Originally Posted by
Looigi
RWGPS uses old USGS topo map data for elevation and grade. There's one stretch I ride over where the road has been routed through a cut in a big hill. RWGPS shows a hill and 30% grade for my uploaded ride because the cut wasn't there and the road went around the hill back when the topo data was generated. Also, the granularity is pretty big in the topo data, so there can be very steep sections and even ups and downs between contours that won't be captured using the topo data.
Strava must have a similar problem. This bridge near me;
Corresponds to this strava segment;
Strava Segment | Maxwell Bridge East
Showing a maximum elevation of less than 30 ft.
To the OP, buying a Garmin or similar is handy for real-time slope info, but if you have a smart phone and don't mind waiting until you get back from your ride to find out how tough that hill was, just download a tracking app like strava, endomondo, etc and keep it in your pocket.