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Geography 281 Map Making with GIS Project Eight: Comparing Map Projections

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Geography 281 Map Making with GIS Project Eight: Comparing Map Projections Geography 281 Map Making with GIS Project Eight: Comparing Map Projections In this activity, you will do a series of projection comparisons using maps at different scales and geographic extents. In this activity you will learn the following ArcMap projection techniques: Using the Data View projection properties Modifying the default projection properties Performing on-the-fly projections In this project you will also learn: How to export a map from ArcMap and import it into a Microsoft Word document. How to set up multiple data frames in one project and display them in a layout view. The data needed for Project 8 is located in the \\geogsrv\data\geog281\proj8\data directory. Project 8 files: Description: Feature Type: cal_county shapefile California County map polygon cal_utm11_83 shapefile California map in UTM coordinates polygon latlong shapefile World Graticules line oc_sp6_83 shapefile Orange County map in SPCS coordinates polygon ocblock_utm11_83 shapefile Orange County map in UTM coordinates polygon us48_states shapefile Lower 48 US States map polygon world shapefile World map polygon states shapefile US States map polygon The first part of the activity guides you through the steps needed when mapping the following geographic extents: world, continental, and local. The second part of the activity lets you apply what you have learned regarding projection techniques as well as how to display multiple data frames in one layout view. Geographic Extent: World One general principle of map projections is that the larger the area being mapped, the greater the amount of distortion. It follows that the differences among projections will have the greatest impact on world maps. Let's explore three views of the world: Geographic Coordinate System (unprojected), Miller cylindrical, and Robinson compromise projections. Since the datasets contain latitude-longitude coordinates, ArcMap can create a projected view of the layers without the need to actually change the data on disk. However, since you can only view one projection per data frame, you will need to export each of the three maps to an image (.bmp) file, then load them into Word so you can compare them. To start, you will save a Geographic Coordinate System (unprojected) map of the world: Add the world outline layer (world.shp) and the latitude-longitude grid layer (latlong.shp) to your project. Arrange the drawing order so that world draws on top of latlong. Set the color of the world layer to light yellow and the color of the latlong layer to black. Next export the Geographic Coordinate System (unprojected) map to an image file: From the menu, choose File - Export Map. Navigate to the c:\temp\proj8 folder. Set the file name to world_GCS and the file type to BMP. Press Save. Page 1 of 7 Now change the projection to Miller Cylindrical: From the menu, choose View - Data Frame Properties - Coordinate System. Double click on Projected Coordinate Systems – World. Scroll down and highlight Miller Cylindrical (World). Press OK. Press the Full Extent button to maximize the world view. Once again, export your map to an image file: From the menu, choose File - Export Map. Navigate to the c:\temp\proj8 folder. Set the file name to world_miller and the file type to BMP. Press Save. Now follow the same steps and change the Data Frame projection to Robinson; save the map to c:\temp\proj8\world_robinson. Comparing Maps in Word In order to facilitate comparison of the three world maps, you will copy them into a word document: Minimize ArcMap. Run Microsoft Word. At the top of the blank page, type: Unprojected World Map and press enter. From the menu, choose Insert - Picture - From File. Navigate to c:\temp\proj8. Highlight world_GCS.bmp and press insert. Press Control - Enter to start a new page. Repeat the process for world_miller.bmp and world_robinson.bmp. Save the Word document to c:\temp\proj8\world_maps.doc. Compare the broad shapes of continents and countries (ignore Antarctica) and think about the answers to the following questions: Which is larger Is Africa or Greenland larger on a globe? on a globe? Africa or Greenland Compare the How does the shape of the world in the Miller view differ from the shape in the projections. Geographic Coordinate System map? How does the size of Africa compare to Greenland on the Miller projection? Compare Which shapes in the Robinson view differ from the shapes in the Geographic Coordinate Robinson System map? How does the size of Africa compare to Greenland on the Robinson projection? Projection Types Is the Miller projection conformal, equivalent or a compromise projection? Think about what property is distorted and which is preserved. Is the Robinson projection conformal, equivalent or a compromise projection? Page 2 of 7 Understanding datums Shapefiles store projection information in a separate file with a .prj extension. ArcMap uses this file to determine the coordinate system for a shapefile. If we were to open the world.prj file in Notepad, we would see the following information: GEOGCS["GCS_WGS_1984", DATUM["D_WGS_1984", SPHEROID["WGS_1984",6378137,298.257223563]], PRIMEM["Greenwich",0], UNIT["Degree",0.0174532925199433]] It tells ArcMap the coordinate system, the datum used, the unit of measurement, etc. In this case the world shapefile uses the GCS_WGS_1984 coordinate system which is based on the World Geodetic Survey 1984 (WGS_1984) datum. What would happen if we tried to add a shapefile based on another geographic coordinate system to the same data frame? Let’s find out. In ArcMap, add the us48_states shapefile to the Robinson projection of the world. You should see a warning stating that us48_states has a geographic coordinate system that differs from other data in the map or from the current map projection. Click OK to add the data despite the warning. Despite the warning, the 48 states appear in the correct location with respect to the world. Look at the coordinate system for the us48_states layer. Open the property sheet for the us48_states layer. Click on the Source tab. Compare the coordinate system of this layer to the one for the world layer. The us48_states uses the GCS_North_American_1983 coordinate system: GEOGCS["GCS_North_American_1983", DATUM["D_North_American_1983", SPHEROID["GRS_1980",6378137,298.257222101]], PRIMEM["Greenwich",0], UNIT["Degree",0.0174532925199433]] Although both the world and us48_states files both use a geographic coordinate system (GCS), they are different. The world shapefile uses the GCS_WGS_1984 coordinate system based on the World Geodetic Survey 1984 datum, while the us48_states uses GCS_North_American_1983 coordinate system based on the North American 1983 datum. Because the datums are different, the same coordinate location, such as 33.87° N, 117.97° W, will fall in a slightly different spot causing a misalignment between the maps. ArcMap supports the automatic reprojection of certain datums and this warning announces that a datum transformation is needed. Bottom line: if you see this warning, stop and think how your project will be affected by possible misalignments. For more information, click on the hyperlink- About the geographic coordinate systems warming. Open a new blank document in ArcMap. From the File menu, select New. Under My Templates on the New dialog box, select Blank Document and OK. You do not need to save the old document. Page 3 of 7 Geographic Extent: Continental Mid-latitude areas with an east-west dimension that is less than a hemisphere (such as the U.S. lower 48 states) can be mapped with considerably less distortion compared with maps of the world. Two commonly used projections are the Albers Equal Area and the Lambert Conformal Conic. In truth, there is little visual difference between them when creating a thematic map. Save the new map file as Proj8a.mxd. Add the us48_states shapefile to your map. In the Table of Contents, right-click on Layers and click on Properties. This is the Data Frame Properties form, NOT the Layer Properties Form. Click on the General tab and set the name to Albers. Click on the Coordinate System tab. In the search box type- USA_Contiguous_Albers_Equal_Area_Conic and click on the search icon directly to the right of this box. You should see the information on the screen shot to the right. You may need to expand the folders to see the information. Highlight USA_Contiguous_Albers_Equal_ Area_Conic and press OK. Now insert a new data frame into your project: From the Insert menu, select Data Frame. Highlight the New Data Frame in the Table of Contents. Again, add the us48_states shape file to the new data frame and give it a different color than the one assigned to the other us48_states in the Albers data frame. Set the name of the data frame to Lambert. Follow the steps that you used to define the Albers projection above, only this time choose USA_Contiguous_Lambert_Conformal_Conic. To compare the two projections: Open the layout view and arrange the two data frames so they are both the same size with one drawing on the top half of the page and the other drawing on the bottom half. Since there are two data frames, you must activate the data frame in which you want to work. To activate a data frame, right click on the data frame name in the Table of Contents. Select Activate. The activated data frame name should now be in bold. Before continuing, save Proj8a.mxd. Page 4 of 7 Geographic Extent: State Level Now set up a new Proj8b.mxd file containing only the Lambert data frame: Save the project as Proj8b.mxd. Restore the data view if necessary. Right-click on Albers in the layer list and remove the Albers data frame from the project, leaving only the Lambert data frame. Default projections optimized for the entire US 48 states have a central meridian that passes through the center of the country (-96 degrees longitude).
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