Introduction
by Winky Merrill

In the spring of 2006, when I became a member of the Board of Weston Forest & Trail, I learned that the organization was about to create a revised map of Weston’s trails using GPS (Global Positioning System) data.

The current map, created using aerial photographs and the input of avid hikers, would become much more accurate thanks to this technology. Steve Fogg, Weston’s Town Engineer, would be hiking all of Weston’s trails with a powerful GPS unit during the winter of 2006/2007. He would be collecting satellite coordinates that would be entered into the town’s mapping database.

As a Girl Scout leader for over ten years, I was simultaneously searching for a “Silver Award” Project for my troop of ninth grade girls. The Silver Award is the second highest award in Girl Scouting and it demands that each girl spend 40 hours completing a community oriented project.

I quickly called Steve. Could our troop accompany him on his walks during the coming winter? He was receptive and very gracious, and I asked if he would come to our next meeting, show us the GPS equipment and discuss the idea with the girls in Troop 3064. He did, and the girls liked the idea – and so it continued to evolve. We decided that after collecting the data, we’d write a trail guide for kids in Weston, so they’d know more about the conservation lands and their trails that are right in their backyard.

This Guide is the result of a year’s worth of work on the part of eight dedicated Girl Scouts. The troop went on seven walks of 4 hours each starting in November 2006 and ending in February 2007.

We had to hike during the winter months because when the leaves are on the trees it is too difficult to receive the data from the satellite.

Each girl hiked 8-12 hours on Weston’s Trails (8-12 miles or so). Steve Fogg went on all of our hikes and was the point person for recording the data. The girls took turns carrying the unit – and listening for the beeping sounds that indicated that we were recording our position.

The girls also helped out by figuring out efficient hiking routes – so that we would cover each and every trail (including the dead ends). After our hikes, Steve took the unit back to his office and “plugged” it into the town’s mapping database. The data literally “drew itself” onto the town map.

Global Positioning System: How it All Works
By Kara Hedges

The GPS (Global Positioning System) is a worldwide radio navigation system used to calculate exact locations on the earth. It was developed by the US Department of Defense because they needed a super precise way to identify positions of nuclear submarines. Fortunately, they made the system available to everyone so that you can use a global positioning unit anywhere on earth to locate where you are and help you find where you are going. Without it, many of us would find ourselves like these girls:

There are always at least 24 satellites orbiting the Earth that work with a GPS. The more satellites that your GPS can connect with, the more accurate your results will be.

One way to conceptualize how GPS works is to think about it two dimensionally. Imagine if you are lost and you asked someone how far you were from Boston. Suppose they said, “You are 10 miles from Boston.” You could draw a circle around Boston with a radius of 10 miles and know that you are somewhere on that circle. Then suppose they said, “You are 60 miles from Providence.” Then you could draw a circle around Providence with a 60 mile radius and you would see that the two circles intersected at two points (probably). In order to know which of those two points was your location, you’d need one more piece of information. If they then told you “You are 30 miles from Worcester”, you could draw a circle around Worcester with a radius of 30 miles and you would be at the point where all three circles intersected. This is how it works except GPS works in three dimensions.

When you use a GPS to find your location, its receiver will send signals up to any and all of the satellites that it can find in the sky. It works best when you have no interfering objects such as trees or tall buildings, so unfortunately sometimes it was hard to find accurate results since we were mostly always deep in the woods covered by towering pines. Once it reaches contact with at least three satellites, it finds the exact distance between the GPS and the satellites. Using that information, the GPS does some trigonometry (complicated math that is almost impossible to understand). The system calculates spheres (like a ball with air inside it) around each of the satellites with the satellite at the center. The radius of each sphere is the distance from the satellite to the point on which you are standing.