A day of kinematic GPS

I really don't mean to be neglecting my blog (or neglecting commenting on your blogs) as much as I have been lately. It's mostly that, after the classes and the rehearsals and the homework and the grading, I put aside the papers and think only of sleep, rather than of spending more time at a screen writing things. I'm accumulating a rather lengthy To-Blog list, which, following Maria's lead> I think I'll have to make a summer break resolution (since New Year's is way too far off) to actually write up.

Anyway, though much of my time this quarter has been spent at a desk, the seminar I'm taking on tectonic geomorphology and quaternary field methods has served well to get me out and about. Many of our "trips" have been into the hills behind campus, mostly for funding reasons, but we have had a few larger outings. The most intensive one so far was when we went to Grass Valley in the San Bernardino Mountains to make a digital elevation map using kinematic GPS. (There was also a USGS/Caltech team out there using LiDAR to image some precariously balanced rocks, but we did not get to actively participate in this part of the work; we were mere observers.)

Grass Valley is located in the San Bernardino Mountains, not far from Lake Arrowhead. It falls into the region of highest possible threat of major earthquake ground motion in the state, due to its proximity to several major faults. The San Andreas Fault is 11 kilometers away, with several well-constrained paleoseismic sites within 20 km. The San Jacinto Fault is only a kilometer or two further away than the San Andreas; the north frontal thrust of the San Bernardino Mountains is about the same distance away to the north. The Cleghorn Fault runs directly through Grass Valley, though its activity isn't as well constrained; there's no current microseismicity, and no evidence of Holocene rupture.

The precariously balanced rock team was clearly there to try and constrain whether or not the worst case scenario of shaking according to the state hazard report had actually ever happened since those rocks became precarious to begin with. The focus of the class exercise, however, was more on a catchment containing a small system of drainages in the process of being captured by the Mojave River. Our goal was to use kinematic GPS to get a good picture of the area, which could then be used to better situate the precariously balanced rocks in the middle of the site.

Kinematic GPS is a method of relative measurement. Actual latitude/longitude figures don't come into play. All measurements are taken relative to a base station that gets set up somewhere central in the site, for the express purpose of kinematic GPS measurement. People then carry portable GPS antennae, either attached to a backpack or on a long pole, all over the area surrounding the base station. The goal is to go back and forth over all of the bumps and dips in topography - even though that does, of course, make for harder hiking at times - to make sure they show up in the DEM. The data has to be corrected for height of the person carrying the antenna (I was not, shockingly, the shortest one!) and for roughness of gait, but the corrections that need to be made can be gauged by having all of the antenna-carryers walk the same path before going their separate ways.

We spent a full afternoon tromping around the catchment, and the four of us managed to cross paths only a couple of times, hopefully implying that we didn't all cover the same ground. In my run-in with one of the professors, I was detailed to walk around the far edge of the site that hadn't yet been covered. I did so, but then I proceeded to overshoot the other edge of the site and get quite confused. I continued to walk through all of the (mostly-dry) drainages I could find for a while, but I started to get worried when I wasn't catching sight of the base station or the precariously balanced rocks. I definitely realized the irony of not having a clue where I was, despite having a GPS antenna strapped to my back, but dark humor wasn't going to get me out of the situation on its own. I did manage to work my way back to the road, then proceeded to go the wrong way on it for about half a mile before I noticed that I was wrong. It turns out I'd found the road only about 200 yards shy of the turnoff that lead straight to the base station. Ah well!

I haven't seen our DEM yet, though I'm told we covered a lot of ground, and I'm told that there was one antenna that went quite a bit further away than all the others did. One of the people in the class is doing all the processing as his final project for said class, so I may ask him if I can show off the results of our day in the field.