Exertion Indices are an attempt to objectify the difficulty ratings of the hikes so that the ratings won't depend on how the surveyor is feeling.

The GPS receiver the surveyor uses records position and elevation every few seconds automatically. The index for any hike is calculated by adding all the recorded up-steps and ignoring all the down-steps. This is more meaningful than minimum and maximum elevations because it accounts for the ups and downs in between.

Naturally there are some inaccuracies. The GPSR's accuracy is limited at best and sometimes tree cover or canyon walls make it worse. And of course the details of the trail profile are unrecorded between recordings.

But as long as the results are used comparatively they seem to be valid. The Cerro Colorado mine hike has an index of 815 and it seems to the surveyor to be barely in the strenuous category compared to hikes listed in guidebooks.

Consider also that the indices don't consider absolute elevation. If you're used to hiking at sea level some of the higher hikes will take longer to complete than if you've had a couple of weeks to acclimate.