The BILBY Geotrail
Basic Information for Locating Benchmarks Yourself
CONSTITUTION AZ MK
THE GEOTRAIL
The Maryland Society of Surveyors (MSS) welcomes everyone to participate in this statewide trail which showcases the different “Benchmarks” used by surveyors and which geocachers enjoy finding. The BILBY trail will include a total of 20 cache locations with geocoins awarded at different levels of caches found.
To most non-surveyors any brass disk they find is a benchmark, however not all benchmarks are created equal. There are approximately a dozen different types of benchmarks, and this trail will introduce you to 5 of the most common. Azimuth Marks, Triangulation/Horizontal Marks, Reference Marks, Special Marks and Bench Marks also known as Vertical Marks.
One MSS geocoin will be awarded when 3 caches are found from EACH of the five different types of marks (15 total finds). We have a limited amount of National Oceanic and Atmospheric Administration geocoins that will be awarded to anyone finding all 20 BILBY caches. Be sure to record the code word from each cache to qualify for geocoins.
To claim your geocoin(s) use this link to access the decoder. If multiple people share one geocaching username, only single coins will be awarded per user.
First of all, we will refer to all these marks as “Control Stations” since the word benchmark really pertains to just one type of control station. Next, we will use “Bench Mark,” with two words, since that is the way the National Geodetic Survey (NGS) defines it and the NGS is the federal agency that maintains this network of control stations.
While Geocaching.com no longer allows you to find and record these in your geocaching statistics, you can still post your finds on Waymarking.com and the NGS would welcome all finds or DNF’s to be recorded in the Survey Mark Recovery section of their web site. The NGS also has an interactive map located on their Map Page to help you find them.
Here are the five types of Control Stations and locations in the BILBY Geotrail:
HISTORY AND WHY THE NAME BILBY
The first set of Control Stations were placed around 1816 when Thomas Jefferson established the “Survey of the Coast” with the mission to map the eastern coastline of the United States. Since boats were the main method of transportation and how to get goods to the citizens of this new nation, accurate maps were needed to identify rivers, bays, harbors, shoals, light houses and any other navigation aids to keep commerce moving safely.
The only way to make measurements, until GPS was invented over 200 years later, was by direct line of sight. Measuring from hilltop to hilltop was a way to cover large areas quickly, however since the hills and mountains were mostly forested, they needed a way to get above the trees. Lumber was obtained either from the forest itself or purchased at a nearby lumber yard to construct towers. Some towers were constructed over 100 feet tall.
Jasper Bilby (1864-1949), who this trail is named, was a surveyor and geodesist that worked for the U.S. Coast and Geodetic Survey. Jasper (Click Here for more information) saw the enormous amount of manpower and cost to build these towers which were typically left behind, due to the time and manpower to dismantle then transport them to a new location. He noticed the steel windmills used by farmers to pump water and thought he could do something with that. He developed a tower within a tower system, so the survey instrument was on one tower and the operator stood on another. That way as the operator moved around to take readings it did not affect the stability of the instrument. The inner and outer towers were color coordinated and bolting the sections together made assembling and disassembling easier and faster.
THIS STATION
AN AZIMUTH MARK control station is one that is used by the surveyor to determine an accurate azimuth direction while their instrument is set up on a Triangulation/Horizontal mark. The Azimuth mark will always have an arrow in the center of the disk pointing towards the Triangulation/Horizontal station. All stations in the NGS database have a unique two-letter, four-number label known as a permanent identifier or PID as names of stations are frequently repeated. Azimuth marks typically do not have an accurate horizontal or vertical value their PID (CK6862) does not have a separate datasheet. Clicking the link below, after the station name, will take you to the datasheet of the main CONSTITUTION station which has a lot of information about the mark and will show the accurate azimuth from the main station to the Azimuth mark.
CONSTITUTION (PID JW1492) is about 0.2 mile away at a geodetic azimuth of 70° 33' 02.2". The main station and azimuth mark were set in 1955. There are also several smokestacks, water tanks, tower masks and a church steeple in the area that could be used to set the azimuth.
THE CACHE
The posted coordinates will take you to the Constitution Az Mk brass disk as shown in the picture above and the focus for this BILBY cache.
Please take a picture of the disk with a signature item and post with your log. While this is not required, it shows your appreciation to those many volunteers who helped create the BILBY trail.
From the Constitution Az Mk brass disk (posted coordinates), walk northwest toward the tennis courts and the sign coordinates in the additional waypoints. Use the information on this sign to find the coordinates to the final.
Final is at: N 39° 39.ABC' and W 078° 44.XYZ'
To solve ABC, from the first five-digit number, take the first number and last number to make a 2-digit number. Add this to the coordinates for the Az Mk brass disk (posted coordinates), then add 1. This is ABC.
To solve for XYZ, take the last two digits of the year that the plaque was donated by the U.S. Coast Guard. Take these numbers and add to the west coordinates of the Az Mk brass disk (posted coordinates), then subtract 4. This is XYZ