EVERY MAP REFLECTS THE interests of its makers and users. A road map guides drivers to their vacation spots. A zoning map shows planners where they may construct houses, parks and shopping plazas. A congressional district map determines a representative's constituency. By depicting information in a graphic way, maps become potent instruments of communication and persuasion [see "The Power of Maps," by Denis Wood, page 88].

We had the opportunity to do some amateur cartography with the Westchester Land Trust, an environmental organization in Westchester County, N.Y. The maps we created illustrated a watershed system that went beyond political boundaries. We undertook the project to show how different types of land use affect the quality of the Mianus River, the main source of water for 130,000 residents in contiguous parts of New York and Connecticut. The maps highlighted environmentally sensitive areas and revealed those sections of the river that are particularly vulnerable to pollution.

A watershed, the area drained by a specific stream or river system, resembles a bowl whose rim is marked by hilltops or ridges and whose base is a river or other body of water. Besides providing water for drinking and recreation, watersheds are essential for wildlife habitats, climate control, agriculture, industry and sometimes even transportation and electric power. Each of us lives in a watershed, although no one actually sees it. A map is probably the best way to make a watershed visible. It does so by shifting our frame of reference from man-made districts to a landscape defined by physical boundaries. It reveals the anatomy and physiology of the region.

Constructing the watershed base map meant combining information from several different kinds of maps. First, we used state highway maps to determine the path and full length of the Mianus River. We found that this 37-mile-long river crosses five towns, two counties and two states before emptying into Long Island Sound.

Once we knew where the river was and where it went, we assembled a large-scale paper map of the Mianus from large-format county road atlases. Such atlases are the ideal choice from which to make watershed maps because, in addition to being inexpensive and readily available, they render information at a useful scale and depict the natural and man-made features in color. We compiled enough pages to show all the town, county and state borders, and we carefully matched grid lines on the borders of the maps and connected the ends of the roads. Our finished base map required 19 atlas pages. Because we used both sides of some pages, we needed two copies of each atlas.

Also used were drainage basin maps, which show the location of the drainage divides. The divides define the watershed boundaries-the rim of the bowl down which water drains into the Mianus River. Drainage basin maps show rivers and streams as solid lines and drainage divides as dotted lines. We needed four drainage basin maps to make a complete composite of the Mianus drainage basin: one Westchester drainage basin map and three U.S. Geological Survey quadrangle basin maps for southwestern Connecticut (the Mianus basin includes part of Fairfield County, Conn.). Drainage basin maps are generally available from some local government agencies. We obtained ours from the Westchester County Planning Department and from the Connecticut Department of Natural Resources.

If drainage basin maps had been unavailable, we could have used topographical maps, or topos. In addition to showing rivers and streams, topos represent the hilltops and ridges as closely spaced lines. Finding the watershed boundaries on a topo requires an extra step. One would have to plot the drainage divides by connecting the highest elevations shown-a laborious and time-consuming task.

Because a drawn river is by nature long and skinny, we had to be careful that our paper map did not get too big to handle. We decided that the maximum size would be 48 by 36 inches. As the Westchester part developed, it became clear that its scale of three inches to one mile would produce a base map of about the right size. We adjusted all other maps to this control scale.

We quickly found out how difficult it is to work with rolled maps. So we made a map storage case by taping together two large pieces of scrap cardboard to keep the work flat. The finished paper maps were kept rolled for easy transport and long-term storage.

Making composite maps is easy if all the pieces are drawn to the same scale. All our maps were drawn to different proportions, so we had three of them rescaled ("statted") to our control scale of three inches to one mile. A copy shop that reproduces blueprints was able to stat large maps as well as copy them. A small map must be enlarged in sections and then reassembled.

To help the copy shop, we drew a precisely measured control line on the bottom of each map, showing the inch-to-mile ratio. One map was not printed at the scale indicated by its legend, so we had to draw a corrected line. Another had no legend at all, so we calculated the scale arithmetically.

The copy shop statted the maps by enlarging the control lines until each was three inches long. Once the scales matched, we cut and taped the pieces together to make two maps: one of the river and one of the drainage basin. We laid a 48- by 36-inch sheet of Mylar over the drainage map and traced the outline of the Mianus watershed.

Although the Fairfield County chunk had been correctly statted to scale, it did not line up with the Westchester chunk at the county border: roads, rivers and even the reservoir were slightly askew. We therefore adjusted the river map to make the pieces fit as closely as possible. We suspect that incorrect scaling is more common than is generally thought; one should maintain a healthy skepticism toward the presumed accuracy and objectivity of maps.

Because the copy shop could not produce statted maps in color, we lost valuable information from the Fairfield section. Blue and green waterways and parks just disappeared from the black-and-white copy. We reoriented ourselves by hand-coloring major features with felt-tip markers.

When we placed the transparent drainage basin Mylar over the paper map of the river, the Mianus watershed immediately appeared on the landscape.

We saw a completely new, more comprehensive picture: the relations among the boundaries of the natural region, the man-made region of roads and the abstract region defined by politics. A1though our simple experiment ended with the creation of this base map, one could build a much more sophisticated watershed map by combining the transparent overlay of the watershed with other kinds of maps, such as those depicting hydrography, topography, soils, property taxes and land use.

The total cost for this project was approximately $145: about $60 for the maps, $24 for the Mylar and $60 for reproduction. You can save the copying costs if you can obtain maps drawn to the same scale. Paper maps from government sources may be free.

A more high-tech mapping procedure, perhaps best suited as a team project, would be to use a computer and software called a geographic information system, or GIS. A GIS can combine maps of wetlands, slopes and soils with such nonmap data as aerial photographs, satellite images, statistical tables, water-quality measurements and census records. GIS technology enables those who have no training in cartography to make maps and models. Its jazzy graphics allow one to see, analyze and anticipate the effects of alternative courses of human activity on the watershed. The GIS used for the Mianus project was ARC/INFO, which is produced by Environmental Systems Research Institute, a software company in Redlands, Calif.

Although GIS technology keeps becoming cheaper and more widely available, it is still so expensive that it is used mainly by government departments and large corporations. Anyone tackling this version of the project must have access to a source of digitized data, a GIS and trained personnel who are willing to dedicate the time to design and generate a watershed base map. Furthermore, amateur mapmakers need a personal computer able to run PC Paintbrush or equivalent graphics software. At least one team member would have to be conversant with the software and be able to design a map with it. Although a color printer would produce the best maps, any printer that can generate graphic patterns can be used to create a base map.

Amateur cartographers should know some GIS jargon. A GIS works with digitized data, not maps. Each layer of data is called a coverage. The U.S. Geological Survey subdivides coverages into geographic regions called quads (short for quadrangles). The Mianus watershed spreads over four quads. We needed four coverages for each quad. The coverages were for municipal boundaries, watershed boundaries, roads and rivers Coverages are available from the county planning department or the state department of natural resources.

Because learning to use any GIS software package requires a significant commitment of time, amateur mapmakers should consider working with a government or academic planning department that has a GIS. Its staff would help design and generate a preliminary watershed base map. After assembling the coverages, they would display the base map on the screen for the project team to review. The base map would then be saved on a floppy disk in a file format on which the mapmakers could work with their own graphics software.

Both the simple and the advanced exercises produced informative maps, but each contained different messages. The simple map was troublesome to make because the data were hard to find and harder still to combine. Even with precise cutting and fitting, major discrepancies are glaringly apparent. The accuracy and clarity of the GIS map are a major leap forward. Unfortunately, it is also much more expensive. Each coverage costs about $20, and we needed 16 of them. Then there are the costs of the personal computer and the graphics software.

Of course, variations on a watershed map can lead to other kinds of applications. A community group can plot recorded sightings of migratory birds on our simple base map to make a persuasive case for preserving or restoring seasonally critical habitats. A town can identify a suitable site for a new well by taking our computer-generated base map and plotting existing water service districts, surface and subsurface geology, area soils, zoning and land cover.

For additional information about making a watershed base map, contact the Westchester Land Trust, 31 Main Street, Bedford Hills, NY 10507.

Bibliography

THE MAP CATALOG: EVERY KIND OF MAP AND CHART ON EARTH AND EVEN SOME ABOVE IT. Joel Makower. Vintage Press, 1986.

GIS WORLD. 12 times per year. 155 East Boardwalk Drive, Suite 250, Eort Collins, CO 80525. Telephone: (303) 223-4848.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.