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
What kind of compass do you prefer?
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13 comments:
I hate quadrant now (well, I guess 'hate' is a strong word) ... even though that's what I learned on ... I love the minimalism of azimuth.
Azimuth all the way!
When I saw your query on type of compass, the first thing that came to mind is the different approach between the Brunton I was taught with in North America and style which seems to be more popular here in Australia (sorry, I don't recall the brand name!).
With the Brunton we measure strike first then use the level to measure dip, and record them in that order in our field notes. With the Australian sort, they set the flat of their compass against the rock and pivot the compass in relation to it till the compass itself is level. They then read the dip direction from the needle, and pick up the compass and look at the angle between the body and flat part to read the amount of dip, in one easy step.
I haven't yet purchased one of these, because I don't yet know upon which continent I am going to settle when I finish my PhD, but I do like the look of them!
(Note: I can't use my Brunton here in Australia because the needle wants to point through the glass if I hold the compass level, it would be necessary to change the position of the weights on the needle itself to use it, but if I'm not staying "down under" I'd just have to switch it back when I head north again.)
Here in the UK we tend to use Suunto or Silva compass-clinometers with our students so azimuth is the natural way to go. It is also easier for entering data into stereonet drawing programs.
My battle here is over planar structures where I think it is easier for students to learn to record dip direction / dip angle rather than strike / dip. However, since tradition (but not on the British survey maps!) is to record the strike on the map then I have to teach strike / dip.
Being a geophysicist, I prefer a systematic way of recording strike and personally use 'left hand rule' (thumb down the dip, forefinger points the direction of strike - i.e. strike always 90° anticlockwise of dip).
However, this is deemed 'too complicated' by my colleagues and I have to teach a mish-mash of strike/dip/dip direction.
Dip direction really does make more sense mathematically (physically?). I can see its usefulness.
I'm never quite sure what to teach about the right-hand/left-hand rule. Some of my colleagues use the left-hand rule, but call it the right-hand rule (because the dip is always to the right of strike). My personal suggestion to students is to be as redundant as possible in field notes, because it's easy to make mistakes (on a steep slope, with the sun beating down on your head and flies swarming), and if their note-taking is redundant, they can recognize bad data later. (It's good to know that geophysicists use the same system in the US and the UK, though - I only started internalizing that system when I had students work with Vince Cronin's focal mechanism lab.)
The next question I should ask is whether strike/dip symbols are good ways to help people visualize the orientation of surfaces on a map. I think so, but it took a lot of practice for me to be able to look at a map and quickly visualize the orientations of the units.
I'm a GIS professional. Azimuth is the ONLY way to use these measurements within a GIS. An obscene amount of my time is spent converting measurements into a machine-readable format. Quadrant notation IS as outdated as the slide rule. Fact is, though, some people aren't willing to learn new techniques - and it falls to my sect to deal with 'em. Job security, I guess.
And when will we learn to use the metric system?!
it's all about azimuth. i'm an american (with a brunton) just back from field work in Mozambique with a Suiss (with one of those dip-azimuth things) and southafricans (with silva compasses). We had this discussion over and over. I think the Brunton is by far the most precise instrument of the options and although the others recognized this, they didn't think such precision made up for the slightly longer time it takes to make a measurement. I find it easier for beginning students to understand surfaces and lineations if they are represented as strike/dip/dir and plunge/trend. If a planar feature is also represented by plunge/trend it can get a bit confusing. But of course we all need to be literate in all the formats, even metric.
I debated this when i bought myself a brunton a few years ago. I ended up buying a quadrant, even though I knew I was being counter-culture by doing so. I learned to use a compass from my mom, who is an ecologist, and the Brunton I used at field camp and for the first part of my PhD belonged to the US Forest Service, so I wonder if there isn't a disciplinary divide at work here. That said, when I used my brunton regularly I got pretty good at converting between the two in my head.
In university were are being taught both ways to understand maps from other countries and it is also tested in exams, however the azimuth method is used as a standard in university field work to make all works more easily comparable. Then again we never use a Brunton but a Geologists compass after Clar that measures dip and dip direction in one step. I also know Bruntons but found them to be horribly inaccurate when comparing results but maybe thats because I am better trained with the first one.
"Are azimuth bearings easier to handle in GIS?"
Yes.
Try writing a spreadsheet function to translate them, to understand why.
I haven't written a spreadsheet function to convert quadrant to azimuth bearings, but I've written detailed instructions for students (and we'll be doing that in class on Tuesday). But my experience with GIS was that azimuth bearings were also difficult to handle. (May have been my own inexperience with GIS, though.)
On the other hand, computers are supposed to be able to do things like "if x is between 90 and 180, then y = 180-x." (On the other hand, Blogger can't tell a "less-than" sign from an html tag...) And do it the same way every time, unlike people doing calculations in their heads.
So, for people who work with azimuth exclusively, did you have to train yourself to recognize bearings that are 180 degrees apart? (I know that if you use a convention like the left-hand rule, you don't need to constantly add or subtract 180 degrees in your head. But if you've got planes that are near vertical, you could have very similar orientations with different strikes. It takes practice to quickly realize that the measurements are similar, even if the numbers look very different.)
Yes... I trained myself to recognize bearings that are 180 degrees apart-- I have to if I want to make sure I don't record erronous data.
I use one of the Brunton Geo Transits (poorly made and falls apart a lot, but half the price of more durable European models) and record data in azimuth with dip and dip direction-- It's much faster and more accurate (especially for lineation measurements) than using a traditional Brunton, and I've found it leaves little room for me to bungle recording a measurement (Which still happens occasionally), and it ends up being a lot easier to enter the data into structural databases... which is very significant, as I'm often taking 3000+ measurements (mostly on joint sets) per map. The problem is that I've yet to master the art of subtracting 90 degrees every time someone asks me what the strike of something is in the field.
I still prefer, "Fault line at 11 o'clock. Cover me; I'm going in."
I am a Turkish structural geologist and I used to benefit from Brunton- or Breithaupt-type compasses in the field until I met my "trésor" in France, in late 80's, a Topochaix compass. It is bigger and heavier than a Brunton, offers only azimuthal readings, and has an advantage the other compasses do not have: its inclinometer allows one to directly measure the pitch angle of a lineation. I think that its french producer does not continue its job. A final comment for those measuring the dip direction: from my measurements(I never use quadrant methodology), azimuth values of the structures bring me to the concept of tectonics, motions and stresses that deform the rocks. In this case, I do not see clearly how raw dip direction data would help me to interpret my structural data in terms of tectonics.
I agree with BrianR: azimuth in all structure !
PS. With new digital compasses, planes are measured by their dip angle and orientations. The next "GIS" generations may probably never use the compasses we now use.
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