Topographic Maps and Digital Elevation Models

116 Part III Maps and Images
g. What angle does this surface make with respect to the horizontal? Use the following graph to sketch the
gradient you just got and either measure the angle with a protractor (less accurate) or use trigonometry to
calculate the angle (more accurate). Label your graph.
If you were on a roof pitched at this angle, you would find it very difficult to keep from slipping off. Thus,
you would have to be a rock climber to scale Devils Tower.
Devils Tower is an interesting place. If you want to see what it looks like, try going to
den2-s11.aqd.nps.gov/grd/parks/deto/index.htm (or link to it through www.rnhhe.com/jones6e).
Climb the Tower: A web search reveals numerous sites dedicated to climbing Devils Tower. It's a classic
place for technical rock climbing. A National Park Service website (www.nps.gov/deto/home.htm) gives some
information on historical climbs of the Tower (before the advent of modern equipment) as well as its geology
(click on "Study the Tower").
6. Topographic maps and DEM data on the web: If you need a map, DEM image, or air photo of a given area,
there are many available web resources. Here is a brief guide to some we've found useful:
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TopoZone (www.topozone.com) delivers map portions centered around the place or coordinate you specify.
You can see your area on maps of scales of I:24,000, I: 100,000, and 1:250,000, and you can see different
areas at each map scale by adjusting the scale at which the map is shown on the screen. A "print" link allows
you to print or save your map.
Terraserver (terraserver-usa.com) has a map interface that isn't as good as TopoZone (you don't know
what scale maps you are looking at), but you can switch to an air photo view of your selected area.
Sam Wormley's GIS Resources (www.edu-observatory.org/gis/gis.html) lists site links that carry
scanned USGS topographic maps. Look under "DRG's Available Free Online"; DRG stands for "digital
raster graphics." The disadvantage of full maps is they are larger files and are difficult to print unless you
have a large plotter.
MapMart (www.mapmart.com) allows you to download USGS DEM files for 7 ~-minute quadrangles. You
can easily learn the name of the quadrangle you need by typing a place name into TopoZone's search engine.
A good MapMart interface allows you to zoom in on the quadrangles around your point of interest and to
quickly and easily download (for free) the DEM files. Note: You will need specialized software to see these
DEMs.
OEM software resources (edc.usgs.gov/geodata/public.html): To view DEMs, you'll need software that
translates the SDTS-format files the USGS provides. This page lists some freeware and shareware programs
that you'll need to download and learn to view DEMs. MacDEM (www.treeswallow.com/macdem) is a nice
shareware program for the MacIntosh. dlgv32Pro (mcmcweb.er.usgs.gov/drc/dlgv32pro/) is a nice freeware
package for pes.
United States Geological Survey Publications Page (www.usgs.gov/pubprod/) lists publications
(including maps) and tells you how to purchase them. There are links to many on-line retailers of USGS
maps (click on Retail Sales Partners to get to an alphabetical list).
You can easily check out all these links by visiting a single web site: www.mhhe.com/jones6e.