If you map out very short sections then they're pretty close to the USGS survey data. Garmins work the same way when they run out of space. basically, it takes so many sample locations based on the total length of the plot. The longer it is, the more detail it loses and it underestimates everything. With the Garmin units, you can record so much data at 1 sample per second (3.5hrs on the 305) and after that time frame, it starts to drop every other second as it records past the limit. It continues to widen the samples and as it does, it too loses detail.

I used mapmyride (free) for the longest time to track my habits until I bought the Garmin unit. Being able to track and map things like cadence and HR alongside the data is way more beneficial for me. Now it's all on GarminConnect.