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Mocavo

Some people eat, sleep and chew gum, I do genealogy and write...

Saturday, February 5, 2011

The Genealogist's View -- The ABCs of GPS Part Two

 http://commons.wikimedia.org/wiki/File:WorldMapLongLat-eq-circles-tropics-non.png Public Domain

Note: This post follows The ABCs of GPS Part One.

I have always loved maps. I used to have hundreds of them in boxes and rolled up in closets. I admit to spending hours and hours pouring over maps looking at all the details and literally memorizing the mountains, rivers, streams and other geographic features. During most of my adult life, I have lived in Utah and Arizona and traveled a very high percentage of all of the roads in the two states. I cannot imagine being lost in either state. It would be like getting lost in my own house. (I admit this possible as I get older, but not quite yet). For that reason, I have never seen the need for GPS device, I always know where I am within zero feet, I don't need a computer to tell me that.

But as a genealogist and traveler, I do have to admit that the new GPS devices are appealing. The first issue is that even though I live in the West, my relatives all come from the East and Europe. I have had to look for locations in Rhode Island, Pennsylvania, Massachusetts and other states and finding locations in the eastern states is much more of a challenge. Lately, finding places in Arizona has turned out to be a challenge. I have run out of all of the familiar locations and am now trying to find really obscure locations. Guess what? I have used a GPS receiver to find places that would be nearly impossible to find in any other way.

Because of my interest in maps, I am very familiar with the topographical variety (sometimes called terrain maps). Even if you purchase a GPS device, your days at looking at maps will not be over. It is true that some of the more arcane map symbols are probably on their way out, especially with online copies of all of the topographical maps in the U.S. You may want to check out The National Map, a compilation of most of the topographical maps of the U.S. online.   Because maps and GPS devices are intimately related, here are a few of the most common map terms and their definitions:

Topographic map: a detailed representation of the earth's surface using contour lines at different elevations, and symbols to represent valleys, hills, mountains, rivers, lakes and other features. Topographical maps may also show roads, buildings, churches, cemeteries and other man-made structures.

Coordinates: a set of numbers representing linear and/or angular distances of a point from a standard point of origin expressed as latitude and longitude. By definition the zero point is the Greenwich meridian for longitude and the Equator for latitude.

Latitude: the angular distance, in degrees, minutes, and seconds of a point north or south of the Equator.

Longitude: the angular distance, in degrees, minutes, and seconds of a point east and west (thanks to a reader for catching my error) of the Greenwich meridian.

Meridian: a great circle on the surface of the earth, passing through both geographical poles. On any given meridian, all points have the same longitude. 

Bearing: the direction of your route. A GPS device will calculate the direction of the way point you want to reach once you enter the coordinates of your actual destination.

Way Points: a set of coordinates representing points along a route which are stored in the memory of the GPS device.

Route leg: the straight line between two way points.

Heading: the direction you want to go, irrespective of the bearing of each way point. Often it is impossible to go directly to your destination but the heading is the ultimate direction you want to go.

Course: the direction from the starting point to the destination determined through angular measurements like degrees, radians or mils. 

Route: the combination of way points, bearing and heading that make up the ultimate objective of your trip.

Track:  your actual movements with respect to the route. Depending on the GPS device, as you track your progress, the device may recalculate the route and try to correct your course.

TrackBack: the GPS automatically stores your route and way points and when you want to return, it will convert your track into a route back to your starting point. 

With a vehicle mounted GPS, all of the above functions may be so automated that you are entirely unaware of them. If you use a basic GPS system, you will immediately be involved in determining your the coordinates of your destination and perhaps plotting way points. Most route finding assumes that the ultimate destination is far out of sight. For an example, let's say you want to go to a cemetery marked on the map with certain coordinates. You enter the coordinates into your GPS device and it tells you the heading and the distance from your present location to the destination. As you move towards the cemetery, you may wish to mark certain important features of the landscape as you pass by them. These become your way points. The screen on the GPS device will show your progress through an animated track and by showing you your bearing like a compass and will keep recalculating the distance and the heading of the destination. By constantly adjusting your bearing to match the heading of the destination, eventually you will reach your destination. The GPS device keeps a log of your route and way points. When you want to return you push whatever buttons are necessary to create a trackback and the GPS will show you a route back to your start point, this route may or may not involve retracing your entire track.

Got it? Probably not the first time or second, but if you persist in working with any particular GPS device, you will eventually get the gist of the terminology and the functions will begin to make sense. If all this seems too much, you might want to enlist the help of a knowledgeable friend or take an online class on orienteering. The above description makes some obvious assumptions, for example, that the cemetery is accurately located on the map and that access is even possible at all. There might be a locked gate or high fence blocking the way, not shown on any of the maps.

In the more advanced GPS devices, as I mentioned above, most of the functions I discussed are automated to a greater or lesser degree. You may initially think that the more the features are automated the more useful the device. But for the same reason that professional photographers seldom use automatic settings on their cameras, route finding in the real world may be better accomplished with a basic GPS unit and a good map. But you say, what about GPS units that have built in maps? They may work very well or not at all. Even U.S. Geological Survey maps are not always entirely accurate. The addition of satellite imagery may, at some point, make all maps acceptably accurate, but we are still a ways from having completely accurate maps. All of the surprises in the natural world have not been mapped out yet.

Watch for Part Three of this series.

2 comments:

  1. Correction: Your Longitude definition has an error. It should read east or west of Greenwich meridian, not north and south.

    Otherwise, very nice two-part intro to GPS!

    ReplyDelete
  2. Thanks for catching my error.
    James

    ReplyDelete