From makeshift encampments beneath overpasses to dank Sixth Street alleys, hundreds of volunteers will scour San Francisco's darkest corners tonight in an effort to take an accurate count of The City's homeless population.

The biannual street count is required in order to qualify for about $10 million The City receives annually from Housing and Urban Development, according to Ed Cabrera, regional coordinator for HUD's council on homelessness.

Cabrera called the count "an important part of ending the problem of the chronically homeless." He said the Bush administration is "focused on results" and without an accurate sense of the number of those sleeping outdoors, it would be impossible to measure the success of individual homelessness programs.

Working in teams of two and three, the 250 volunteers will search between 8 p.m. and midnight, and will cover 90 percent of The City's geography. Unlike the last count, done in 2002, surveyors this year will collect an array of specific information, including age, gender, ethnicity, and exact geographical location, according to Trent Rhorer, executive director of the Department of Human Services.

The size ofThe City's homeless population is generally thought to be between 8,000 and 12,000. The 2002 count found 8,640 people living on the streets.

Homeless people currently residing in hospitals, jails, and treatment centers will not be part of the count, although Rhorer said The City has accurate information on those populations.

All of the information gathered in the count will be entered into a geo-mapping system that will help street outreach workers focus on problem spots.

"It's just smart. If you are going to deploy outreach workers, you're going to want to know where to deploy them," Rhorer said.

The count will also give city officials a yardstick by which to measure Care Not Cash, Mayor Gavin Newsom's program that took cash welfare payments away from the homeless, replacing the money with services and housing. Thus far, 700 homeless have been housed.

Angela Alioto, the civil rights attorney who is Mayor Gavin Newsom's advisor on homeless issues, said the specific information would help house the chronically homeless.

"It's going to be the most accurate count The City has ever had," she said.