First I want to say how impressed I am by the knowledge and dedication all the members have here. Being the noob that I am, it's really cool to read what everyone does here.
So, my question: I'm wondering how simple it would be to use an APM2 for only data logging? To clarify, it wouldn't be used to control anything, it would just be powered and ride along with me and record data.
The second, related question: If this is doable, how long can the onboard 16mb store of data logs? I read one post where it said about 3.5 hrs (I'd imagine this varies by what you are logging and how).
Of course if anyone wants to offer a brief explanation of how this would be done or what issues might be encountered that would be very helpful. From what I've read I would need to power the card, and then go through the similar pre-flight procedures as if it was attached to a drone, and go from there.
While you could, it seems a little overkill with the APM. Do you need altitude?
it would just be powered and ride along with me and record data.
The ArduiIMU board with an add-on GPS is better suited, smaller, and just easier to deal with generally. It doesn't have an altimeter, but is does have the same Invensense gyro/accelerometer, and a GPS port, and the output is human readable. Being that the 328p cpu is basically an Arduino Uno minus the USB interface, it wouldn't take a lot of effort to add a shield with a SD card slot and depending on the libraries used to read the format of the card, you could have minimally 2GB and maybe as much as 32Gb.
Some info on what this is kicking out.
Now, let's get deeper into your project. If you are concerned with how long it logs for, then it sounds like you are traveling some distance (more than a mile). Do you really need the accelerometer and orientation data? Seems to me standard GPS data logging gives you everything you need (position, speed, and rough altitude). So in my mind, you are capturing way more data than you need/want, paying too much for too many sensors, and just a bit of overkill.
Key here is that for any real distance, you need GPS, and in order to get GPS, you need line of site to the sky. So you can't put this in a box in your pocket and be assured of solid GPS signal, especially not in any vehicle, unless it is next to a window.
I might even lean towards this as a lower cost solution http://www.adafruit.com/products/98 (that's a board that has the SD shield but requires the GPS module and Arduino(or clone).
GPS Module http://www.adafruit.com/products/99
And any standard Arduino and a battery source.
Again, I have the feeling you would be better suited with a cheaper solution, but that all depends on what you want to do with the data, how you want to visualize or use it. Almost any of these solutions could work but all will crap out it you toss it in a bag or backpack that is buried and cannot see the sky to get GPS. You'll get orientation data, but to accurately determine where it's been is a bit of a stretch if you don't have GPS.
The instructions and more data http://ladyada.net/make/gpsshield/index.html
Update, this may even be a better solution
The other cool feature of the new MTK3339-based module (which we have tested with great success) is the built in datalogging ability. Since there is a microcontroller inside the module, with some empty FLASH memory, the newest firmware now allows sending commands to do internal logging to that FLASH. The only thing is that you do need to have a microcontroller send the "Start Logging" command. However, after that message is sent, the microcontroller can go to sleep and does not need to wake up to talk to the GPS anymore to reduce power consumption. The time, date, longitude, latitude, and height is logged every 15 seconds and only when there is a fix. The internal FLASH can store about 16 hours of data, it will automatically append data so you don't have to worry about accidentally losing data if power is lost. It is not possible to change what is logged and how often, as its hardcoded into the module but we found that this arrangement covers many of the most common GPS datalogging requirements.