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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:

• points• lines• polygons• large database

each object

• Raster GIS– AREA represented

by:• Grid cells• one value per cell• Large number

thematic layers

Forest road

Cropland stream

Looks like a map Looks like

what?

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Vector Data

Line

Area

Point

“Kinds” of GI data

Aerial Photo

Landsat 7 image

2’ resolution rectified aerial photograph

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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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Syracuse

-76.19 W 43.07N

Where are we?

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Earth’s Coordinate SystemX 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

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X, Y = Longitude, Latitude

Lines of constant LongitudeLines of constant Latitude

0-90 +90-180 +180

0

-30

30

-90

90

-60

60

Equator

Stretch the top

Stretch the bottom

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X, Y = Longitude, Latitude

Lines of constant LongitudeLines of constant Latitude

0-90 +90-180 +180

0

-30

30

-90

90

-60

60

Equator

90E, 30N

90W, 30S

+90, +30

-90 -30

W76.57° N42.93°

-76.57° 42.93°

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The world in Geographic Coordinates

IsAntarcticaReally 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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The Mercator Projection Making a Projection

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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 …

• HOWEVER• ArcGIS will project on the fly so

the problem is not great• But only IF there is a metadata

file for the data.

Data abou

t 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 thecylinder is nowhorizontal and sois tangent to theearth along a meridian which passes through the Poles

Central MeridianErrors 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

Easting(X)

Northing(Y)

•The units in UTM are usually Meters•The coordinates are Eastings & Northings•The zone has to be specified

• 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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State Plane ZonesNY West

Zone 4851

NY CentralZone 4826

NY EastZone 4801

NY Long IslandZone 4876

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State Plane ZonesNY West

Zone 4851

NY CentralZone 4826

NY EastZone 4801

NY Long IslandZone 4876

Transverse

Mercator

Lambert Conform

al

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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

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Earth

Spheroids & Datums• A spheroid can be moved mathematically to

fit different parts of the earth…

FITFit

SpheroidNow 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

• These are the common datums• For Coordinate Systems the spatial

properties are given in statements like…

NAD 27 NAD 83 WGS 84

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!