A Concurrent Session on GIS
Last week I attended an excellent session at Sweet Briar on GIS for
Biologists, and what I saw and heard there has influenced what I want to
explore here. I am a long-time beginner with GIS --long-time because I've
been thinking about computer mapping for about 30 years, and have played
with it in various forms, including line-printer maps (SYMAP), CAD
(AutoCAD, in an early incarnation of the product) and, during the last
year, ArcView. I'm entirely self-taught, which means that I've learned
bits and pieces as I needed them, and also means that I have blind spots
about a lot of features and possibilities that people with real training
would consider obvious. But I have this long-run conviction that
spatial distribution is a vital pedagogical and research
perspective for many of the fields I've been professionally involved in
as an anthropologist (and soi disant human geographer,
ethnomusicologist, and ecologist), and now as science librarian. Simply
put, we are awash in spatial data from remote sensing sources, data
applicable to the interests of many of the sciences and to disciplines
outside the sciences as well. Indeed, many of these data are of
transdisciplinary interest and relevance. We have to learn --and to
teach--
effective ways of analyzing these data and presenting the stories they
tell. And desktop GIS packages like ArcView realize the
possibility of the sorts of exploration and analysis we need to develop.
So where do we get the maps in the first place? Lots are
available on the web, in various formats. And maps can be
digitized in various ways. And various forms of images can be
used as layers upon which new layers can be drawn. So, for example,
geoTIFF images are available for topographic quads (the one for Lexington
is \miley\gis\vfic\O37079g4.tif)
I want to offer a pretty simple example, whose greatest advantage is
that the maps we can make are inherently meaningful to us --we can look
at patterns and start to interpret and explain them. We'll use a
readily-available (downloadable via the web) dataset on agriculture in
continental US counties (it's in \miley\gis\vfic). Look at this for a
few minutes:
Even without knowing what data are mapped, we can immediately start to
interpret the image:
- a mesh of county boundaries
- different
values, in 10 classes
- whatever it is concentrates in the
Mississippi-Ohio drainage basin
- many counties have none of whatever
it is(code "-99").
As a matter of fact, the map shows "Percentage of county as soybeans for
beans, harvested, 1987" (the variable's name is 'pct2307').
Here's an image of some of the data that lie behind the image:
So what we're seeing is an association between a .dbf file (manipulatable
via any spreadsheet program) and units (county boundaries) represented in
Cartesian
space --in this case, we're looking at a surface [% soybean area]
approximated with vector data.
The simple point here: we can make all sorts of maps by assigning values
to predefined units. If we had other data for U.S. counties we
could import those data and show them; if we wanted to manipulate the
data via the spreadsheet, we could show the results as new maps. No big
deal, unless you try to do the same thing by hand.
So we need a guide to the available
variables of the U.S. agriculture database, so that you can try out
some of them yourselves.
But first we'll need to document some of the basic moves for using
ArcView. I want to keep these simple, since the object here isn't to
teach the use of ArcView, but to let you try out some of the basic
features. Once ArcView is running (it's on the START menu on the lab
machines)
- click on the New button to open a View
- Find the button just below Edit that has a + sign on it --the
Add Theme button, click it, and navigate to P:\GIS\VFIC. You'll see 5
'feature data sources' open. Double-click any one of them to open it.
- click in the upper-left-hand box of the View1 window to display the
base map
- double-click on the colored rectangle to Edit the Legend
- choose 'Graduated color' from the pulldown Legend Type menu
- use the Classification Field pulldown to select [by clicking once]
one of the NUMBERED
variables for display (see the list to translate their cryptic numbering...)
- click the Apply button to see the variable mapped.
- try other variables, use the Classify... button to change the number
of classes
- and/or try adding other themes