Thursday, July 15, 2010

semi-log paper

The light table I mentioned earlier is a bit of a dinosaur. But it’s a reliable dinosaur. It sits in the corner and is forgotten about 99.9% of the time. You don’t need another light table, you just need to keep (unearth?) the one you have tucked away somewhere.

Semi-log paper is consumable. It’s getting harder and harder to find, too.

You may ask, “Why the hell would anyone need semi-log paper? Doesn’t everyone have a spreadsheet/graphing program?”

There are some problems with computer-only semi-log graphing:

1. When you graph something in excel, the points are immutable unless you actually track down the point in the spreadsheet and adjust it. In order to see patterns in the data (for example, what slope you need to use to analyze the time-drawdown data for an aquifer test), it’s often easier to adjust points so that you can figure out what actually happened.

2. If you’re in the field and you’re trying to decide if you have enough data to continue a drawdown test, it’s often easiest to do a quick sketch of your manual measurements rather than reconnecting multiple transducers to a laptop or hand-held device and fiddling with the data so you can put everything together.

3. Excel is becoming a black box. Does a student actually understand what a logarithmic scale is? If they’re using a published graph of data, can they compare their results and see what they have? In both my instructing gig and my TA days in grad school, I’ve found that students are stymied by creating simple graphs by hand. If you can’t put points on a graph, how can you interpret the graphs that excel made for you?

Maybe I’m old and too suspicious of computer gadgetry. But I can plot something up and figure out trends to bolster an argument in 30 seconds using my trusty semi-log paper.

5 comments:

Erik said...

You can drag plotted points around in excel by clicking on the line once to select it, then on the individual point once to select it, then click and drag to move that point around. The data cells for that point in the spreadsheet will update with the new value.

Lockwood said...

I totally agree that if a student hasn't done it by hand enough to be pretty facile with it, it's questionable whether they really understand what they're doing. As for finding the stuff, it's getting harder and more expensive. Probably the easiest way is to download PDF images and print as needed. Here's one source that came up in a google search-
http://www.printfreegraphpaper.com/
-and I'm sure there are numerous others.

I keep meaning to try and track down a wet/dry adiabatic lapse chart- a very cool graphic tool that can lead to some fascinating conclusions about the behavior of moisture in the atmosphere.

Marciepooh said...

I completely agree with your 3rd point.

If you are near a college campus look in their book store, then make copies. Keep several originals 'hidden', just in case you use up the copies and originals. (This assumes your copier doesn't skew things much.) I've also found (on waterproof-paper.com) some pdfs of different graph paper (polar, triangular, stereonet), there may be a free source our there for semi-log pdfs, too.

Silver Fox said...

These are great posts about possibly "obsolete" items. I still have a light table (and maybe use it most often for art work rather than geologic work), don't use semi-log or log-log paper often but use regular graph paper all the time. Plotting points up on paper would help everyone understand what they are actually doing, I think. Plus, you can just sit down and do it without having to get everything set up as with Excel.

martinamc said...

This kind of graph paper allows you to graph exponential data without having to translate your data into logarithms. The paper does it for you! At stationery and university bookstores, you can buy semilog graph paper with anywhere from 1 to 5 or 7 cycles.

decal paper