Geodesy and Coast Defense

Geodesy is the science of representing the shape of the earth and defining the positions of points on its surface through surveying. Coast defense involves siting batteries of weapons in known positions, plotting the locations of targets so that these weapons can be fired at them with accuracy, and then observing and correcting fires for best effect in hitting these targets. Thus geodesy and surveying are essential parts of an effective coast defense system.

In 1807, President Thomas Jefferson created the Survey of the Coast, whose purpose was to map the U.S. coastline and create nautical charts of our coastal waters. In 1878, this agency became the U.S. Coast and Geodetic Survey (USCGS), which lasted until 1970, when its duties were assumed by the National Geodetic Survey (NGS), a part of the National Oceanic and Atmospheric Administration (NOAA).

Beginning in the late 1800s and continuing at an accelerating pace into the 20th Century, the USCGS carried out surveys that located a network of points throughout the United States. These points were marked by the brass disks familiar to many [see Slides 1 and 2 in the Gallery at left] and in other ways. These marks marks were then used in measuring the distances between known points through a process known as triangulation. This enabled accurate mapmaking, and was a crucial underpinning of coast artillery fire control.

When coast defense forts and batteries began to be constructed in earnest (1895-1915), this network of geodetic markers was not as widespread as it is today, so a number of ad hoc surveys had to be carried out to site coast artillery guns and observation stations. Some of these surveys weredone by Battery personnel in the Coast Artillery Corps itself. Others were assigned to the USCGS or the U.S. Engineers ("USE", as the Army Corps of Engineers was often called). These ad hoc surveys often did not leave permanent markers, but instead referenced the "pintle centers" (balance or mounting points) of individual guns or observation instruments (e.g., spotting scopes) as their marks. Slides 3 through 5 at left show types of ad hoc survey marks used to aim mortars at Fort Andrews and 6-inch guns at Fort Revere.

As federal and state survey work expanded in the 1930s, the network of geodetic points grew, and many parts of the coast defense system (i.e., forts, gun batteries, and observation towers) received their own permanent geodetic markers. As pre-war tensions increased from 1938 on, the work of locating new coast defenses (particularly the new fire control towers needed to direct the longer-range guns) gave rise to specific surveys of these defenses. [NOTE: It appears that almost all the coast defense-related survey marks set by the USCGS or the USE in MA, NH, and ME during this period were handled by teams under the control of one man, Lt. (later Capt.) Percy (Peter) L. Bernstein, USCGS, USE. Known by the initials "PLB" on geodetic Datasheets and by his careful field work, Bernstein died in 1974. His brief biography can be found here, but his legacy persists today, inscribed in brass throughout the region.]

A cursory review of other U.S. harbors indicates that Boston coast defenses possess what is probably the most extensive network of geodetic marks of any harbor the United States. This is a big help to any student of coast defense in Boston, since the latitude and longitude of its assets (even those which have been destroyed) can more often than not be pinpointed precisely. For example, here is a modern Datasheet from the National Geodetic Survey that describes a geodetic mark (from 1934) that was set in the roof of a base end station associated with Battery Winthrop, the 12-inch guns at Fort Heath in Winthrop, MA.. This station was one of three in identical brick fire control structures that sat side-by-side in echelon at Fort Heath in Winthrop (see map in Wikipedia article linked to above). All three structures (and the battery itself) were later leveled during the redevelopment of the site (for a radar station) and the area was later re-leveled to create a public park.

Beginning in the 1920s, the U.S. Army began to identify fire control structures and other assets in each harbor's defenses by developing harbor-specific X-Y coordinate grid systems. The position of an asset was specified in terms of the number of yards north and east of a chosen origin that it lay. In many Coast Artillery Corps (and Army Engineers) reports from the WW2 era, the positions of fire control structures and radars, and some rangefinder stations and guns were identified by these X-Y coordinates.

Some Geodetic Marks in Coast Defense

This brass disk, buried under the soil for most of 66 years, was set during a general statewide survey in 1934 atop Btty Gardner in Nahant. The original disk was reset in 1943, because construction of the casemates for the Gardner guns had buried it too deeply, and it now goes by the designation RUCKMAN RESET. The triangle identifies this as a "station disk" (as oppsed to a reference mark disk, as in Slide 2). The plotting room fir the battery was located nearby and beneath this mark. (PG 2009)
This disk can be identified as a reference mark by the arrow cast into its surface. It was set in 1934 and was stamped as Reference Mark No. 1 for station RUCKMAN (see Slide 1). The arrow points (approximately) to the location of the station to which it refers, and the precise compass bearing and distance from the station to this mark are given in the Datasheet for the station. If a station disk is somehow destroyed, its position can often be reconstructed by taking bearings and measurements from its reference marks. (PG 2009)
This photo looks northerly from the top of the bank between mortar pits A and B of Battery Whitman at Fort Andrews, Peddock's Island. At the bottom of the photo is visible a geodetic mark, likely set by the Coast Artillery Corps, that consists of a brass pin set in a concrete-filled section of soil pipe. This mark, which is not in the NGS database, was likely set as a directing point (DP) for the mortars in both mortar pits. It was used used to aim the mortars in azimuth (compass direction). This mark was pointed out to the author by a resident of the island. (PG 2010)
This image shows the top of the concrete pylon behind the next to last gun of Battery Sanders at Fort Revere in Hull. The boring on the left was likely the mount point for a portable fire control instrument (perhaps a Depression Position Finder-DPF) that could be used by the Battery Commander as a secondary (back-up) means of directing the fire of its 6-inch guns. [The primary and secondary fire control stations for this Battery were located in a building atop the bluff to the rear of the Battery and in a bunker over at Fort Dawes, on its central hill.] The greenish item on the right with the cross scored into its surface is likely a local geodetic mark that was set following a survey conducted by Battery personnel themselves. This inscribed brass bol head, about the size of a quarter, might have marked the Directing Point (DP) for the three (and later four) guns of the battery. The location of the pylon is explained in the next slide, below. (PG 2010)
This photo looks ENE along the gun line of Battery Sanders at Fort Revere in Hull. The concrete pylon described in the previous slide can be seen at the rear of the gun platform at the right-center of the image, in the shadow cast by the tree behind it. In the distance, roughly over the pylon, can be seen the silhouette of the tall WW2-era fire control tower at Point Allerton. Downtown Boston is to the left, and Nantasket Beach to the right. Originally a three-gun battery, Battery Sanders later "absorbed" one of the three guns of nearby Battery Pope, just visible in the distance at the right (behind the tree's trunk). (PG 2010)
This disk, marking a traverse station, was set in the step at the entry to the Point Allerton tall fire control tower in 1943, as part of a coast defense-specific survey done by the USCGS. From here, precise coordinates were tansferred by survey to the "instrument centers" of the observation instruments on the top levels of the tower. This disk is unusual because, as a concession to wartime rationing, it was fabricated made of steel instead of the more traditional brass. Unfortunately, this arrangement was short-sighted, since the stampings on almost all disks of this type have rusted away or been rendered illegible in the 70 years or so since they were set. (PG 2010)
In many fire control structures, the point of the geodetic mark is specified as the center of the instrument base, or mounting block, and their is no disk set the mark that point. This is a photo of an octagonal base for an instrument called a Depression Position Finder (DPF), on the 6th (top) floor of the tall fire control tower at Point Allerton in Hull. In this particular case, the position of the tower's instruments (which were located on three levels, one on top of another) was also specified by a brass disk set into the surface of the roof above them all (see Slide 8). Today, many of the original instrument base blocks have been jackhammered away by the new postwar civilian owners, to make the floor space in the towers more useful. (PG 2010)
This is the disk that was set in 1943 in the roof of the Point Allerton tall fire control tower. Its bronze surface is now well weathered and the mounting cement around it is cracked. Its stamping reads B 5/10 S 5/10, indicating that it was intended to be the fifth base end station (B) and spotting station (S) for tactical battery No. 10 (Battery Whipple at Ft. Standish). However, the decisions regarding which battery/ies a tower was to serve often changed after the disks were set, so the initial stampings are not always reliable. Note how the stampings are often ambiguous, with the "3s" resembling "5s" or "6s". (PG 2010)
This bunker, which housed a coincidence rangefinder (CRF) station for Battery Basinger at Fort Strong on Long Island, Boston Harbor, is unique in the area. Access was through an armored door at the rear (since blocked by the slumping earth of the slope), and both levels of the bunker were used for observation. The bunker looked out over the harbor entrance channel between Long Island and Deer Island and directed the two 3-inch guns which were emplaced about 75 ft. further west along the shore. The geodetic marker shown in the next slide was emplaced in the floor of the lower level of this bunker. (PG 2010)
The greenish circlular object roughly at the center of this image (at the top edge of the cleared area) is is the geodetic disk that was set into the floor of the lower level of the bunker pictured in the previous slide. The cement-filled steel pipe lying on the floor below the marker was the mounting pylon (since tipped over by vandals) for an azimuth scope inside the bunker that was used to fix the bearings of approaching ships. But for a thick covering of mud, rocks, and debris, the disk would have likely been vandalized as well. The rocks ans dirt were tossed through the bunker's viewing slit from the harbor by wave action. (PG 2010)
This disk is an example of a U.S. Engineers (USE) "Map Control Station" mark, used for a short time just before WW2. This is the disk that was set in 1940 roughly in the center of the floor of the lower level of the two-level CRF bunker shown in the two previous slides. Disks of this sort have been found marking several fire control and radar towers erected near Boston just before WW2. (PG 2010)
The red arrow indicates the top portion of the granite post containing the brass disk marking the directing point for the guns of Battery Stevenson, a pre-WW2 battery of 12-inch guns at Fort Warren on Georges Island, Boston Harbor. The distance of each gun from this marker was measured and then used to adjust the firing coordinates for that gun. The water tower on Telegraph Hill at Fort Revere in Hull can be seen on the horizon at left. (PG 2010)

The Boston Harbor Coast Defense X-Y Grid

This map illustrates a square grid, dimensioned in yards with its origin putside the Boston area, which produced X-Y coordinates (instead of latitude and longitude) for use in the harbor defense system. As the text describes, a converter between these two methods of plotting (X-Y and lat-long) has been developed by Jerry Wahl, a professional surveyor, in cooperation with the author.
This snippet from a 1945 Army report on Boston's harbor defenses tabulates the X-Y coordinates of a number of guns, Battery Commander's stations, rangefinder stations and fire control towers associated with two Boston batteries, Bty Long on Hog Island, and Bty Stevens at Ft. Strong. Location numbers refer to Army designations for clusters of defense assets, and the Sites are sub-areas within these Locations. "DP No. 1 Gun" refers to the pintle center of that gun, usually the rightmost gun of the battery as you look seaward, which is the Directing Point for that battery (the gun other guns in the battery adjust their aim to). "BC" means Battery Command (post), and "CRF" refers to a rangefinder instrument station. An entry of the form "B1 S1" refers to base end and spotting station No. 1 for this battery, or a designated fire control tower or structure (identifiable by its Loc/Site designation). The X-Y coordinates are in numbers of yards.