### Map Projections (PPT)

```Introduction to GIS
» Surprise! It’s not a
perfect sphere
» Earth is an…
˃ Oblate spheroid
˃ Oblate ellipsoid
˃ Geoidal
» An unprojected coordinate system
» uses latitude and longitude to define the
locations of points on a sphere or spheroid
» Covers entire earth with one system and a
single origin
» Used when a single coordinate system is
needed for entire earth
» Since latitude and longitude are angular
measurements they are not suitable for
measuring distances.
» LATITUDE
˃ the equator is the origin for latitude
˃ 1 degree of latitude is ALWAYS 69
miles (111km)
» LONGITUDE
˃ Greenwich, England, is the origin
for longitude.
˃ 1 degree of longitude is 69 miles
(111km) at the equator
˃ 1 degree of longitude is 0 miles at
the poles
» ORIGIN
˃ single origin is off the coast of Africa
» Latitude and longitude are angles expressed in
degrees, minutes, and seconds
˃ 1 degree = 60 minutes
˃ 1 minute = 60 seconds
» 125°30’ 45’’
˃
˃
= 125 + 30/60 + 45/60/60
= 125.5125
» A datum is a
mathematical
representation (model,
a set of reference
points) of the shape of
the Earth’s surface
» It serves as the
reference or base for
calculating the
Geographic Coordinate
of a location.
http://gis.washington.edu/esrm250/lessons/projection/
» Two kinds of datum:
˃ A global/geocentric datum: is centered on the earth's center of mass.
+ World Geodetic System of 1984 (WGS 84).
˃ A local datum: is slightly offset to a convenient location in order to
accommodate a particular region of study.
+ The North American Datum of 1927 (NAD27)
+ The North American Datum of 1983 (NAD83)
» North American Datum of 1927 (NAD27) is a datum based on
the Clarke ellipsoid of 1866. The reference or base station is
located at Meades Ranch in Kansas. There are over 50,000
surveying monuments throughout the US and these have
served as starting points for more local surveying and
mapping efforts. Use of this datum is gradually being
replaced by the North American Datum of 1983.
» North American Datum of 1983 (NAD83) is an earthcentered datum based on the Geodetic Reference System of
1980. The size and shape of the earth was determined
through measurements made by satellites and other
sophisticated electronic equipment; the measurements
accurately represent the earth to within two meters.
» Projection is the process that transforms threedimensional space onto a two-dimensional map.
» a Map Projection
˃ this is what gets us flat
» Scale change
˃ zoom into the area of
interest
» Map Coordinates
˃ this helps us locate
things on the map
» ANY projected flat map distorts reality by compromising
on one of the following:
˃
˃
˃
˃
Shape
Area
Distance
Direction
[S]
[A]
[D]
[D]
» This makes geographers and GIS experts very [SADD]…
» There are names for the different classes of projections
that minimize distortion. Those that minimize
distortion:
˃
˃
˃
˃
in shape:
conformal.
in distance:
equidistant.
in area:
equal-area.
in direction: true-direction.
» “A set of techniques developed by
cartographers to depict with reasonable
accuracy the spherical earth in two-dimensional
media.”
Cylindrical (regular or transverse)
conic
azimuthal
The Mercator projection is also known as
the cylindricalprojection and is shown in the illustration here.
Note that it is as if a cylindrical piece of paper is wrapped
around the globe. If a light bulb were inside the globe, the
outlines of the continents and latitude and longitude lines will
be "projected" onto the paper. You would then trace these
lines and create the map when you open up the paper. It is
important to see that where the paper is in contact with the
globe the transfer of information to the map is very accurate.
As the distance between the paper and the globe increases,
the amount of distortion in the shapes of the landmasses also
increases. Therefore, with the Mercator projection, the map is
most accurate at the equator and the worst at the poles. Look
how excessively large Antarctica is!
»
»
»
»
parallels and meridians are perpendicular on this map
best accuracy at equator, worst representation at the
poles
map is used in navigation: straight lines on this map are
rhumb lines - lines of constant compass bearing (not the
shortest route). Rhumb lines are also known as
loxodromes.
Great circle routes (shortest route between two points)
are curved on this map.
The Polar Projection (also known as
the Plane or Gnomonic
Projection). Produced by placing our
piece of paper on either pole.
» parallels are shown as circles
around the pole
» meridians are straight lines
radiating from the pole like the
spokes of a wheel
» distorts near the equator
» only shows one hemisphere at a
time so if you want to show the
entire world, you would need two
maps
» Great circle routes on this map
are straight lines, rhumb lines are
curved.
The Conic Projection is
centered on the mid-latitudes
so there is least distortion
there. Make a cone and place
it on the globe.
» parallels are curved
» meridians appear straight
and converge towards the
pole
» good map projection if you
are going to show features
in the mid-latitudes, like the
United States.
» Different coordinate
systems have different
origins (0,0)
» Different coordinate
systems cover all or
only part of the earth
» Different coordinate
systems have different
units of measurement
(e.g. feet, meters,
degrees)
A coordinate system is a means
for identifying a point on the
earth on a planimetric map.
» Geographic Coordinate
System (GCS);
˃ map units in decimal degrees.
» Universal Transverse
Mercator (UTM);
˃ map units in meters.
» State Plane Coordinate
System (SPCS);
˃ preferred map units in U.S.
Survey Foot.
» UTM provides georeferencing at high levels of
precision for the entire globe
» covers the entire earth in 60 zones, each 6
degrees wide
» commonly used by environmental scientists,
the military and other professions who need to
work at the local level but also need their maps
to coordinate with other areas on earth.
» 80 degrees S to 84 degrees N used due to
deviation on the two ends
» 10 zones (10-19) cover the continental US
» Each 6 degree zone is split in two: North Zone and
South Zone
» Origin (0,0) is at South West of each zone (avoids
negative coordinates
» UTM is measured in meters, i.e. how many meters
east and north from origin?
» developed by the US Coast
Guard and Geodetic Survey
» used only in the US
» about 120 zones to cover
the entire United States
» each zone has its own
coordinate system
» similar coordinate systems
are used around the world
» Wisconsin (left) has 3 zones
» each zone has its origin (0,0) in
the south west corner so that all
coordinates in the zone have
positive values
» SPCS is measured in feet
» a point is located based on how
many feet east and north or
origin
» zones are defined by political
boundaries
» CALIFORNIA ZONE I FIPSZONE: 0401 UTM ZONE: 10
˃
DEL NORTE, HUMBOLDT, LASSEN, MODOC, PLUMAS, SHASTA, SISKIYOU, TEHAMA, TRINITY
» CALIFORNIA ZONE II FIPSZONE: 0402 UTM ZONES: 10 & 11
˃
NEVADA, PLACER, SACRAMENTO, SIERRA, SOLANO, SONOMA, SUTTER, YOLO, YUBA
» CALIFORNIA ZONE III FIPSZONE: 0403 UTM ZONES: 10 & 11
˃
ALAMEDA, CALAVERAS, CONTRA COSTA, MADERA, MARIN, MARIPOSA, MERCED, MONO,
SAN FRANCISCO
SAN JOAQUIN, SAN MATEO, SANTA CLARA, SANTA CRUZ, STANISLAUS, TUOLUMNE
» CALIFORNIA ZONE IV FIPSZONE: 0404 UTM ZONES: 10 & 11
˃
FRESNO, INYO, KINGS, MONTEREY, SAN BENITO, TULARE
» CALIFORNIA ZONE V FIPSZONE: 0405 UTM ZONES: 10 & 11
˃
KERN, LOS ANGELES, SAN BERNARDINO, SAN LUIS OBISPO, SANTA BARBARA, VENTURA
» CALIFORNIA ZONE VI FIPSZONE: 0406 UTM ZONE: 11
˃
IMPERIAL, ORANGE, RIVERSIDE, SAN DIEGO
» Both based on zones
» UTM zones follow lines of latitude and
longitude
» state plane zones generally follow political
boundaries
» UTM not as accurate as SPCS
Who cares? I’m not a cartographer, I’m a planner!
» Everything in ArcGIS is based on an “X” and a
“Y”
» If your data does not have a defined projection,
you can “define” it (but you need to know what
it is!)
» If your data has a different projection than your
other data, you need to “project” it to one
common projection
» In other words… one project = one projection
» The “Define Projection”
tool:
˃ Assign the correct projection to
the data layer
˃ ArcMap, select Catalog window >
Toolboxes > System Toolboxes
» The “Feature/Project”
tool:
˃ Creates a NEW copy of the data in
the coordinate system you have
selected
» On-the-fly projection
eliminates the need to
change the projection
of the data in ArcGIS
» A data frame’s
projection can be preset by the user or
ArcMap will default to
the projection of the
» DeMers, Michael N., 1997. Fundamentals of Geographic
Information Systems. New York: John Wiley & Sons, Inc.
» Krygier, John and Wood, Denis, 2005. Making Maps. New York:
The Guilford Press.
» http://krygier.owu.edu/krygier_html/geog_222/geog_222_lo/
geog_222_lo13.html