Are homeless population numbers real?

One of the first things I noticed when I arrived in San Diego, was this oft quoted article www.kpbs.org/news/2012/dec/11/san-diegos-homeless-population-third-nation/ What I found most intriguing is that this article compared San Diego's "County" population with other "Metro" populations, when determining that San Diego had the 3rd highest homeless population in the country. This comparison seems illogical. Metro for metro comparisons and county for country comparisons would somewhat better - except that every county and every metro area in the country is of different sizes and are calculated differently. San Diego County is one of the largest counties in the country. The city of Vista CA has a total size of 18.6 sq miles. And the city of San Diego has 372+ square miles. Can there really be an accurate comparison between homeless populations between the two? fyi: the county of San Diego is 4526 square miles of space. In such large spaces is it really possible to get a handle on the number of homeless people within it?