Coordinate Systems, Datums and Map Projections reviewed

Coordinate Systems, Datums and
Map Projections
The study of the size and shape of the
The Earth is…
3000 BP Bablonians
An oyster
600 BP Greeks
500 BP a perfect shape A sphere
300 BP Circumference ~ 25,000 miles,
Dark ages
Flat again
1492 Columbus
A pear
1753 French
Oblate ellipsoid
Geoid to describe the deviations from a
The Earth as a Geoid
Geographic Coordinate System
•Great and Small Circles
Geographic Coordinate System
GCS uses a 3-D spherical
surface to define locations
on Earth.
GCS includes an angular unit
of measure, a prime meridian
and a datum.
Geographic Coordinate System
• Longitude and Latitude
– Degrees, minutes, seconds
– 1o latitude = ~110.5 km (equator)
– 1o longitude = cosine of the
– 1 minute of latitude = ~1852
How to convert form DMS to DD
• Example: 37°36’30”
• Divide each value by the number of
minutes or seconds in a degree
• 36 minutes = .60 degrees (36/60)
• 30 seconds = .00833 degree (30/3600)
• Add it all up
• 37° + .60 + .00833 = 37.60833 DD
Sphere vs. Speroid
A sphere is okay for small scale maps (<1:5,000,000).
For larger scale maps a spheroid is necessary, the spheroid used
will depend upon the purpose, location, and accuracy of the data.
• Reference frame for locating points on
Earth’s surface
• Defines origin & orientation of
latitude/longitude lines
• Defined by spheroid and spheroid’s
position relative to Earth’s center.
Position of the Capital of Texas
N. American Datums
• NAD27
– Clarke 1866 spheroid
– Meades Ranch, KS
– Local datum
• NAD83
– GRS80 spheroid
– Earth-centered datum
– GPS-compatible
• NAD27
NAD83 up to 500’ shift
Datums and Elevation
• Horizontal and Vertical Datums
• Sea - level?
– Panama Canal
• Height Above Ellipsoid (HAE)
• Height Above Geoid (HAG)
A spherical planet in plane terms
Projected coordinate systems are any coordinate system designed for
a flat surface.
Map Projection
• Distortions are inherent in maps
– Earth is round, map is flat
• Projection is the term used to describe
the process of mapping a round surface to
flat paper
Projection & Datum Overview
Earth is three-dimensional
Map (screen) is 2-D
Projections convert 3-D to 2-D
3-D to 2-D causes distortions
Datums locate in 3-D
Map Projections
• Map projections always introduce
distortion of some kind
• Distance
• Direction
• Shape
• Area
Map Projections, cont.
• There are many kinds of projections
for different parts of the world
• Different projections produce
different distortions
• Key is choosing appropriate
projection for a particular situation
Map Projections Types
Map Projections and GIS, cont.
• Should be considered before starting to
automate GIS data or starting a GIS
• Must also be considered if you obtain
data from other sources (who doesn’t?)
• Map projections are central to insuring
different GIS databases properly register
to one another (really important in GIS!)
• Important for valid spatial analysis
The State Plane Coordinate System (SPCS)
• Not a true map projection but rather a set
of coordinate systems for every state.
• Designed in the 1930s to provide a local
reference system tied to a national datum.
• Most USGS 7.5 minute maps indicate state
plane coordinates (as well as UTM, and
lat-long grid)
• Some states (depending on size and
latitudinal extent) have multiple zones
State Plane Zones for NAD 1983
The State Plane Coordinate System (SPCS), cont.
• East-West oriented States use the Lambert
Conformal Conic projection (ex. Tennesse)
• North-South oriented States use the
Transverse Mercator projection (ex.
• One zone in Alaska uses Oblique Mercator
• Based on the North American Datums:
NAD27 and NAD83
• Reference Eastings and Northings
Universal Transverse Mercator
• Transverse Cylindrical (Mercator) Projection
• 60 zones
– 6o wide
– Central meridian is 500,000 m
• Eastings
– 0 is 500,000m west of central meridian
• Northing
– depends on hemisphere and is measured in meters from the
• We are in Zone 10 N
UTM Zones
A Generic UTM Zone
Central Meridian
Easting = 500,000
Northern Hemisphere:
Northing = 0 meters
Southern Hemisphere:
Northing = 10,000,000
We are in UTM zone 10 north
Basic Feature of Maps
• Scale can never be totally accurate all over the
map, unless the map covers a very small area.
• Features: Title, Date, Legend (key), North
Arrow (magnetic vs. true north)
Map Characteristics
• Scale
– Ratio between distance on map to distance on Earth
– Small scale - cover large area
• e.g. 1 : 1,000,000
– Large scale - shows more detail
• e.g. the 1 : 2,400
– Types: Representative fraction, linear, verbal
• Resolution
– accuracy that location and shape can be depicted
– Diminishes with scale
Topographical vs. Thematic Maps
• Topographical
– Maps whose primary purpose is to indicate the
general lay of the land
– outline natural features
– topographical maps often show other features
such as roads, boundaries, rail links
Topographical vs Thematic Maps
• Thematic Maps
– displays geographic concepts
population density
land use
Types of Thematic Maps
• Chloropleth Maps
– maps that display information using man-made
reporting areas. Areas shaped according to
their value.
• census tracts, ZIP codes, counties, etc.
• Area Class Maps
– maps that display information by constant
• e.g. coniferous Vs deciduous forest
Types of Thematic Maps
• Isopleth (Isoline) Maps
– maps showing imaginary surfaces -connects all
points with equal value.
• contours
• pressure
• temperature
• pollution levels
• Isolines are:
drawn at regular intervals
never cross
they are closed lines
values inside are higher than outside