What is a coordinate system and why is it so important?

Or

Why is my school map showing up somewhere in the Gulf of Mexico?

Ever have this happen? You bring in a map of your state, try to overlay your city map and they just don’t match up? Maybe your map view comes up looking something like this one.

This should be

Here.

If your data is not overlaying properly, its most likely due to data that is stored in two different coordinate systems.

So back to our first question, what is a coordinate system? If you have had a class in Algebra or Geometry, this figure to the right will look familiar.

Not all coordinate systems are alike. The two most common coordinate systems used by EAST students are the

Universal Transverse Mercator (UTM) and the

Geographic coordinate systems.

X

Y

Equator: X

Prime Meridian: Y

(+x, +y)(East, North)

(+x, -y)(East, South)

(-x, -y)(West, North)

(-x, +y)(West, North)

The intersection of the Prime Meridian and the Equator form the axes of the geographic coordinate system.

“Flatten out” the axes and the coordinate plane is like the one to the right.

Longitude is the X coordinate. Latitude is the Y coordinate.

Geographic coordinates may be written in one of the following ways:

Degrees, Minutes, and Seconds (DMS) Example

Little Rock, Arkansas

X = Longitude: 92 degrees W 45’ 33”Y = Latitude: 34 degrees N 32’ 52”

Decimal Degrees: Example

X = -92.2579 Y = 34.834

Equator: X

Prime Meridian: Y

(+x, +y)(East, North)

(+x, -y)(East, South)

(-x, -y)(West, North)

(-x, +y)(West, North)

The UTM coordinate system uses a “flat earth,” model and is divided up into the zones as shown below.The X axis is the Equator, and there is a different Y axis for each zone.

This map illustrates the UTM zone 15 North. The UTM coordinate system uses only one quadrant.

Rather than using degrees, minutes, and seconds to measure distance from the axes, the UTM system is measured in Meters.

The UTM coordinates Little Rock, Arkansas are:

X= 559,713.08 Meters EastY= 3,843,632.12 Meters North

Even though the following sets of coordinate pairs represent the same geographic location, notice how different the numbers are.

X = -92.2579 Y = 34.834

X= 559,713.08 Meters EastY= 3,843,632.12 Meters North

If you tried to place features stored in two different coordinate systems in the same map you might end up with something like this.

Little Rock