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?
1
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
2
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
324
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
5
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
6
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 …
7
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
10
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
11
The world in Geographic
Coordinates
Is
Antarctica
Really that
big?
12
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…
13
The Problem
14
15
16
The Mercator Projection
Making a
Projection
17
Some Projections
18
Some Projections
It is pretty obvious that if
you have data in different
projections they are NOT
going to “line up” with each
other
19
Projections: Distortion
• In going from spherical coordinates
(surface) to a flat surface THERE
WILL BE DISTORTIONS of one or
more of the following
–
–
–
–
Shape
Area
Distance
Direction
20
Projections: Distortion
• In going from spherical coordinates
(surface) to a flat surface THERE
WILL BE DISTORTIONS of one or
more of the following
–
–
–
–
Shape
Area
Distance
Direction
21
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
25
These systems are…
• The UTM coordinate system or
Universal Transverse Mercator
coordinate system
• And
• The State Plane coordinate system
– Unique to each state
26
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!
27
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
28
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
30
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.
31
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
32
NY East
Zone 4801
NY Central
Zone 4826
State Plane Zones
NY West
Zone 4851
NY Long Island
Zone 4876
33
NY East
Zone 4801
NY Central
Zone 4826
State Plane Zones
NY West
Zone 4851
Transverse
Mercator
Lambert
Conformal
NY Long Island
Zone 4876
34
ArcMap Problem (or NOT)
• ArcGIS will project on-the-fly
• By that, we mean that if you add a
layer that is NOT in the same
Coordinate System, Projection, etc. as
the data in the map ArcMap will
project (verb) the new data to match
that of the data already loaded
• Is there a problem??
• Data has to have a metadata file
35
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
36
Spheroids & Datums
• A spheroid can be moved mathematically
to fit different parts of the earth…
Earth
Spheroid
Now we have 2
different datums
37
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
38
BUT…
• This neat functionality of ArcGIS
only works if you have a metadata file
for each layer
• Sometimes that is a problem!
39
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 so far
1. There are a number of GIS data types
2. Spherical earth (globe)
a. Longitude, Latitude (X,Y)
3. Flat maps
a. Projections
b. Coordinate Systems
•
•
•
UTM
SP
Spheroids & Datums
4. There is one other factor influencing data
accuracy - scale
41
Scale
Scale =
distance on map
distance on ground
A Scale of 1/24,000 Means
1 inch (or foot, or furlong) on the map =
24,000 inches (or feet or furlongs) on the
ground.
42
New York
Numeric Scale
=
1/5,198,769
3.9
320 mi
43
3.6”
2.63mi
Scale =
1/46,288
44
2.6” / 25’
Scale = 1/115
Living Room
Dining Room
Kitchen
45
Scale
1/47,000,000
Is a smaller
number than
Small Scale data
Large area/sheet
Least accurate
1/46,000
Is a smaller
number than
Living Room
Dinning Rm.
Kitchen
1/115
Large Scale data
Small area / sheet
Most accurate
46
So what’s all the fuss?
• Scale is a very important property of maps
and digital data derived from maps.
• Why?
• Because it stands for accuracy in the data
• A small scale map is less accurate than a
large scale map.
• Why is that?
• Generalization
47
Accuracy & Generalization
• Take the case of a
winding stream
• Shrink it to a Smaller
scale
• Now it is hard to see what
is there
• So the cartographer
simplifies the stream
48
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!
49
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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Acronyms
• NAD – North American
datum
• GCS – Geographic
Coordinate System
• WGS – World Geodetic
System
• UTM – Universal
Transverse Mercator
corrdinate sysem
• SPC – State Plane
coordinate system
• GRS –Geodetic
Reference System
• DD – Decimal Degrees
• DMS – Degrees, minutes,
seconds
• HARN – High Accuracy
Reference Network
(State Level)
• NADCON – North
American Datum
Conversion between
NAD27 & 83
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