1. Referencing Data to Real Locations Module 3
ESRI Virtual Campus
Learning ArcGIS Desktop Training Course
ESRI ArcGIS
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2. Referencing Data to Real Locations
A GIS represents reality, it is not reality.
A GIS map must accurately represent feature locations.
To determine location of features in real world or on map need a reference system
A set of lines of known location that can be used to determine the locations of features that fall between the lines
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3. Referencing Data to Real Locations
Coordinate Systems
Reference systems used to determine feature locations
In this module learn about
different coordinate systems
how they work
how to change the coordinate system of a map.
Understanding coordinate systems  manage your data to increase accuracy
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4. Learning Objectives Name two types of coordinate systems.
Identify components of each type of coordinate system.
Assign coordinate system information to a dataset.
Set display units for a data frame and measure distances on a map.
Explain what a map projection is.
List the major categories of map projections.
List spatial properties that may be distorted when different map projections are applied.
Change the map projection for a data frame and describe its effects.
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5. Understanding Coordinate Systems
Two types of coordinate systems
Geographic
Used to locate objects on the curved surface of the earth
Projected
Used to locate objects on a flat surface
a paper map or a digital GIS map displayed on a flat computer screen.
Each attempts to model earth and feature locations accurately
But no system is completely accurate
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6. Geographic Coordinate System
Reference system for identifying locations and measuring features on the curved surface of the earth
Consists of a network of intersecting lines called a graticule
Intersecting lines = longitude and latitude
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7. Geographic Coordinate System
Graticule
Longitude
Vertical lines
Latitude
Horizontal lines
Because earth is spherical, these lines form circles
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8. Geographic Coordinate System
Measurements expressed in
Degrees
1/360th of a circle.
Can be divided into 60 minutes
Minutes
Can be divided into 60 seconds
Seconds
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9. Geographic Coordinate System
Lines of longitude
Called meridians
Measures of longitude begin at the prime meridian
Defines zero value for longitude
Range from 0° to 180° going east
Range from 0° to -180° going west
Lines of latitude
Called parallels.
Measures of latitude begin at the equator
Range from 0° to 90° from the equator to the north pole
Range from 0° to -90° from the equator to the south pole
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10. Geographic Coordinate System
Prime meridian
Green line
Starting point for longitude
Has a value of 0
Equator
Red line
Starting point for latitude
Runs midway between the north and south poles
Dividing earth into northern and southern hemispheres.
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11. Geographic Coordinate System
Longitude and latitude actually angles measured from earth's center to point on earth's surface
For example, consider these coordinates:
Longitude: 60 degrees East (60° 00' 00")
Latitude: 55 degrees, 30 minutes North (55° 30' 00")
Longitude coordinate refers to angle formed by two lines
one at the prime meridian
the other extending east along the equator.
Latitude coordinate refers to angle formed by two lines
one on the equator
the other extending north along the 60° meridian.
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12. Geographic Coordinate System
Longitude and latitude are angles measured from the earth's center to a point on the earth's surface.
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13. Understanding Spheroids
Many models of the earth's shape
Each has its own geographic coordinate system
All based on
degrees of latitude and longitude
Exact latitude-longitude values assigned to individual locations will vary
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14. Understanding Spheroids
Two shapes commonly used to model earth
Sphere
Spheroid
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15. Sphere vs Spheroid
Assuming the earth is a sphere greatly simplifies mathematical calculations
Works well for small-scale maps
Maps that show large area of the earth
A sphere does not provide enough accuracy for large-scale maps
maps that show smaller area of earth in more detail
For those, it is preferable to use a spheroid
A spheroid is a more accurate model of the earth, but it's not perfect.
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16. Understanding Spheroids
Planet Earth
slightly pear-shaped and bumpy
has several dents and undulations
south pole is closer to the equator than north pole
Geoid
Model for complicated of earth
Too mathematically complicated to use for practical purposes, so spheroid is used as a compromise
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17. Understanding Spheroids
Different spheroids currently in use
Some spheroids were developed to
Model the entire earth
Model specific regions more accurately
World Geodetic System of 1972 (WGS72) and 1984 (WGS84)
Used to represent the whole world
Clarke 1866 and Geodetic Reference System of 1980 (GRS80)
Most commonly used in North America
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18. Understanding Spheroids
Why do you need to know about spheroids?
Because ignoring deviations and using the same spheroid for all locations on the earth could lead to measurement errors of several meters or, in extreme cases, hundreds of meters.
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19. Understanding Datums
A spheroid doesn't describe the earth's shape exactly.
A geographic coordinate system needs a way to align the spheroid being used to the surface of the earth for the region being studied.
For this purpose, a geographic coordinate system uses a datum.
A datum specifies which spheroid you are using as your earth model and at which exact location (a single point) you are aligning that spheroid to the earth's surface.
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20. Understanding Datums Red spheroid Blue spheroid
Aligned to the earth to preserve accurate measurements for North America
Blue spheroid
Aligned to the earth to preserve accurate measurements for Europe
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21. Understanding Datums Datum
Defines origin of geographic coordinate system
The point where the spheroid matches up perfectly with the surface of the earth and where the latitude-longitude coordinates on the spheroid are true and accurate.
All other points in the system are referenced to the origin.
In this way, a datum determines how your geographic coordinate system assigns latitude-longitude values to feature locations.
There are different datums to help align the spheroid to the surface of the earth in different regions
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22. Does Changing Datums Affect Your Data
If you change the datum of the geographic coordinate system, you should know that the coordinate values of your data will also change.
For example, consider a location in Redlands, California, that is based on the North American Datum of 1983
The coordinate values of this location are:
–117° 12' " (longitude) 34° 01' " (latitude)
Now consider the same point on the North American Datum of 1927
–117° 12' " (longitude) 34° 01' " (latitude)
The longitude value differs by about three seconds, while the latitude value differs by about 0.05 seconds.
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23. Does Changing Datums Affect Your Data
In both NAD 1927 and the NAD 1983 datums
Spheroid matches the earth closely in North America
Is quite a bit off in other areas
Notice that the datums use different spheroids and different origins
NAD 1927
origin aligns the Clark 1866 spheroid with a point in North America
NAD 1983
Origin aligns the center of the spheroid with the center of the earth
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24. Does Changing Datums Affect Your Data
The most recently developed and widely used datum for locational measurement worldwide is
World Geodetic System of 1984 (WGS 1984)
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25. Projected Coordinate Systems
The surface of the earth is curved but maps are flat.
To convert feature locations from the spherical earth to a flat map
Latitude and longitude coordinates from a geographic coordinate system must be converted, or projected, to planar coordinates
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26. Projected Coordinate Systems
A map projection uses mathematical formulas to convert geographic coordinates on the spherical globe to planar coordinates on a flat map.
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27. Projected Coordinate Systems
A reference system for identifying locations and measuring features on a flat (map) surface
Consists of lines that intersect at right angles, forming a grid
Based on Cartesian coordinates
Have an origin, an x and a y axis, and a unit for measuring distance
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28. Projected Coordinate Systems
Based on Cartesian coordinates which use a grid.
Feature locations are measured using x and y coordinate values from the point of origin.
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29. Projected Coordinate Systems
The origin of the projected coordinate system
(0,0)
commonly coincides with the center of the map.
This means that x and y coordinate values will be positive only in one quadrant of the map (the upper right).
On published maps, however, it is desirable to have all the coordinate values be positive numbers.
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30. Projected Coordinate Systems
To offset this problem
Mapmakers add 2 numbers to each x and y value
Numbers are big enough to ensure that all coordinate values, at least in the area of interest, are positive values.
False easting
Number added to the x coordinate
False northing
Number added to the y coordinate
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31. Projected Coordinate Systems
A false easting value of 7,000,000 was added to each x coordinate.
A false northing value of 2,000,000 was added to each y coordinate.
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32. Working with Coordinate Systems in ArcGIS
All geographic datasets have a geographic coordinate system (GCS).
Some datasets also have a projected coordinate system (PCS).
When you add a dataset to ArcMap it detects the geographic coordinate system and the projected coordinate system if there is one
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33. Working with Coordinate Systems in ArcGIS
If all the data you want to display on a map is stored in the same geographic coordinate system, you can just add it to the map—the layers will overlay properly.
If some of the datasets also have projected coordinate systems, even if they are different, you can also just add them to the map without data alignment worries—
ArcMap will automatically make the layers overlay using a process called "on-the fly projection."
The geographic coordinate system is the common language.
ArcMap can convert the geographic coordinate system to any projected coordinate system and it can convert any projected coordinate system back to the geographic coordinate system.
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34. Working with Coordinate Systems in ArcGIS
An issue arises when you want to display datasets that have different geographic coordinate systems on the same map.
The first layer you add to an empty data frame determines the coordinate system for the data frame.
If that layer has a projected coordinate system, the data frame will have that same projected coordinate system.
If you add a layer that has the same geographic coordinate system but a different projected coordinate system (or no projected coordinate system at all), ArcMap will perform an on-the-fly projection and convert the data to the data frame's projected coordinate system.
The layers will overlay properly.
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35. Working with Coordinate Systems in ArcGIS
If, however, you try to add a layer that has a different geographic coordinate system, ArcMap will display a warning message telling you that it may not be able to properly align the data.
ArcMap can still project the data on the fly, but it can no longer guarantee perfect alignment.
For perfect data alignment, you need to apply a transformation to make the geographic coordinate systems match
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36. Working with Coordinate Systems in ArcGIS
How do you know what coordinate system your data is stored in?
You can view the coordinate system information for a dataset in ArcCatalog™, in its metadata.
If a dataset has no coordinate system information in its metadata (it's missing), you may not be able to display the data in ArcMap.
You may need to do some research to find out the coordinate system, then define the coordinate system using the ArcGIS tools provided.
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37. Understanding Map Units and Display Units
Units in which coordinates for a dataset are stored
Determined by the coordinate system
If data is stored in a geographic coordinate system
Map units are usually decimal degrees
If data is stored in a projected coordinate system
Map units are usually meters or feet
Units can be changed only by changing the data's coordinate system.
Display unit
Independent of map units
Are a property of a data frame
The units in which ArcMap displays coordinate values and reports measurements.
You can set the display units for any data frame and change them at any time.
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38. Decimal Degrees Latitude and longitude are angle measurements
Angles are measured in degrees.
Degrees can be expressed two ways:
degrees, minutes, seconds (DMS)
decimal degrees (DD).
In a GIS, decimal degrees are more efficient because they make digital storage of coordinates easier and computations faster.
Convert from DMS to DD
Latitude of London expressed in DMS
51° 29' 16" North.
To convert this location to DD,
Divide each value by the number of minutes (60) or seconds (3600) in a degree:
29 minutes = 29/60 = degrees 16 seconds = 16/3600 = degrees
Add up the degrees to get the answer:
51° ° ° = DD
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39. Exercise
View and modify coordinate system information
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40. Working with Map Projections
Used to convert data from a geographic coordinate system to a projected (planar) coordinate system
There are many different map projections
Each preserves the spatial properties of data (shape, area, distance, and direction) differently
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41. Work with Map Projections
Maps are always flat, so do you always need a map projection?
Maybe—it depends on what you want to do.
For example, suppose your project doesn't require a high level of locational accuracy—you won't be performing analysis based on location and distance or you just want to make a quick map. In these situations, there is probably no need to convert your data to a projected coordinate system.
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42. Work with Map Projections
Use a map projection to convert data to a projected coordinate system
If you need to perform analysis
measure distances, calculate areas and perimeters, determine the shortest route between two points
If you need to show a particular spatial property for features on a map as it really exists on the earth
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43. Types of Map Projections
The term "map projection" comes from the concept of projecting a light source through the earth's surface onto a two-dimensional surface (a map).
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44. Types of Map Projections
Map projections are created using mathematical formulas
There are three types of surfaces that a map can be projected onto:
Cylinder
Cone
Plane
Each of these surfaces can be laid flat without distortion.
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45. Types of Map Projections
Projections based on each surface can be used for mapping particular parts of the world
Cylinder
wrapped around the earth so that it touches the equator
accurate in the equatorial zone
Cone
placed over the earth so it touches midway between the equator and the pole
accurate in the mid-latitude zone
Plane
touches the earth at a pole
accurate in the polar region.
Knowing the surface used helps determine if the map projection is right for purpose
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46. Projections Based on a Cylinder
Produce maps with
straight, evenly-spaced meridians
straight parallels that intersect meridians at right angles
Created by
wrapping a cylinder around a globe
projecting a light source through the globe onto the cylinder
cutting along a line of longitude
Being laid flat
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47. Projections Based on a Cone
Produce maps with
straight converging longitude lines
concentric circular arcs for latitude lines
Created by
setting a cone over a globe
projecting light from the center of the globe onto the cone
cutting along a longitude line
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48. Projections Based on Plane
Produce maps on which
longitude lines converge at the north pole and radiate outward
Latitude lines appear as a series of concentric circles
Created by
passing a light source through the earth onto a flat surface (plane).
In this example, the plane touches the earth at the north pole.
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49. Understanding Distortion
Converting locations from a spherical surface to a flat surface causes distortion
Four spatial properties subject to distortion:
Shape
Area
Distance
Direction
Each map projection is good at preserving one or more (but not all)
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50. Understanding Distortion
Different map projections preserve different spatial properties and produce different-looking maps.
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51. Understanding Distortion
Shape
Shapes, such as outlines of countries, look the same on the map as they do on the earth.
Called "conformal”
Compass directions are true for a limited distance around any given location
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52. Understanding Distortion
Area
Size of a feature on the map is the same relative to its size on the earth
If you draw a shape and move it around the map, no matter where you place it, its size will be the same
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53. Understanding Distortion
Distance
A line between one point on the map and another is the same distance as it is on the earth (taking scale into consideration).
Most maps have one or two lines of true scale.
An equidistant map preserves true scale for all straight lines passing through a single specified location
i.e. if the map is centered on Moscow, a linear measurement from Moscow to any other point on the map would be correct
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54. Understanding Distortion
Direction
Direction, or azimuth, is measured in degrees of angle from north
preserves direction for all straight lines passing through a single, specified location
Directions from one central location to all other points on the map will be shown correctly
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55. Understanding Distortion
The azimuth from A to B is 22 degrees. If the azimuth value from A to B is the same on a map as it is on the earth, then the map preserves direction from A to B.
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56. Understanding Distortion
Anyone who uses maps should know which projections are being used and which spatial properties are distorted and to what extent.
When choosing a map projection, think about which properties you want to preserve.
If your map is large-scale (shows a relatively small area of the earth), the effect of a map projection will be much less than if your map is small-scale (shows a large portion of the earth's surface).
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57. Choosing a Map Projection
Which map projection you choose for a particular map depends on
Map's purpose
Spatial properties you want to preserve
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58. Map Purpose If map will be used for general reference or in an atlas
Want to balance shape and area distortion.
In this case, a compromise projection such as the Robinson projection may be the best choice.
If map has a specific purpose
May need to use a projection that preserves a specific spatial property
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59. Preserving Spatial Properties
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60. Choosing a Map Projection
Other factors to consider when choosing a map projection
the size of the area you're mapping,
the orientation (east-west or north-south)
the particular portion of the earth that is covered.
When working at a large scale
distortion doesn't play a big role
almost any projection centered on your area will be appropriate
In some situations, decision of which map projection to use has already been made
State Plane and UTM are standard for mapping U.S. states
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61. Exercise
Compare different map projections
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62. Review
All geographic datasets have a geographic coordinate system (GCS).
There are different spheroids for different parts of the world, and there are different datums to help align the spheroid with the surface of the earth in different regions.
As long as they have a common geographic coordinate system, ArcMap can properly display multiple datasets in the same data frame using a process called "on-the-fly projection."
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63. Review
In ArcMap, the first layer you add to an empty data frame sets the coordinate system for the data frame.
You should use a map projection when you need to perform analysis, take accurate measurements, or when you need to show a particular spatial property as it really exists on the earth.
Anyone who uses maps should know which projections are being used and which properties are distorted and to what extent.
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64. Review Questions
What are the components of a geographic coordinate system?
Name one way you can get information about the coordinate system of a dataset in ArcCatalog.
What are the two most important factors to consider when choosing a map projection?
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65. Review Answers
A geographic coordinate system is defined by a prime meridian (usually Greenwich), a datum (which includes a spheroid), and an angular unit of measure (degrees).
You can use the Metadata tab to view the coordinate system information for a dataset. You can also look at the dataset's Properties dialog box.
When choosing a map projection, you should consider the purpose of the map and which spatial properties you want to preserve.
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