Tuesday, October 30, 2018

Just how much is this going to hurt? A Strategy for Interpreting Raw Trail Data

A casual glance at this hike shows that it crosses an awful lot of contour lines:

But what does that mean in terms of level of difficulty? Is this going to be an exhausting slog, or a casual ascent?

Zooming in, it's possible to use the contour lines to derive the starting and ending altitude:

Which turns out to be 640ft and 1,040ft respectively. Also, using Backcountry Navigator it's possible to measure the distance between the two points.

Using this data, I now know that if we tackle this hike we'll be gaining 400 feet of elevation in 2.2 miles. But I'm still left with the original question: is this going to be an exhausting slog, or a casual ascent?

To answer this question, I decided I needed a PHC, a Personal Hill Catalog. That is, the stats on a number of local hills I'm intimately familiar with, that I can use to help interpret raw trail data.

My first attempt to setting up a PHC involved going to the hills in question and using my phone to collect the relevant info. You can see my attempt at doing this here. The problem arose when I compared the stats on my phone with Shira's phone for one particular hill. The numbers were way off, which lead me to question how accurately my phone can measure altitude.

Fortunately, there is a far more reliable way to get the elevation data I was after. Arlington County publishes a series of online maps, one of which includes topography. It's possible to know, within 2 feet, the altitude of any point in Arlington.

Using the topography map for elevation and Google Maps to measure distance, I was able to create my Personal Hill Catalog:

This spreadsheet consists of three base values: low elevation, high elevation and distance measured in feet, and a number of derived values (such as distance in miles, or slope percentage).

With my catalog in place, I was now ready to answer the question at hand: just how brutal would this hike to the summit be?

The slope of a 400 foot ascent in 2.2 miles is about 3.4%, which is surprisingly close to the Glebe, from 26th st to Walter Reed hill. I've walked this hill countless times, and while it's noticeable, it has a more or less gentle feel to it. This tells me that despite the crossing of so many contour lines, we have a do-able hike in front of us, and not some monster. I won't be able to declare the catalog a success until I've verified my assumptions in the field, but I'm amazed at the promise it shows with only a handful of local hills.

Incidentally, comparing my GPS readings with the Arlington County Map values shows that my phone wasn't especially off. In most cases, there was just a few feet difference, not enough to influence the catalog. Verifying the accuracy of my phone's GPS has been yet another benefit of this little project.

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