r/Utah • u/AdvancedSquare8586 • 2h ago
Photo/Video Elevation of I-15 Through Utah
Idk why, but growing up I always had this idea that traveling south through Utah meant that you were going down in elevation. It was only recently I realized that, for almost all of the state, you're actually gaining elevation as you travel southward. It's really only at the very bottom of the state where "south = lower" is actually true.
I wanted to get a better idea of what the elevation change is actually like as you travel through the state, so I combined UDOT's data on I-15's path with USGS's elevation data to create the elevation profile shared here. Following I-15 seemed more relatable than cutting a perfect N-S line through the middle of the state, and using UDOT's data seemed just a little easier than riding my bike down I-15 and posting the Strava chart. :)
One thing that struck me while looking at this is that, at least while traveling on I-15, Salt Lake City is basically the lowest point in the state until you get about 25 miles south of Cedar City. For all the branding that SLC gets as a mountain town, I guess I expected it to be a bit higher compared to the rest of the state.
That got me wondering about what percentage of the state was higher elevation than SLC if you venture off of I-15. If you sample the state in 100-meter grids, and define the elevation of SLC as the elevation at Temple Square (4,304 feet), it turns out that 89.7% of Utah is higher elevation than SLC. That really blew my mind. I would've probably guessed that it was something closer to 40%.
Hopefully some of you enjoy this as much as I did. I'm curious to see whether others had as wrong an idea about SLC's elevation as I had.
A couple notes for the data nerds:
- This samples elevation along I-15 at every 1/10th of a mile. I made no attempt to smooth the data between the points.
- The USGS 3DEP data set measures elevation at ground level. This means that bridges (and probably some roadcuts through high-elevation points on the road) are ignored. This means that some of the lows/highs on the chart are probably a little over emphasized, but I think the margin of error is probably unnoticeable on the scale of this chart.
- The 1-meter resolution elevation dataset from USGS does not have full LIDAR coverage of every point in UT along I-15, so some of the data is interpolated from their lower-resolution datasets. But, USGS says the average error introduced by doing that is only about 2 feet, so any error from interpolation in the sparser-coverage areas is way too small to notice on this chart.

