Samuel T. Freeman & Co. The value of the 1784 map by surveyor George Woods has been estimated to be between $60,000 and $90,000.

If it's authentic, it would be the earliest known draft and only surviving copy of Pittsburgh's town plan, produced in 1784 by surveyor George Woods for the Penn family, which owned the land.

But the map, drawn in faded ink on yellowed parchment, is shrouded in mystery, with an unknown provenance and a gap of almost 150 years in its whereabouts.

Its owner, said to be a private collector with a passion for early Western Pennsylvania documents, has consigned it for auction next month in Philadelphia and wishes to remain anonymous. He told the auction house he purchased it at a house sale on the North Side in about 1988.

"He said at first he didn't fully understand what it was," said David Bloom, vice president of books, prints and manuscripts for Samuel T. Freeman & Co., which estimates the map's value at $60,000 to $90,000. "It was folded up among miscellaneous house sale items. I don't think he had a eureka moment."

But in 1994, the owner began to research the map, and what he found convinced him he had purchased the sole survivor of three known original drawings of the city's 1784 plan, whose whereabouts were last documented in 1842.

"This document stands as the Penn's charter of Pittsburgh," the auction house's description reads. "Every deed issued by Penn's Philadelphia Land Office referred to it. All subsequent real estate ownership in Pittsburgh's 'Triangle' is based on this document, and it [and its facsimiles] have been presented as evidence in many court boundary disputes in the 19th century."

The map measures 13 1/2 inches by 17 1/4 inches and has some darkening and creasing. Street names are assigned and blocks are divided into numbered lots, but no names appear on the lots.

The Historical Society of Western Pennsylvania, which operates the Senator John Heinz Pittsburgh Regional History Center and owns a large 1805 map of Pittsburgh, is weighing a bid on this one.

"Our archivists have been in touch with the auction house looking for more information determining its provenance and authenticity and seeing what they could find out about where it originated and who owns it right now," said Andrew E. Masich, the society's president. "We're not sure if we're going to bid on it or what our interest is yet."

The map's authenticity rests on the authority and signature of James Ross, Pittsburgh attorney and U.S. senator who happened to be surveyor Woods' son-in-law.

Written on the map and signed by Mr. Ross is a two-line testament of authenticity: "This is the parchment draft referred to in my deposition," followed by "this 29th day of December, 1841."

The deposition was taken by city solicitor Moses Hampton for the purpose of authenticating the map. It also is signed by W. B. Lowrie, likely Walter B. Lowrie, another U.S. senator from Western Pennsylvania, as a witness to Mr. Ross' signature.

According to the owner's research, Mr. Ross had seen the map 40 years earlier in the Penns' office and later asked that it be sent to him during the Supreme Court of Pennsylvania's Commonwealth v. McDonald case.

An Allegheny County clerk's notation, "Recorded in the office for recording deeds" and dated Feb. 19, 1842, appears on the back of the map. A copy of the document, complete with Mr. Ross' statement of authenticity, appears in the Allegheny County plat book "Survey and General Plan -- Town of Pittsburgh 1784."

The owner said the plat book mentions two other copies of the map, one in the mayor's office and another in the possession of Alex Wilson. He thinks both may have been lost in the Great Fire of 1845, which destroyed about a third of the city. The third copy of the map was saved, the owner said, because it was in Mr. Ross' house outside the city.

Mr. Bloom, who said the map has never been published, did not feel the need to have its age verified by a conservator, which he does if a piece doesn't look right.

"I think it's a pretty unlikely forgery," he said. Under magnification, "It looked OK. It didn't look suspicious."

Fort Pitt's Col. John Campbell first laid out a street plan for Pittsburgh in 1764, but it included only four blocks adjacent to the fort, along the Monongahela. Twenty years later, Mr. Woods and his assistant, Thomas Vickroy, continued the plan to Grant Street and added two other streets, Penn and Liberty, parallel to the Allegheny, and a public square to hold a market house and courthouse.

The earliest known published map of the city is the 1787 survey by John Hill, in the collection of the University of Pittsburgh's Darlington Memorial Library.

The parchment map will be sold in a Nov. 19 auction of Pennsylvania furniture, decorative arts, folk art, books and manuscripts. The sale will benefit the Library Company of Philadelphia, co-founded by Benjamin Franklin, and celebrate the auction house's 200th birthday. An image of the map will be available online by Nov. 9 at www.freemansauction.com.

If the map and its owner's story are authentic, they pose another mystery: How did the map make its way from the hands of Mr. Ross, who died in 1847, to a North Side house sale?

And where had it been for 146 years?