For three days and nights, several thousand volunteers have fanned out across Los Angeles County trying to get an accurate tally of the county's immense homeless population. The Greater Los Angeles Homeless Count is considered the largest such effort in the nation. The count is now done annually, one of several changes intended to make it more efficient and effective.

Conducting a homeless count is not easy. Volunteers traipse into dry river bottoms, push through thick brush at public parks and cruise along dimly lit city streets.

Volunteer counters are discouraged from having too much interaction with people -- there’s a more comprehensive census-style survey that is done separately from the actual counting -- or from disturbing people holed up in makeshift outdoor shelters or vehicles.

So volunteers, typically led by members of the Los Angeles County Homeless Authority’s street outreach team, typically rely on their own intuition, instinct and internal extrapolation.

It’s an imperfect science, to be sure.

"The way the count is conducted, it is often done drive-by, right. You're in a car looking for people, but really the people are not to be found,” says Wade Trimmer, director of the San Fernando Rescue Mission, about 30 miles northeast of downtown L.A.

Trimmer says there’s been a significant spike in the San Fernando Valley’s homeless population. But he doesn’t think it’s accurately reflected in recent L.A. County homeless counts, partly because of how the count is done: mostly at night by relatively inexperienced volunteers.

"If you look at downtown [Los Angeles], it makes a lot of sense to count at night because people are congregated downtown at night," says Trimmer.

“It doesn't make as much sense to me to do the count at night in the San Fernando Valley because people are not congregating on the street," he says. "They’re finding a place to sleep, they're in the canyons or behind a building.”

It can be frustrating, says Trimmer, because shelters and other services depend on the data collected during the count. It’s the number cited by local leaders to justify spending more or less of their own resources. It’s how the U.S. Department of Housing and Urban Development figures how much money to give L.A. County.

“It helps us advocate for our fair share of funding,” explains L.A. County Homeless Authority spokeswoman Naomi Goldman. "It's also data that helps us work with the community-based organizations and the homeless service providers to figure out where best to allocate the dollars so that'll do the most impact."

Last year's L.A. County homeless count recorded a 12 percent increase from 2013, or around 41,000 people living on the streets and in shelters across the region.

More than half of them were living in the city of Los Angeles alone, with the greatest number concentrated in and around downtown’s Skid Row.

But the raw numbers are just part of the picture. Just as important, says Todd Palmquist, is a survey portion that works a little like a census survey.

"Certain numbers of the homeless are actually interviewed," says Palmquist, former director of the San Gabriel Valley Consortium on Homelessness and a volunteer homeless count coordinator and trainer.

The survey helps identify trends, such as more people living in cars, for example, or spikes in the number of kids or military veterans living on the street.

"It can really give a good idea of populations that are in need of help, and hopefully eventually be able to be used to identify and set up programs to prevent homelessness rather than just help people get out of homelessness,” says Palmquist.

This year L.A. County shifts to an annual count, a move it hopes results in more consistent data. The count used to be done every other year.

It's also deploying more teams to far-flung areas of the county where homeless people are harder to find, and expanding efforts to link people to services on the spot. But counts are still done mostly the old-fashioned way, says Wade Trimmer.

“Why can't we do the count on the smartphone or do it on a tablet?” wonders Trimmer. “It’s (still) pencil and paper and I have volunteers who are always surprised, who come back and say: 'This is how we do the count?' ”

LAHSA’s Naomi Goldman says the count is starting to go high tech, though slowly.

"We're always looking to refine and see what can we improve," says Goldman. "We do have parts of the count that are using iPads and Survey Monkey, which will make some of the counting and some of the process a little more streamlined, a little bit faster."

Results of the 2016 Greater Los Angeles Homeless Count should be available by spring.