A great way to make this more accurate is to enter a starting elevation. First you can find your starting elevation here:
Then enter a new elevation point on the Garmin by pressing Menu > GPS > Set Elevation and follow the prompts.
Next time you start from that point, you will see an additional dialog when you press Start on your Garmin 500: "Elevation Point Found".
Note to my fellow Colavita Riders: The starting elevation of the Deerfield train station lot is 673 feet.