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An Introduction to GPS (Global Positioning System) in Locating Cemeteries

Copyright 2003 by Steve Reid

 

 

Q: What is GPS?

A: A technology introduced to the public in the 1990's, Global Positioning System (GPS) tracks satellites orbiting the earth to calculate precise location. This is done often through means of a handheld, portable GPS device which monitors the satellites to determine exact latitude and longitude.

 

Q: What is latitude and how is it measured?

A: Latitude is measured as distance from the equator, given in degrees. Think of these latitude degrees or lines as "tomato slices" of the globe. The equator is assigned the value of 0o (read as "zero degrees") latitude. The north pole is assigned the value of 90o North latitude while the south pole sits at 90o South latitude. These north and south pole values establish the extreme limits of latitude.

Latitude has been used in establishing U.S. state boundaries. For example, the line that forms the southern border of Tennessee is the 35o North latitude line, Kansas' northern border is the 40o North latitude line, and Wyoming's northern border is the 45o North latitude line--or half the distance between the equator and the north pole. Interestingly, the 40o North latitude line cuts right through Daviess County so we know that all latitudes in the county are going to be very close to 40o North latitude.

North latitude is also commonly referred to with a positive value while latitudes south of the equator are often referred with a negative value. Thus the city of New Orleans, Louisiana, could be approximated at 30o North latitude or simply + 30o latitude while the city of Durban in South Africa far south of the equator could be approximated at 30o South latitude or simply -30o latitude.

 

Q: What is longitude and how is it measured?

A: Longitude is measured as distance east or west from an imaginary line drawn from the north pole to the south through Greenwich, England (the home of the person who made up the system of longitude), given in degrees. Think of these degrees or lines like "orange quarters" of the globe. There are 180o (read as "one hundred eighty degrees") in either direction--east or west--of the line through Greenwich. At 180o the east and west longitude lines merge in the Pacific Ocean, forming another well-known line called the international date line.

Longitude lines have also been used in establishing U.S. state boundaries. For example, the western border of Nevada north of Carson City is the 120o West longitude line. The 94o West longitude line cuts right through Daviess County so all longitudes in the county will be very close to 94o West longitude.

Like latitude, longitude can be referred to with a positive or negative value instead of east or west. If longitude is positive, it is a place east of Greenwich; if longitude is negative, it is a place west of Greenwich. Therefore, all longitudes in the U.S. are west, or negative, longitudes.

 

Q: How are latitude and longitude used together?

A: By converging a latitude line (a horizontal line) with a longitude line (a vertical line), a unique and precise spot on the globe is located.

 

Q: How do you get precise latitudes and longitudes with only a hundred or two degrees to work with?

A: Both latitudes and longitudes are further broken down into minutes and seconds. Each degree has from 0 to a maximum of 59 minutes and each minute has from 0 to a maximum of 59 seconds. For example, Mt. Ayr Cemetery in Daviess County is located precisely at 39 degrees, 52 minutes, and 35 seconds North latitude and 94 degrees, 5 minutes, and 37 seconds West longitude. (Notice that the latitude is close to the 40 degrees mentioned earlier and the longitude is close to the 94 degrees previously discussed.) Rarely is a latitude/longitude written like this but rather with the degree sign (o) the minute sign (') and the second sign ("). Thus Mt. Ayr's latitude may often be written +39o 52' 35" and the longitude as -94o 5' 37". The chart below summarizes this:

Latitude: +39o 52' 46" read as 39 degrees, 52 minutes, 46 seconds North latitude
Longitude: -94o 05' 37" read as 94 degrees, 5 minutes, 37 seconds West longitude

 

Q: What other forms of latitude/longitude notation are there?

A: Besides using the degree-minute-second (d-m-s) notation, a decimal notation and a combination notation can be used. Decimal notation still uses degrees, but converts the minutes and seconds to a decimal number. For example, Mt. Ayr's latitude could also be noted as +39.8794o. On the other hand, the combination notation keeps degrees and minutes, but converts the seconds into a decimal part of the minutes. Therefore, Mount Ayr's latitude could also be noted as +39o 52.77'. The chart below compares these three different notations for Mt. Ayr Cemetery:

  d-m-s form decimal form combo form
Latitude:

+39o 52' 46"

+39.8794o

+39o 52.77'

Longitude:

-94o 05' 37"

-94.0936o

-94o 05.62'

 

Q: How do you convert from d-m-s to the other forms?

A: Take the seconds of d-m-s form, divide by 60. If you add that decimal to the minutes, you get the combo form. Take the minutes (including decimal) of combo form and divide by 60 and you get the decimal to add to your degrees to make the decimal form.

 

Q: What are some applications of these different forms?

A: The United States Geological Survey's Geographic Names Information Server (GNIS) is an excellent source for finding coordinates given in d-m-s form for cemeteries. If you query by feature type: cemetery in Daviess County, Missouri, GNIS will pull up a list of 15 cemeteries and their coordinates.

The U.S. Census Bureau's Tiger maps require the decimal form for placing markers on maps. An example of this is Mt. Ayr Cemetery red stickpin I have placed on a small green area. (Green areas are usually parks or cemeteries on Tiger's maps.) Notice towards the bottom of this page the area entitled "Place a marker on this map:" and that I have filled in Mt. Ayr's latitude and longitude in decimal form.

My personal GPS locator gives me a choice of either d-m-s or combo form. I choose the combo form when I read a cemetery's coordinates because it is just slightly more precise than the d-m-s form simply because you have more values to work with (that is, less rounding.)

 

Q: Specifically, how does a GPS locator help in finding a cemetery?

A: By entering latitude and longitude, most GPS locators will give you a "bird's way" direction to a cemetery or other location--that is how far you have to travel in the closest direction. This is useful particularly to the novice GPS user when the cemetery is close--for example within a half mile--and the cemetery lies in a field or forest away from established roads.

 

Q: Are GPS locators easy to use?

A: Once the GPS is initially set up, garnishing the precise coordinates for a place is easy. For navigating to an unknown spot or cemetery it definitely requires some study time of the manual and practice.

 

Q: How precise is GPS?

A: Although I am no scientist, it is my understanding that the GPS was so precise that the government generated a small random error so that the public would never have as accurate information as the military. With regards to cemeteries, GPS can accurately locate a cemetery or sections of a large cemetery, but because of the random error is unable to yield accurate results measuring from tombstone to tombstone.

 

Q: How much does a GPS device cost?

A: Useful GPS devices under $100 can be found at major retailers.

 

Q:  Where can I find a map to a cemetery based on GPS coordinates?

A: Topozone.com provides free topographic maps that are able plot GPS coordinates such as those found on this website.  Just plug in the coordinates in d-m-s format and you have a map!  CAUTION: To completely understand this topic, an understanding of Map Datums must be had.  A future update to this page will deal with this topic, but for now try the following URL:  http://www.gpswaypoints.co.za/FAQ_map_datums.htm 

 

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