Before I bought my gpsr, I used my cell phone successfully by downloading an offline map that I could direct my phone GPS to and then put my phone in airplane mode, with GPS still on, while outside of range. If you don't put it in airplane mode, you'll drain your battery quick.
It's accuracy varied when outside of cell range, I used it very successfully on Hebo Mt deep in the Tillamook Forest, however in another section of the Tillamook Forest, it was so bouncy the coords were going 500' from one direction to the next.
It'll get you to the general area, but won't be as precise deep in the woods.
