Percentage of grade - gotta love these hills!

[Crankin]

I routinely ride several short, steep hills that are 18-20% in places. I am in my granniest of granny, but I am not standing. So, I don't think you can say most recreational cyclists can't do this. It's all what you are used to. My driveway has a spot that is 15% and my street has some 10-12% grades. Believe me, I am not a racer, etc., just someone who lives around hills.
Now, while I can handle this, I don't like long climbs, i.e. mountains that go on forever, even if they are less steep. And the only time I got off my bike and walked was a sustained 18-22% grade for about a mile. I rode about 1/4 of it.
I have also found that my DH's Garmin estimates the grade as about the same as any other method we have used.

[mudmucker]

Just as a note, most of these "12-18%" grades that are encountered here in MA points east, are only sustained anywhere from 200 to 900 feet of horizontal (* generally mostly), maybe a little more in some circumstances, where the grades either drop to flat or changes to gently rolling topography thereafter and so there is much time for recovery. So yes these are quite do-able. And yes, I ride the same hills as Crankin on occasion and these are some of the ones I'm talking about. Most of the time, there may be 2 or 3 of the "steep sessions" but they occur within a short 1/4 mile stretch where again, the topography levels out to half that grade and less but there is still an incline maybe for a mile or so or not. Many times I look down at my Garmin, and the 13 or 15% only lasts a few seconds, or that amount of time it takes for the unit to register a few times and then it drops down a bit. Furthermore I have seen the Garmin register unlikely high spikes when I look at the data afterwards so don't take the max reading as gospel.

I am a GIS analyst and work with contours and USGS data and have had to take contour measurements in the field in my younger days. Most contour data used in county/state GIS offices come from USGS digital terrain models (the same contour lines you see on the topo maps) with vertical accuracies of several feet (can't remember exactly and it is a range so we don't know). Not only that, depending on the number of data points, the contours are interpolated so we are only good as the number of datapoints, the method of measurement and the interpolation method, all of which may have varying degrees of inaccuracy at any given location in the US. If you are looking for absolute data vs trend. So the answer to RolliePollie is these mapping sites are using USGS data that is standard and overall very good, but even some areas may have a "degree" of inaccuracy due to the aforementioned concepts. These surveys were done in the 1940's-70's with only slight updates in small areas - just as a note, in some county/states that can afford it, more accurate LIDAR data has been flown and I would trust that more but I don't believe the mapping sites are using this yet because there is not full coverage of the US. I still wonder about correlating Garmin measurements against places like MapMyRide but it's inconsistent - accurate in some areas, not in others. The same with the mapping sites - might be accurate in some places, or in the ballpark within 1 or several percentage numbers.

[Mr. Bloom]

Thanks mud - it's good to have authoritative feedback offered.

For the record on standing - I generally don't stand on the steepest hills unless I'm racing the clock...

[mudmucker]

Yes, it was quite a long winded way to say kinda sorta maybe. Basically saying, the USGS data is an excellent data source but as with anything there are always inherent inaccuracies with any measurement.

Standing/sitting: I'll stand in the pedals frequently on these steep parts I just mentioned but not all the time or maybe just stand on 1 or 2 of the incremental parts. I'm used to the short steep and I love them and don't seem to be able to muster the kind of power/speed I would like to on lower longer grades.

[msincredible]

I think it's certainly possible in the Sierras.

[mudmucker]

Actually you reminded me of a point for those who don't live in the "up and down varied terrain with short distance" scenario. I would expect the data to be more accurate under less varied conditions: so if you have a long consistent, non-undulating grade I would expect that data to be more consistently accurate. And, road grade cut into the mt/hill is going to be different than the adjacent hillside slope of that part of the terrain not affected by road construction, and how accurately might that road grade be depicted based on the rest of the surrounding survey data. And if it's new road construction after the USGS survey was done.....

As for Garmin measurements, SadieKate did a number of comparisons of different cycling computers but I think this was for ascent or amount of climbing vs determining actual grade. As to her comparisons, I think she was comparing barometric devices and also measured against a barometric ciclosport device but we are back to understanding which is the correct result. She may see this and correct me if I'm wrong. Generally I recall on some other forums that people find the Garmin may register higher than what is and I think for SadieKate's area of the US she realized she might need to incorporate some data smoothing (within Sporttracks) to bring it down within a reasonable and believable number for feet climbed.

[Jiffer]

Well, I can comment on elevation gain on mapmyride verses Garmin. Dh just did a ride yesterday that mapmyride said would be 3,500 feet of climbing and his Garmin said it was over 6,000. Almost double.