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Step 1 Start ArcMap If you have a shortcut icon for ArcMap™ on your desktop, double-click it to start ArcMap. You can also start ArcMap by clicking the Start menu, then choosing Programs, ESRI, ArcGIS, ArcMap (your ArcMap location may be different, depending on the program location specified when the software was installed). You see the interface of the ArcMap application.

Sizing UP Earth Exercise

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Page 1: Sizing UP Earth Exercise

Step 1 Start ArcMap

If you have a shortcut icon for ArcMap™ on your desktop, double-click it to start ArcMap. You can also start ArcMap by clicking the Start menu, then choosing Programs, ESRI, ArcGIS, ArcMap (your ArcMap location may be different, depending on the program location specified when the software was installed). You see the interface of the ArcMap application.

Your ArcMap interface may look different from the View Result graphic. For example, your ArcMap window may be larger or smaller than the View Result graphic. You may or may not see the ArcMap opening dialog box. Your ArcMap may have different toolbars

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displayed, a particular toolbar may be in another location or not displayed at all. These differences have no effect on the overall results of the exercise.

Don't worry about understanding everything you see on the ArcMap interface. Learning ArcMap is not the focus of the course.

Step 2 Open an ArcMap document

If you see the ArcMap opening dialog box, select the option to open an existing map then click OK. Otherwise, from the File menu, choose Open. In the Open dialog box, navigate to your module folder (e.g., C:\Temp\VirtualCampus\MapProjections\Roundearth). Click Spheroids.mxd, then click Open.

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Of the five data layers in the ArcMap document, two are currently displayed (i.e., checked). The Graticule 30 layer consists of 30-degree latitude by 30-degree longitude lines using the WGS84 spheroid. The Countries layer shows the generalized outlines of world countries using the WGS84 spheroid. Three layers are not currently displayed. The Clarke 1866 layer shows a detailed outline of New York using the Clarke 1866 spheroid. The International 1924 layer shows a detailed outline of New York using the International 1924 spheroid. The WGS 1984 layer shows a detailed outline of New York using the WGS84 spheroid.

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Step 3 Zoom in to the area of interest

Differences in spheroids are not noticeable at scales smaller than 1:5,000,000, so you will need to zoom in to a specific area. From the View menu, point to Bookmarks and click Long Island.

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The view zooms into the area around Long Island, New York.

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Step 4

Display a more detailed layer

The feature shapes in the Countries layer are too generalized to let you see the differences between spheroids. You need to use more detailed data. Check the box next to the WGS 1984 layer to display it.

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You see much more detail along the coastline and many smaller islands that weren't visible before.

Step 5

Compare the spheroids

Check the boxes next to the International 1924 and the Clarke 1866 layers to display them.

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Turn these two layers, along with the WGS 1984 layer, on and off so you can make a visual comparison. Depending on the size of your ArcMap window, the current display scale is smaller than 1:5,000,000. At this scale, you shouldn't see any differences among the three spheroids.

Step 6

Zoom in closer

Click the check box next to Countries to turn off its display.

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You are interested in the detailed feature shapes around Long Island. From the View menu, point to Bookmarks and click Detail 1.

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The view zooms in. Your map scale is probably greater than 1:350,000 and you should just be able to see some differences between features as represented by the three spheroids.

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From the View menu, point to Bookmarks and click Detail 2.

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Your scale is now probably greater than 1:50,000. The differences between spheroids should be obvious. The Clarke 1866 and the WGS 1984 spheroids vary the most with the International 1924 spheroid lying approximately halfway between them.

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Step 7

Measure the differences

Click the Measure tool . The Measure window opens and you can move it as needed. ArcGIS 9.0 and 9.1 will not see a Measure window. ArcGIS 9.0 or 9.1 users can skip this line of instruction. In the Measure window, click the Choose Units button, then choose Distance. From the list of distance units, choose Meters. Now all your measurements will be in meters

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The map display shows a small island above a horizontal strip of land. Move the mouse pointer over the northernmost point on the small island in the WGS 1984 layer and click once.

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Move the mouse pointer (which looks like a measure tool) over the northernmost point on the small island in the Clarke 1866 layer and double-click.

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The distance is reported in the Measure Window, or in the lower left corner of the ArcMap window if you are using ArcGIS 9.0 or 9.1.

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Click the Pan tool . Move the mouse pointer to the top of the map display. Click and hold the mouse pointer and drag it to the bottom of the display, then release it. The graphic area refreshes. Repeat the Pan operation until a land mass comes into view. This is the coastline of Long Island.

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You can use the Pan tool to move the map display in any direction. Use the Pan and Measure tools to examine the coastline of Long Island. Are the differences uniform throughout the layer? Note: The datasets for this exercise use different spheroids within the same datum. This may not mean much to you now, but it is something you should never do with your database! Part of a datum's definition is its spheroid. We changed spheroids solely for educational purposes so we could demonstrate their differences. You will learn about datums later in the course.

Step 8

Exit ArcMap

From the File menu, choose Exit. Click No when prompted to save your changes.

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Review

The ancient Greeks conjectured that the earth was round based on observations such as the way ships disappear on the horizon. Pythagoras was the first to advance this proposition. Later, Eratosthenes estimated the circumference of the earth by measuring the angles of shadows cast in Syene and Alexandria and finding the distance between the two cities. In the eighteenth century, it was determined that the earth is not a perfect sphere, but an ellipsoid that bulges slightly at the equator. Because this bulge is very slight, the earth's shape is often called a spheroid—an ellipsoid that approximates a sphere. Below are some important points for you to remember.

Because of surface irregularities, the earth cannot be adequately modeled by a single spheroid. Instead, different spheroids are applied to different parts of the world to achieve local accuracy.

Locations on the earth's surface are measured with lines of latitude and longitude. The mesh of intersecting lines of latitude and longitude is called a graticule.

Latitude and longitude are measured in degrees, minutes, and seconds.

Decimal degrees (DD) are similar to degrees/minutes/seconds (DMS) except that minutes and seconds are expressed as decimal values.

Review questions1. At which scales can you assume that the earth is a sphere, and what is the term for a sphere that has the same area as a given spheroid? 2. What term is used to describe an ellipsoid of rotation that, like the earth, rotates about its shortest axis? 3. What primary purpose do decimal degrees serve as compared to measurements in degrees, minutes, and seconds?

What are the correct responses?Answer

1. For scales smaller than 1:5,000,000, it can be assumed that the earth is a sphere. An authalic sphere is a perfect sphere that has the same area as a given spheroid. 2. Oblate ellipsoid.

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3. Decimal degrees are decimal equivalents of measurements in degrees, minutes, and seconds. Decimal degrees make computer calculations faster.

Exam (Exercise 1)

Name: _________________________________________________________________

1. The earth's surface is not uniform; therefore, a spheroid that is the best model for one part of the earth is not necessarily the best model for another part.

True

False

2. An ellipsoid that approximates the shape of a sphere is often called a spheroid.

True

False

3. Which of the following terms describes a sphere that has the same surface area as the earth's spheroid?

Equiaxis sphere

Major-axis sphere

Authalic sphere

Equidistant sphere

4. You can use a sphere instead of a spheroid when your map's scale is less than 1:5,000,000.

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True

False

5. The earth's spherical coordinate system is an angular coordinate system.

True

False

6. Which of the following spheroids is now regarded as the best model of the earth for the region of North America?

Clarke 1866

International 1924

GRS80

Bessel 1841

7. Which lines on the graticule are parallel, run east-west, and mark angular distance north and south of the equator?

Meridians

Lines of longitude

The minor axes

Lines of latitude

8. Which of the following terms is used to describe an ellipsoid of rotation that rotates about its shortest axis?

Shortened spheroid

Oblate ellipsoid

Prolate ellipsoid

Compressed ellipsoid

9. Which of the following decimal degree values is equivalent to the degrees/minutes/seconds location 45°30'18"?

45.3045

45.5050

45.6050

45.9045

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10. The distance between meridians is a constant value.

True

False