far north as Canada and as far west as the Montana-North Dakota line. Later meridians were established farther west as settlement progressed _ except for Texas, whose land distribution varied under its multiple governments.

Each township is further divided into squares one mile on each side and, from there, property can be easily cut up into 160-acre tracts. The terms ``the north 40'' and ``40 acres and a mule'' are directly tied to how tracts were developed.

Records of Robbins and Brown's initial surveys, and those from other teams that helped fill in the gaps, are kept at the state land commissioner's office in Little Rock. Inside 268 metal cabinet drawers are descriptions of everything the surveyors found on their lines _ trees, swamps, rivers and, at least once, ``an amplitude of briars and mosquitoes.''

Because the land was available for settlement immediately, surveyors were required to note whether the land was suitable for farming, and much of it was. Just off the swamp, farmland rich with cotton, rice and soybeans stretches in all directions today.

But many soldiers didn't take advantage of the land and lost it when the Arkansas Territory began taxing it in the 1820s.

``If the tax wasn't paid, the land would be forfeited to the (territory) and then could be sold for back taxes,'' Bolton said. ``If you were in New Hampshire or in Georgia, you probably had no idea this was going on.''

Records from Independence County showed that, in 1825, the sheriff sold 169 160-acre tracts to satisfy tax liens of $3.20 each _ $2.40 to the state and 80 cents to the county. The state in turn sold the land to others.

Robbins and Brown's initial point remained relatively obscure until new survey crews revisited the site in 1921 to settle a boundary dispute between two of the three counties that meet here. The surveyors found the original witness trees, the nearest permanent markers to the site, and the Daughters of the American Revolution, realizing the site's significance, purchased it and made plans for a monument.

``There had been a couple rumors, proven false, that when they brought the monument down here they just dropped the monument and said, 'This is the spot,''' said Wayne McPhink, curator of exhibits at the Delta Cultural Center in Helena.

But with something as historical as the starting point of the Louisiana Purchase surveys, it is desired to know for sure that the monument was placed in the right spot.

Mickie Warwick, a surveying instructor at the University of Arkansas at Monticello, said students plan to conduct additional research this winter, after the leaves are off most of the trees.

As the survey spread from Arkansas, crews encountered previous land grants made by Spain. When necessary, boards of land commissioners settled disputes, but in Arkansas there were few to resolve because, until 1815, the region was mostly home to hunters and trappers and a few Indians, Bolton said.

``I'd be willing to bet that Congress decided to give veterans land in Arkansas because it figured no one else was here and it wouldn't infringe on anyone else's rights,'' Bolton said. ``Arkansas was essentially defined as a place at an uncomfortable distance from both St. Louis and New Orleans.''