Schematic
Software
The biggest challenge with this system was making correct usage of the SRAM memory space in the Arduino Uno. With 2k bytes available, this space disappeared fast, and online searches showed that developers have had problems with the Adafruit GPS / SD card libraries and the I2C library. In order to get around this, we dove into the provided libraries and stripped out as much as possible. In order to enable developers to use these platforms in all kinds of projects, the libraries provided for all of our modules contained quite a bit of unnecessary #defines and other code that took up needed space. While the compiler should remove this for us, I found that doing this manually had some drastic effects on memory usage.
The outcome is a sketch that will load the GPS module, SD card module, MP115A2 Module, and (optionally) the OneWire module, read from the sensors every 30 seconds, and record that data in a CSV file on the Micro SD card. There is very little error handling in this sketch, so you'll want to watch the pin 13 LED for a quick blink to see that it is writing to the card before you seal it up. Even though the sketch comments advise using the OneWire on Leonardo or Mega only, I've had success with enabling this on my R3 when compiling using Arduino 1.0.5 on my Mac. I've seen behavior differences in other sketches when compiled on a Linux system vs. a Mac, so YMMV. In addition, you'll notice that I pulled the modified libraries into this sketch directly. This way, you do not have to replace the libraries included into your Arduino IDE to build this project.
Download
Download the code here if you're interested in using it for your own project or if you need to use the MPL115A2 library with the GPS / SD logger library.