A map projection is a system of lines on a planar surface that represents a corresponding referencing system or datum. A datum is a surveying or referencing system used as a basis for calculating and/or measuring geographic locations.  

The cartesian coordinate system is used to visualize the relationship of two dimensional points positioned in mathematical space. GIS coordinates represent earth ground locations in spherical space. There are numerous coordinate types with elegant and unfamiliar sounding names such as:  Albers Conical Equal-Area Projection Lambert Confromal Conic State Plane Transverse Mercator Polyconic Universal Transverse Mercator Clark 1866.

Both Universal Transverse Mercator and latitude/longitude projections are used by United States Geological Survey. These two map projections will provide the necessary beginning experience to comprehend any other projections (no matter the country or region). Generally speaking, any good GIS system will easily interchange the mathematically complex map projections one with another.  

Thinking in "GIS terms" one imagines the application of a huge uniform grid applied mathematically to the surface of the earth, and will reference precisely any geographic point on that surface. The UTM projection allows one to think of the world as a flat surface, or, enough of the earth's surface to accomplish most projects under fifty thousand acres. In the 1991 Gulf War, troops, vehicles, airplanes, and guided missiles were directed to their intended destinations using the military version of the UTM projection. In analyzing the earth's surface features it is important to visualize that each identifiable feature can be correctly coded for mapping purposes within the context of mathematical space using these spatial referencing systems.

The Global Positioning System (GPS) depends on a network of 24 satellites. Each orbits every 12 hours at an altitude of approximately 12,000 miles and transmits its exact location and the precise time (determined by an atomic clock onboard the satellite).    The GPS receiver (on the ground or water) picks up three (or four is better) of these satellite signals and then determine a precise location on the surface of the earth through the technique known as "triangulation."

Reference:  http://tiger.census.gov/cgi-bin/mapbrowse-tbl  

Go to the tiger server (above) and familiarize yourself with the basic map creation components. Scroll down toward the center of the page.  Look for the two form fields where you have the opportunity to "Enter the precise (lat/lon decimal degree) coordinates".     Simply change the field information to access the maps of your choice.

The default map page is 38.89 degrees, Latitude, and -77.02 degrees Longitude.  The location displayed is of Washington, D.C.    Note:  The Map width and height establishes the relative size of the map display (and of course, map scale).

Paul Charles Construction Company, McGregor, Texas  76657