Feature Requests: Altitude Feature Does Not Seem to Work

I've tried to use the altitude feature and it does not appear to provide accurate data.  I created on Bainbridge Island WA that is similar to the Chilly Hilly and it indicated total gain of about 650 feet.  Now anyone who has been to Bainbridge Island will know this is way off!  I also looked at two existing rides that were the same and one said the altitude gain was about 350 feet and the other about 650 feet.

Is there a problem with this function or how can one make it reasonably accurate.

 Thanks.