T WI S T’ S G eog ra ph i c D ata M anu al
--------------------------------------------------------------------------------------------------------------------On the spherical earth the location of features, towns, countries, etc. are given in Longitude and
Latitude. Longitude is measured in degrees East and West from Greenwich, England that is the zero
point. Going Eastward from Greenwich Longitude is increasingly negative until the value equals -180
degrees. Going Westward from Greenwich the
values are positive until they reach 180 degrees
at the same point as where the Eastward and
Westward values are |180| degrees.
Latitude is measure North and South from the
equator to the poles. Latitude is zero at the
equator and 90 degrees at the poles. In the
Southern hemisphere the values are negative.
These values are shown in the globe to the right.
Since you can’t Xerox a globe (ever tried it?)
some math has to be applied to the Long, Lat
values to allow the spatial data to be displayed
on a flat piece of paper or on a computer screen.
The process of doing this adjustment is called
Projecting.
PROJECTIONS
The long, lat coordinates from the globe can be plotted on a Cartesian coordinate system. When this
is done to the earth the figure below shows what happens. Features on the earth are stretched
horizontally. Is Antarctica really that big? Is it really that shape? Answer to both those questions is
NO!
This map is in Geographic Coordinates
and although technically it is a
mathematical process is NOT called a
Projection.
All Projections of a spherical surface on
to a flat surface produce distortions.
These are summarized on the back of
the Mercator Poster in your materials.
There are many projections but they and
the data in them are less important now
since most GIS systems will automatically
project data “on-the-fly” for any data
that does not have the same spatial
properties as the first data layer add to a given project. Small-scale data (lots of information in a
small piece of paper) like that for continents or even large states is usually in a specified projection.
The Power Point in your materials on Spatial Data gives a good demonstration of what happens
when you flatten a sphere.
(Need a graphic of Scale here)
G eog ra phi c Dat a M anua l
Copyright L. Herrington & A. Badruddin, Syracuse 2011
Coordinate Systems
Local data, or what we call Large Scale data (sparse information in a large piece of Paper) will
usually be in one of two Coordinate Systems. There are two of these: State Plane and UTM
(Universal Transverse Mercator).
UTM
The UTM coordinate system is worldwide and the
Zones in the coordinate system run from pole to
pole. The figure to the right shows the zones for
the US. The earth is divided into 60 Zones
starting just West of Alaska’s West most point
(180 degrees west) and continues until 180 degrees
East is reached. Each zone is 6 degrees wide but
there are exceptions to this in Northern Europe.
Wikipedia is an excellent source of information on
UTM. Notice that NY is mostly in Zone 18 and so this
zone is used for most state wide work and work for
smaller areas. The figure to the left, from the Power
Point shows how coordinates are measured in UTM.
The X coordinate is called an Eastings and the Y coordinate is called a Northing. The origin of the
coordinate system is 4 million meters to the west of west boundary of the zone. This is done so that
the Eastings are never negative. The figure shows the coordinates for Syracuse along with
“projection” information as it is recorded for data in Zone 18.
State Plane
Every state in the US has a set (one or
more) of local coordinate systems. The
figure to the right shows the State
Plane zones for all the states in the
lower 48. You can see that NY has 4
separate Zones with slightly different
coordinate systems. Texas has 5
zones. Each Zone has a projection (in
NY most use the Transverse Mercator
Projection (this is NOT the same as
UTM.) Long Island, however, uses the
Lambert Conformal Projection. The
Zones in the state plane system do not
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G eog ra phi c Dat a M anua l
Copyright L. Herrington & A. Badruddin, Syracuse 2011
have mathematical origins (0,0 coordinates) of distance from Greenwich and distance from the
equator. For that reason you cannot display or make maps using data from different zones – the
data will NOT line up at the boundary of the zones. In NY most statewide maps in the State Plane
coordinate system use only zone 18, the central zone. This causes some displacements of data for the
west and east ends of the state.
As with UTM the coordinates for State Plane data are called Northings and Eastings. You can usually
tell if you have UTM or State Plane if you are in the Northern US because the for UTM, since the
origin for Northings is the Equator, Northings are of the order of 5 million meters while for State
Plane the Northings are usually the order of 100,000 meters or 300,000 feet.
Spatial Data
Kinds of Spatial Data
Vector
Raster
Scale
????
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