# FOR 557

Document Sample

```					 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                        • Raster GIS
– Objects                               – AREA represented
represented by:                         by:
•   points                            • Grid cells
•   lines                             • one value per cell
•   polygons                          • Large number
•   large database each                 thematic layers
object

Looks like a
map                                                       Looks like
what?

Cropland   stream
3
324
“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
• So you need a 2D map
• Thus you have to convert 3D data to
2D

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
Stretch the top
60
30
Equator                                      0
-30
-60
-90
-180     Stretch the bottom
-90        0       +90       +180
Lines of constant Longitude
Lines of constant Latitude

10
X, Y = Longitude, Latitude
90E, 30N         +90, +30
90
60
30
Equator                                      0
-30
-60
-90 -30
90W, 30S                                     -90
-180   -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
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)
20
That means…
• Data in different projections will not
line up or be congruent!
• This is something you have to be
aware of …
• HOWEVER                             Data
• ArcGIS will project on the fly so the
Data
problem is not great
• But only IF there is a metadata file
for the data.                          21
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

24
These systems are…
• The UTM coordinate system or
Universal Transverse Mercator
coordinate system
• And
• The State Plane coordinate system
– Unique to each state

25
UTM Coordinate Systems
• The UTM Coordinate system is –
– based on the Mercator projection
– A world wide system                Central Meridian
• Except that the                      Errors are Zero!
cylinder is now
horizontal and so
is tangent to the
earth along a
meridian
which passes
through the Poles

26
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         27
Most of NY is in
UTM Zone 18
UTM Zones

28
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
29
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.

30
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
31
NY East
Zone 4801

State Plane Zones
NY Central
Zone 4826
NY West
Zone 4851

NY Long Island
Zone 4876

32
NY East
Zone 4801

State Plane Zones
NY Central
Zone 4826
NY West
Zone 4851

Transverse
Mercator

NY Long Island
Lambert           Zone 4876
Conformal

33
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

Now we have 2
Spheroid   different datums
35
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
36
Some Datums

• These are the common datums
• For Coordinate Systems the spatial
properties are given in statements like…

37
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!
38
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!

39

```
DOCUMENT INFO
Shared By:
Categories:
Tags:
Stats:
 views: 4 posted: 12/4/2011 language: English pages: 37