The gift by the Evelyn and Walter Haas Jr. Fund accelerates by several years the rebirth of the historic Army base by simultaneously launching three projects at the top of the park's to-do list.

"People will begin to enjoy this park in a way they never dreamed possible," said Craig Middleton, executive director of the Presidio Trust. "This is a pivotal moment for the Presidio."

The announcement opens a new chapter in the transformation of the former Army base into a financially self-sufficient urban park at the foot of the Golden Gate Bridge.

It also cements the Haas family's place among the Mellon and Rockefeller families as great supporters of public parks. Together, the Presidio donation and the family's previous $18 million contribution to the renovation of Crissy Field is the largest cash donation ever made to the National Park Service.

"Our intention with this gift is to help ensure that the Presidio will be a place that is used and enjoyed by the entire community," Robert D. Haas said. "Our national parks belong to all of us."

The donation thrilled park officials and advocates for the Presidio, including House Speaker Nancy Pelosi, who was instrumental in establishing the park when the Army abandoned the post in 1994. She will be among those announcing the grant in San Francisco.

"This gift will help the Presidio realize its potential as a park for all people -- a shining jewel for our city," Pelosi said in a statement.

Work will begin almost immediately with a $4 million renovation of Rob Hill campground.

The campground, open only to youth groups and other organizations, is one of the city's best-kept secrets. It sits in a stand of eucalyptus trees overlooking Baker Beach from the Presidio's highest peak. There isn't much there beyond picnic benches, wood chips and a spectacular view.

Plans call for doubling to four the number of campsites to accommodate 120 people. A public green will include a large stone fire circle. Campers will enter Rob Hill through a stone entryway instead of an ugly gate fashioned from steel pipes.

"It's a wonderful site, and it will be much more beautiful and usable," Middleton said. "By this time next year, it should be done."

Park officials will erect six scenic overlooks throughout the park, each as grand as those at the Presidio's Inspiration and Immigrant points.

A new Crissy Field Overlook will offer one of the best views of Alcatraz Avenue and beyond, but now it is little more than a wide strip of pavement along Lincoln Boulevard. Plans call for stone benches along a stone wall set among a landscaped terrace. Park officials hope to have the overlook finished within a year.

Other overlooks will offer views from Presidio Gate, Lobos Valley, a clearing overlooking National Cemetery and other locales.

The most ambitious proposal is for a network of 24 miles of trails. Plans call for renovating many of the trails, paths and sidewalks that cross the Presidio and building several new ones.

Central to the trail project is a plan to finish the Presidio Promenade, which will allow visitors to walk from the Lombard Gate to the Golden Gate Bridge. The trail, popular with joggers and noontime walkers, now ends at the Main Post in the center of the Presidio.

The $10 million designated for the trails is contingent upon the Golden Gate National Parks Conservancy, the park's fundraising arm, generating another $7 million for the project.

Greg Moore, conservancy executive director, called it "a call to action" that will demonstrate "the civic spirit and generosity of our community."

The Presidio, which covers 1,168 acres, must be self-sufficient by 2013 and so has a commercial component not found in other parks. Filmmaker George Lucas set up shop in the Letterman Digital Arts Complex. A museum celebrating the life of Walt Disney is slated for the Main Post. An Asian spa, a French bistro and a giant sporting goods store are set among nonprofit agencies of every description.

In addition to making the Presidio financially stable, park officials want to make sure it is a place people want to go. They are especially eager to bring children into the park, and they believe Rob Hill will build on the success of the Crissy Center and other programs for kids.

"It will ensure that every child in the city has an opportunity to go camping," said Brian O'Neill, superintendent of the Golden Gate National Recreation Area. "It's a wonderful opportunity to introduce people to the outdoors. You can say, 'Oh, isn't that marvelous -- a $15 million donation.' But it's really about what that money will do."