In all the excitement around mobile local advertising, most location based ad impressions are way off their intended target. This is according Thinknear, whose new Location Score Index, is the topic of our latest podcast interview with founder and GM Eli Portnoy (embedded below).

This is a message we’ve been hearing for a while (see “10% problem”), but is quantified further in Thinknear’s report through testing user-reported locations versus ad-network reported locations. The difference between the two — scaled to a sample of 53 million impressions — was revealed.

— The overall mobile industry scored 49 out of 100 total points for location accuracy

— 67 percent of industry ad requests included latitude and longitude data

— 34 percent of ad requests with location data were accurate to within 100 meters of the user’s true location

— 45 percent lift in mobile time-on-site metrics for campaigns leveraging top tier location data

— 30 percent to 51 percent lift in store visitation rates fore campaigns leveraging top tier location data

Excluding some of the leaders in location based ad targeting (see our post yesterday about xAd), these faulty location impressions are mostly coming from larger ad networks that are latching on to the excitement around location targeted ad relevance.

This is a double edged sword as we discuss in the podcast: The interest in location targeting is growing among advertisers… a good thing. But many publishers and ad networks are opportunistically filling that demand by “inferring” lat longs from less reliable means like reverse IP lookups.

This is a big deal, as it hampers the true performance of location based ad targeting. As we’ve been saying for years, genuine location targeted ad strategies can boost performance and return on ad spend. It’s also great for mobile publishers who can see higher premiums on their ad inventory.

But the disingenuous practices being uncovered by Thinknear’s study degrade the value of this ad inventory over time, and give location targeting a bad name. We covered this closely in our mobile ad attribution white paper and the conversation continues this week with Eli.