Spatial Data
Spatial data comes in many
forms. So How does a GIS
work with the data so that it
can put the data in the right
place on a map?
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Outline
• Kinds of spatial data
– Vector data
• Points, lines, and polygons
• Highway maps
– Raster data and image data
•
•
•
•
Raster representation of data
Satellite data (Landsat)
Aerial photographs
Digital elevation data
• Real world problems associated converting
data from earth’s spherical coordinates to
coordinates on a flat surface
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Compare Raster & Vector
• Vector GIS
– Objects
represented by:
•
•
•
•
• Raster GIS
– AREA represented
by:
points
lines
polygons
large database each
object
• Grid cells
• one value per cell
• Large number
thematic layers
Looks like a
map
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Looks like
what?
Forest
road
Cropland
stream
3
“Kinds” of GI data
Point
2’ resolution
rectified aerial
photograph
Aerial Photo
Landsat 7 image
4
The Digital Elevation Raster
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Uses
• Vector data is most common because
you can tie huge databases to
features
• BUT
• Raster data is very good for
continuous surfaces like
– Elevation
• Images (dumb picture) data
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OK…
• Data on the globe is not very useful
because you can’t put the globe in
your report
• So you need a 2D map
• Thus you have to convert 3D data to
2D
• Lets start with the spherical earth …
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Where are we?
8
Earth’s Coordinate System
X is Longitude and is measured
E and W from Greenwich,
England. West is negative, East
is positive
Y is latitude and is measured N
and S from the equator. North
is positive and S is negative.
These are called Geographic Coordinates
9
X, Y = Longitude, Latitude
90
60
30
0
-30
-60
-90
-90Stretch the
0 bottom
+90
+180
Lines of constant Longitude
Lines of constant Latitude
Stretch the top
Equator
-180
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X, Y = Longitude, Latitude
90E, 30N
Equator
90W, 30S
-180
+90, +3090
60
30
0
-30
-60
-90 -30
-90
-90
0
+90
+180
Lines of constant Longitude
Lines of constant Latitude
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The world in Geographic
Coordinates
Is
Antarctica
Really that
big?
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3D to 2D
• Geographic coordinates introduce too
much distortion to be useful
• So we need to convert 3D coordinates
into 2D coordinates
• But, there is a problem…
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The Problem
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15
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The Mercator Projection
Making a
Projection
17
Some Projections
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Some Projections
It is pretty obvious that if
you have data in different
projections they are NOT
going to “line up” with each
other
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Why different Projections?
• Spherical coordinates to flat surface
produces distortions in:
–
–
–
–
Shape
Area
Distance
Direction
• So different projections for
different purposes (Mercator for
transportation (rumb line stright)
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That means…
• Data in different projections will not
line up or be congruent!
• This is something you have to be
aware of …
Data
• HOWEVER
about
• ArcGIS will project on the fly so the
Data
problem is not great
• But only IF there is a metadata file
for the data.
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Lets make life a bit more difficult
• In addition to the many projections
that 2D data can be in…
• There are two Coordinate Systems
that are in common use …
• For smaller areas (like ½ a state)
• Much of the data you will find useful
will be in one of these systems
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These systems are…
• The UTM coordinate system or
Universal Transverse Mercator
coordinate system
• And
• The State Plane coordinate system
– Unique to each state
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UTM Coordinate Systems
• The UTM Coordinate system is –
– based on the Mercator projection
– A world wide system
• Except that the
cylinder is now
horizontal and so
is tangent to the
earth along a
meridian
which passes
through the Poles
Central Meridian
Errors are Zero!
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UTM coordinate system
• Is a projected coordinate system that divides
the world into 60 north and south zones, each
six degrees wide.
• Why bother?
• Increase Accuracy and decrease distortion
• Because all the data for a zone is within 3
degrees of the Central meridian it is pretty
accurate!
• Can’t map within multiple zones
• New York is usually mapped in one zone
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UTM Zones
Most of NY is in
UTM Zone 18
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UTM Coordinates
Northing(Y)
•The units in UTM are usually Meters
•The coordinates are Eastings & Northings
•The zone has to be specified
Easting(X)
• Example: Location of CCC is:
373,800 Meters E & 4,756,000 Meters N
in Zone 18, N
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The State Plane Coordinate System
• A projected coordinate system used
in the United States
• Divides each state into one or more
zones
• Also known as SPCS and SPC.
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State Plane
• Horizontal zones (Tenn) are in Lambert
Conformal projections
• Vertical zones are in Transverse Mercator
projections
• Each state has its own origins for its own
system
• States may have multiple zones in
different projections
• UNITS are usually feet BUT NOT
ALWAYS
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NY East
Zone 4801
NY Central
Zone 4826
State Plane Zones
NY West
Zone 4851
NY Long Island
Zone 4876
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NY East
Zone 4801
NY Central
Zone 4826
State Plane Zones
NY West
Zone 4851
Transverse
Mercator
Lambert
Conformal
NY Long Island
Zone 4876
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Another niggling Problem
• The earth is only approximately spherical
• We can mathematically convert features
on the 3D earth to a 2D map easily if the
surface is spherical and smooth
• Oops - earth is pear shaped and rough
• So we have to introduce the idea of a
datum
34
Spheroids & Datums
• A spheroid can be moved mathematically
to fit different parts of the earth…
Earth
Spheroid
Now we have 2
different datums
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So what?
• The spatial properties of a GIS data
layer specify both the projection or
Coordinate system and the Datum
• Different datums will cause shifts in
location of the order of 100 meters
• Not big but troublesome
• In ArcGIS on-the-fly projection takes
care of both projection and datum
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Some Datums
NAD 27
NAD 83
WGS 84
• These are the common datums
• For Coordinate Systems the spatial
properties are given in statements like…
NAD_27_UTM _ZONE_18N
NAD_83_SPC _ZONE_4826
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Summary
• There are a variety of spatial data types
• Spherical Geographic Coordinate Systems
are based on Spheroids
• Spherical data is projected onto 2D maps
• There are many Projections
• More commonly, you will run into the class
of Projections called Coordinate Systems
(UTM, SP)
• Projected data is based on a datum and
data in different datums will not (usually)
line up!
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Summary
• The subject of projections and
datums is the most confusing and
complex area of using GIS.
• Take good notes and do your best to
understand it.
• At GIS conferences sessions on this
topic are always very crowded! That
tells you something!
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