One-way GPS device (PND) sales decline as two-way GPS smartphones offer turn by turn navigation and other personal location services.

PNDs are fast being cannibalized as Google and smartphone makers such as Nokia offer free turn-by-turn navigation.

Google’s no-cost Maps app, bundled with Android smartphones, includes voice directions and turn-by-turn navigation, just like the stand-alone big boys. (This is a category in which Androids clearly trump iPhones. The iPhone’s Google Maps app does not offer these features.)

A total of about 41.5 million PND units were shipped in 2010 and that will dip to 39.2 million units this year, according to market research firm IHS iSuppli. Berg Insight sees shipments dropping by a fifth, to 34 million units, by 2015. IHS expects more than 135 million navigation-enabled smartphones to be sold this year, while such phones already in use will almost double to 269 million by 2013.

If you were wondering where the answer to the “where is” the 2-way GPS smartphone business going, skip the Google search and go straight to GTX Corp and its LOCiMOBILE subsidiary.