Or, after Colvin with rod and alidade
by Tony Goodwin
Techniques for mapping have come a long way since 1837, when Ebeneezer Emmons carried a barometer to the summit of the “High Peaks of Essex” and decided to name the lofty eminence after Governor Marcy. Since then, this area has been leveled, photographed, and contoured with all manner of sophisticated instruments from land and air, but even today the production of an accurate map still requires leg work, elevation checks, and name verification.
The most recent remapping of the central Adirondacks was begun last spring by the U.S. Geological Survey which will remap much of the High Peaks in 7½-minute quadrangles to replace the current 15-minute quadrangles. The present quadrangles cover an area of 15 minutes of latitude and longitude on a scale of 1:62,500, with one inch equaling approximately one mile. Most of the rest of New York State has already been mapped in 7½-minute quadrangles on a scale of 1:24,000, or 2½ inches to the mile; and it is the eventual goal of USGS to map the entire country on this scale. The quadrangles included in this current project are the Elizabethtown, Ausable Forks, Mt. Marcy, Lake Placid, Santanoni, Saranac Lake, Long Lake, and St. Regis. In the new series, each of these quadrangles will be four separate sheets, each bearing an appropriate name derived from the outstanding feature on the sheet.
While much new work was done in the preparation of these maps, every survey relies heavily on the work of previous surveyors—unless, of course, it is the first. The first comprehensive survey of the Adirondacks was done by Verplanck Colvin, who in 1873 began the monumental task of accurately mapping the entire Adirondack region, much of which was then trackless wilderness. Colvin began at Lake Champlain, the nearest accurately surveyed point, and worked westward, using transits and triangulation techniques to find first the horizontal positions (in latitude and longitude) of the major peaks and then their elevations. To sight on peaks, Colvin needed signal stations, a mirror raised on a pole and supported by a tripod, and the rusted eyebolts hikers find on many summits date from Colvin’s survey. After accurately determining the location and elevation of each point, Colvin’s crews placed a copper bolt in the rock as a record for future surveys to build on, and many of these bolts are also still in place on Adirondack summits.
For elevations, however, triangulation does have its limits in accuracy, so to really tie down crucial elevations Colvin used a level and rod to run a so-called “line of levels” from a known benchmark to several points, including the summit of Marcy. The leveling of Marcy started at Westport and took over a month to compete, but Colvin’s party bravely finished in a snowstorm Nov. 4, 1875, when they determined Marcy to be 5,344.311 feet. This technique is still the most accurate way to determine elevations, and Colvin’s leveling work on Marcy has gone unchallenged for 100 years.
By 1900, 15-minute topographic maps based on Colvin’s work had been published for most of the Adirondacks, remaining in use until World War II provided the impetus for further surveying. Perhaps fearing that the Axis try to repeat Gen. Burgoyne’s invasion through the Champlain Valley, surveyors took to the field in 1942 and greatly expanded Colvin’s level lines. They also checked the horizontal positions of the peaks, placing the familiar brass benchmarks as they went.
Using the work done for the Army and photographs taken a few years later, the USGS published between 1953 and 1956 the 15-minute quadrangles now in use. These were the first maps in this are on which the contours were drawn by photogrammetry techniques. In this process, aerial photographs are taken with at least a 50 percent overlap so that every point on the ground appears on at least two photos. A photo-interpreter looking at these photos with a stereoscope, an instrument much like a stereopticon, sees a three-dimensional image of the land.
Photogrammetry instruments magnify this image five times, and from this enlarged image a technician draws in the contour lines. Though computers can draw contour lines directly from photos, computers cannot yet meet national standards for map accuracy, the standards to which all USGS maps made. These standards require that on a 1:24,000 scale map, 90 percent of points be within 40 feet of actual ground position and that 90 percent of all elevations interpolated from contour lines must be within half the contour interval. So far, only hand drawing and careful checking allow the mapmaker to meet these standards.
The current mapping project uses photographs taken on May 13, 1976, from a Lear jet flying at 16,000 feet. The jet must fly a perfectly regular east-west pattern to insure the desired 50 percent overlapping coverage and a precise elevation so the scale of the photograph will be consistent. The camera exposes a 9 x 9-inch negative, from which contact prints are made. The early May insured maximum visibility, since the hardwoods are still bare.
In preparation for the field checking, certain recognizable points, called “picture points,” are selected by the photogrammetry technicians as points for which accurate elevations are needed so the contouring can be checked. A picture point is usually a road or stream junction, but it can be any feature readily identifiable in the photographs. The field surveyors must also find a way of identifying each benchmark by finding a nearby, photo-recognizable point and determining its elevation.
Finding the benchmarks so that they could be identified, however, was not always as easy as it sounds. The surveyors who placed them in 1942 wrote such descriptions as “… in an imbedded, gray granite boulder 4′ by 6′ projecting 2′ located 20′ west and 30′ south of the west end of the bridge over Calamity Brook outlet to Flowed Lands. . . .” Most benchmarks were readily located by using these descriptions, but we futilely spent most of a day looking for one placed “… 55′ south of the center of the south shore of the only pond in the notch between Cheney Cobble and Allen Mountain, in a rock ledge. . . .” Since 1942, the beavers have created two more ponds and the blowdown and subsequent regrowth have obscured any sign of a rock ledge, although we did grope around under the moss growing on top of several boulders. We shall just have to hope that the photogrammetry technicians already have enough information to contour the area; otherwise, a crew will have to spend several days next summer running a line from the next-nearest benchmark, at White Lily Pond.
We also searched without success for a benchmark located where Sand Brook is crossed by the Dividing Ridge Trail, a path barely existent in 1942 and now long gone. Six inches of snow didn’t help the search, either. These, however, were our only two real failures at finding benchmarks; after we had found all the Forty-Sixer registers, searching for benchmarks added a new dimension to exploring the Adirondacks.
Once the photos are printed and the picture points selected, the field surveyors are dispatched to determine elevations and to check for roads, trails, and houses not visible in the photos. These features are drawn on the photo for later transfer to the map. It was at this phase of field checking that I became involved with the survey, working primarily on the Mt. Marcy quadrangle.
The technique for finding the elevations is to start at a known elevation, usually a 1942 vintage benchmark, and use a stadia rod and alidade to run a line to the desired point. Someone is needed to hold the rod and Jim Dearing, the surveyor of the Mt. Marcy quadrangle, hired me as his rodman.
The stadia rod is 13.3 feet long with diamonds every one-tenth of a foot. Looking through two horizontal lines in the alidade, the surveyor can determine the distance to the rod by the number of diamonds between the two lines. The surveyor then notes the angle above or below horizontal at which he is viewing the rod. This angle is converted by the alidade to a “Bemin number” which, when multiplied by the distance to the rod, performs the necessary trigonometry to find the difference in elevation between the alidade and the rod. By leapfrogging, the line is run to the desired point and the elevation determined to within two feet. As they were in Colvin’s day, actual benchmarks are still set with levels and rods to an accuracy within two-tenths of a foot, but the alidade is very useful for quickly determining elevations in rough terrain, and it is accurate enough for contouring.
Trails do not usually show up in the aerial photography, but since hikers are major users of topographic maps in the Adirondacks, it was very important that the trails be mapped accurately. After all necessary elevations had been found, I was dispatched to walk the trails. For this work I carried a compass, a portable stereoscope, and the appropriate photos. By noting the direction traveled, the terrain covered, and transitions encountered between hardwood and softwood forests, I could usually plot the trail on the photo quite easily. Some, however, were easier than others. The Lake Arnold Trail up Colden was very easy because it follows an obvious ridge and a quick look at the stereo image sufficed to locate it; but the “scenic trail” up Sawtooth required five hours and many stops to look at the photos to sort out its numerous twists and turns.
After he checks elevations and such man-made features as trails, the field surveyor has a third important job— checking place names. The USGS criterion simply says, use the name in most common local use, but sometimes it becomes more complicated. For the most part, one simply checks names already on the map while keeping an ear cocked for locally accepted names for features not named on the map. The old-time surveyors may have had to endure greater hardships, but at least they had the fun of inventing names immortalizing themselves and their friends. We had to settle for perusing area guidebooks and histories to find names like Rainbow Falls at Lower Ausable Lake and Uphill Brook on Mt. Redfield. We also talked to local residents to find names like Laramore for the northeast ridge of Cascade and Teakettle Falls at Upper Ausable Lake.
More difficult were places with two names like Avalanche Mountain, between Colden and Algonquin, which is also called Caribou by some. The most famous (and almost the only) local resident, Charley Nolan, calls it Caribou; but since it has been Avalanche on the last two maps, and many use that name, there seemed to be no justification to change it: Avalanche it remains. Even more interesting was one name, Railroad Notch, applied to two separate spots, with a different explanation for each location. The name has been applied to the notch between Yard and Tabletop, supposedly because of the Underground Railroad, and to the notch between Porter and Big Slide because of an actual proposed railroad. With opinions about evenly divided, we decided to determine if one explanation had any more merit than the other. Mary McKenzie, North Elba town historian, said that John Brown’s Farm was never a stop on the Underground Railroad, which put that explanation into the area of fantasy; but with the help of Dot Plum of the Adirondack Center Museum in Elizabethtown, I found two newspaper articles referring to railroads actually proposed and surveyed through the notch between Porter and Big Slide, so it won the prize. Getting a person to change what he calls a place is like the proverbial teaching of new tricks to an old dog, so I don’t expect any great change in what people call these places. But at least those who have the apparently correct placement of the name will be happy to see the new map confirm what they knew all along.
After spending six months gathering all this data on elevations, man-made features, and names, the surveyors send their work to the Eastern Mapping Center in Reston, Va., and then move on to other projects. Should problems arise with the drawing of the maps, spot checks may be necessary next summer, but the bulk of the field work has been done and in two years or so the maps should be published and available to the public.
From my work with the USGS, I learned to appreciate the tremendous amount of work which goes into the preparation of maps and even more the effort required of previous surveyors like Colvin, who had to start from scratch without benefit of roads, trails, airplanes, or previous maps. The old-timers did very well with their primitive techniques, and I would like to believe that with our modern techniques I helped to produce a perfect map. But I suspect that one day I will encounter some lost hikers, green sheets spread before them, who will mutter unprintable oaths and call down punishments of the gods on the oaf who made this map.