How about the roads near N37° 23.235' W80° 23.584' and N37° 21.247' W80° 32.022' First, the roads
are not where the map says they are and this may be its biggest problem. Second, none of the cuts
and fills are expressed in the topography or the underlying data points. I believe the contour lines
are derived by TopoUSA from selected input points (which are not necessarily closely spaced or even
in a uniform grid.)
Since you are a pilot, I think you should get out of the clouds and get your feet on the ground.
Remember that those airport elevations that TopoUSA agrees with are flat places and near known
input points.
I have written computer contouring mapping programs and have spent the last 40 years intimately
acquainted with geological and topographical maps. I think the most accurate (in the US) topo maps
are those published by the USGS. They are only accurate at the surveyed Bench Marks. Even these are
sometimes in error. And especially those in remote mountainous areas where these grades are being
determined. Everything on a USGS topo map between Bench Marks are guesses. The guesses may be based
upon stereo photographic information and made by a computer, but they are still guesses. The closer
a point is to a BM, the more accurate it tends to be. I believe TopoUSA uses a similar photographic
method to suppliment known points.
As for GPS. I registered my GPS two days earlier at a USGS BM about 40 miles from the point where
this picture was taken.
http://www.clee.org/MoM2002/image029.htm
http://www.clee.org/MoM2002/image030.htm
The Blueridge Parkway sign called the elevation of the overlook 2400' the GPS registered 2370' the
Topo map in the GPS memory (Not TopoUSA) said that I was on the 2100' contour. The location of the
GPS in relation to the road on the map said that I was about 50' NW of the BRP. IIRC, the data
point from the GPS when plotted on TopoUSA was about as inaccurate as the topo data in the GPS. I
will try to find the track for that trip and give you the coordinates for that point so you can
compare for yourself.
BTW, TopoUSA (v4.0) is a fine product as long as you consider it's limitations. Here are some
impressive jpegs of the kind of things that TopoUSA can do well.
http://www.clee.org/BikeTrips/default.cfm?main=MoM.cfm Click on each one to enlarge.
You also may be interested in this review of TopoUSA 4.0 by Alex Wetmore:
http://phred.org/~alex/bikes/topo4.html
In article <
[email protected]>,
[email protected] says...
> I don't know what you're talking about. Topo-USA works in 1' increments and has nothing to do with
> contour lines when calculating elevation. When calculating the grade of a hill, you can measure
> the elevation at the bottom and the elevation at the top, all within 1 foot, then measure the
> distance to calculate %grade. I've found Topo to be very accurate especially compared to GPS and
> cycle altimeters. When Topo gives an altitude of a point it compares exactly to all references I
> can find, which is far more than I can say for any consumer GPS I've seen. The references to which
> I refer are official altitude markers at passes also, since I'm a pilot, I have charts for airport
> elevations and Topo agrees exactly with those. I've seen GPS units vary by 50 feet in a few
> minutes without moving an inch. That goes for both Magellan and Garmin. What evidence do you have
> for the inaccuracy of Topo? I will agree that road position for very short and curvy sections are
> not especially accurate in Topo, but for the length of the hill from bottom to top the total
> elevation gained, and that's what is important in this calculation, it is extremely accurate.
>
> Gene
>
> "Cletus Lee" <
[email protected]> wrote in message
>
news:[email protected]...
> > In article <
[email protected]>,
[email protected]
> > says...
> > > I use "Topo-USA" Delorme's map program, it calculates %grade for me. It will also tell me the
> > > elevations at any point in a route, so I could calculate it myself.
> >
> > TopoUSA only reports what it can measure from the data (topo contours and
> road position).
> > Usually both are very wrong. Road cuts and fills (used to level out the
> road)usually do not
> > warrant a shift in contour lines and are invisible to TopoUSA.
> >
> > Most GPS have a vertical accuracy of +/- 10-20m So using the relative
> difference on elevation
> > as some have mentioned may or may not always work either.
> >
> > At best you will only get an approximation.
> > >
> > >
> >
> > --
> >
> > Cletus D. Lee Bacchetta Giro Lightning Voyager
http://www.clee.org
> > - Bellaire, TX USA -
>
>
--
Cletus D. Lee Bacchetta Giro Lightning Voyager
http://www.clee.org
- Bellaire, TX USA -
