I see that
eight nine people have already voted in the poll I put up yesterday, even though I haven't found time to blog about it until tonight.
I'm putting together my classes for this semester. (I don't start until Sept. 1, so I've got a few more days.) One of the classes I'm teaching is our sophomore-level field methods course. The title might be misleading - the course is an introduction to geologic mapping, and leaves out many other things that geoscientists do in the field. (In fact, my intro class will be doing some of them in their stream lab.) On the other hand, geology isn't just a set of technical skills that one is trained to do - a geologist should be able to imagine what's hidden underground*, as well, and the skills in this class are designed as one step towards developing the ability to think spatially.
Geologic maps show what rocks are found near (or at) the surface, but they also provide the information needed to make a good guess at what might be hidden underground. That information comes from the orientations of layers of rock, and of fractures and cleavages and faults. And to find that information, we still use big, clunky, old-school geologic compasses. (GPS is great for figuring out where you are, and we use those, too. But the compass is still a basic piece of equipment.)
When I was young (*shakes cane*), I learned to measure bearings by measuring angles from north or south:
If that arrow in the middle of the circle was... oh, let's say it's the direction a current used to flow. I would have measured the direction of the arrow as being 45 degrees east of south (or in my field notes, S 45 E).
But that's not the only way to measure angles. A circle can be divided into 360 degrees, and compasses can measure bearings around a circle, starting with north and then spinning through east (at 90 degrees), south (at 180 degrees), and west (at 270 degrees), and finally getting back to north, if they didn't get dizzy and fall down in the meantime. Those kinds of bearings are known as azimuths. They're easier to deal with mathematically, because they're one number, always measured in the same direction. They confused me when I first dealt with them, because I'm lousy at memorizing numbers. (I hate combination locks.) I've gotten used to them, though, because I've worked with people who prefer them.
I always used to think that quadrant measurements were more intuitive. I think about directions as "southwest", not as "a bearing between 180 and 270 degrees." And I carry a protractor that measures angles between 0 and 180 degrees easily, but which requires me to subtract larger numbers in my head. (And I make mistakes when I do arithmetic in my head.) I prefer to make my maps in the field - they are tools for helping me think about what's going on, and if I wait until evening (or later) to compile them, I might have to hike miles and miles back to the same spot to collect more data. And that means that my favorite compass is one that gives me numbers less than 180 degrees.
I've seen more and more students who are at least as comfortable with azimuth measurements as with quadrants, though. And at least three of my colleagues prefer them. (And I've got a sinking feeling that I'm just hideously old-fashioned for carrying a quadrant compass.)
I'm not going to trade in my compass. It was expensive, and it still works, and I'm capable of converting the different measurements. (I can almost do it in my head. If I practice enough. Grading 20 labs every week gives me lots of practice.) And I always make students convert their measurements back and forth. (I still need to do the math in my own head, though, to check them.) But I'm curious what the current trends really are. Does anyone prefer quadrant compasses, or are people switching entirely to azimuth? Are the azimuth people primarily geophysicists? (Paleomag is easier in azimuth, I believe.) Are azimuth bearings easier to handle in GIS?
And if I carry a quadrant compass, should I carry a sliderule, too?
*Yes, I realize that may be a structural geologist's bias, but you know... in grad school, I could have been a geochronologist or a metamorphic petrologist. I've become a structural geologist by necessity, because my departments needed someone to teach spatial thinking more than they needed someone to unravel the exhumation histories of metamorphic rocks.
Tuesday, August 26, 2008
I see that