The trouble with TOPO!

Lots of great comments...

I was using an older version of Topo. I upgraded to version 3.4.3 and it seems to behave about the same. I think following contours has inherent problems of 1) using a crude pointing device to draw the route, and 2) possible mismatch between the topographic image and the underlying elevation dataset. Some of these maps were drawn many years ago. I wonder, were the maps and the elevation datasets compiled independently? The slightest mismatch would cause errors in the elevation profile.

My mental picture is of a big array of elevation points underlying the topo map image. If there is any mismatch, then you'll accumulate errors, even if you are able to perfectly follow the contour, which most people cannot do.

In his article, Tom Chester wrote that in Topo "they implemented a software change to ignore changes in elevation between 0.026 mile steps of less than 50 feet." This reduced the noise, but obviously didn't eliminate it. I'd like to see an option that would use a stronger filter. Maybe require the elevation to change by X contours before it registers. (I'd try two contours or 80 feet.) I think that would clean up most of the noise I'm seeing and would have almost no negative effect on the routes that I profile.

Another approach would be to allow you to do "point-to-point" elevation plotting. After drawing the route, you could move along it and click on the high points and low points and have the software calculate the elevation totals from those points only. That's how I do it by hand.

Ethan suggested reducing the vertical exaggeration on the Miners Ridge profile that I posted. I don't have any control over that, as far as I know. I just posted what Topo displayed for the route.

To "compress" the profile a little, click and drag down the grey border-line between the profile and the map.  If you drag it tooo far down though the profile area disappears; there's like a minimum height it has to be... about 1.5 inch, on my screen at least.

As far as I can tell, there's no way to expand it horizontally, so it goes beyond the margins of the screen.  That'd be ideal. 

Which reminds me of something I've been sort of wondering.... any other TAY-ers work in cartography or GIS stuff?

That would be me.

My degree is in Geography with emphasis in Cartography/GIS/Remote Sensing and I've been doing it since 1995..

I had never heard this error explained before, but it doesn't surprise me. Whenever I bring my GPS, the distance it records is always greater then when I import my GPS track into TOPO. I figure the GPS is likely to over calculate my distance, but since it's always been so far above TOPO's figure, I was starting to feel the TOPO distance was short. In addition, I've drawn USATF road running race courses several times before in TOPO; everytime I've come up short of the USATF distance of the race.

Thanks for another informative post Lowell.

After spending more time with TOPO, I've got a better idea of how to use it and what its limitations are.  I decided that my previous conclusions about elevation profiling were too hasty. 

I redrew my route maps using the 7.5' map series at 200% magnification.  Using magnification is critical.  Otherwise, it is too hard to draw a precise route with a mouse.  Even with magnification, drawing routes is a slow, tedious process. Surprisingly, I found that TOPO calculates the route profile differently depending on which map series you're displaying when you profile the route.  Even the magnification makes a difference.  (It doesn't seem like it should.) I decided to compute profiles using the 7.5' map series at 100% magnification. 

Using this approach, I found that TOPO's elevation gain/loss statistics are better than I previously thought.  Even routes that contour slopes were profiled reasonably well.  I found some elevation totals that seemed about 10% too high.  These were generally routes that followed undulating ridge crests for a significant distance.  The TOPO profile seemed more jagged than the actual ridge in these cases. 

To evaluate the mileage totals, I consulted Spring and Manning's "100 Classic Hikes in Washington." I looked at the Crater-Jackita Ridge-Devils Loop hike because it has a profile somewhat like a high level ski route.  I found that book mileage was higher than the TOPO mileage by a factor of 1.13.  That seems about right. 

The purpose of all this effort was to analyze a large sample of past ski trips to create a formula for estimating how many days future trips should take.  I think I've come up with a reasonable system for use with TOPO profiles.  My previous system was based on hand measurements of a small number of trips.

I have come to figure on my use of TOPO generating distances that are discounted from actual by about 15%. The discount is slightly higher on traverses and apparent 'flats' and slightly lower on more direct ascents.

Some of this is due to the smoothing inherent to the software. The rest is due to the impatience of the user in crafting the route. As you noted, the use of the magnification feature produces truer results. For most day trips this is of small consequence. For the longer trips it really matters.