In this exercise, you will learn about map projections and see the results of importing data in different coordinate systems. An important resource that supports this exercise is the reference handbook titled Wisconsin Coordinate Systems , which is available in hard copy format from the Wisconsin State Cartographer’s Office for $10 or on-line at: http://feature.geography.wisc.edu/sco/pubs/wiscoord.html
If necessary, start ArcView . From the File menu, choose Open Project . Doubleclick the directory folders and navigate to c:\esritrn\arcview\wiscexer . Open the project wiscexer5.apr
by clicking on the project name, then clicking OK .
When the project opens, you will be in the Blue Lake DRG view. This view includes a scanned topographic map (Digital Raster Graph) for the Minocqua area in Oneida
County.
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Local governments produce and maintain GIS data that may be useful to water management specialists at the Department of Natural Resources. An example is hydrography derived from detailed planimetric and topographic mapping produced for the county.
Add a hydrography theme depicting Blue Lake obtained from Oneida County.
Select the Add Theme button and navigate to the bluelake.shp
theme at c:\esritrn\arcview\wiscdata\bluelake.shp. Select OK . When the hydrography theme has been added to the table of contents in the view, check the box to make the theme visible. Although Blue Lake appears in the view on the western edge of the DRG, the bluelake.shp theme does not display at that location.
Make the bluelake.shp
theme active by clicking on the theme in the table of contents. Select the Zoom to Active Theme button. Note the hydrography theme appears, but not adjacent to the DRG.
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Next, Select the Zoom to Full Extent button. Note the DRG theme appears in the upper right portion of the view window, while the hydrography theme appears in the lower left.
This is because they are each based on different coordinate systems. The DRG is a DNR product and is referenced to the Wisconsin Transverse Mercator coordinate system based on the North American Datum of 1983, 1991 adjustment (WTM83/91). The map units for WTM83/91 are meters. The hydrography theme is referenced to Oneida County
Coordinates with map units in feet. They do not match up, requiring that the Oneida
County data be projected to WTM83/91. This is a service that can be provided by the
Geographic Services Unit of DNR.
Some data sets of interest to water management specialists are available on-line and are often represented in degrees of latitude and longitude, also referred to as geographic coordinates. When creating a map, the location of features are measured using x and y coordinates. The process of converting locations from the curved surface of the earth to the flat surface of a map requires a mathematical formula called a map projection.
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Navigate to the Project Window by selecting wiscexer5.apr
from the Window pull-down menu . Create new view by selecting New with the View icon highlighted.
Use the Add Theme button to add a new theme to the view. When the Add
Theme dialog box appears, navigate to the c:\esritrn\arcview\wiscdata directory and select the wi_cnty_dd.shp
file and click OK .
Make the new theme visible and note the squished appearance of the State of
Wisconsin.
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While themes can be displayed in ArcView using decimal degrees of latitude and longitude, they often appear deformed. We can apply the parameters of the Wisconsin
Transverse Mercator projection to give the state a different appearance.
Open the View Properties dialog box by selecting Properties… from the View drop-down menu.
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Click on the Projection button to open the Projection Properties dialog box.
Choose Custom at the top of the dialog box to define a custom projection. For
Projection, select Transverse Mercator from the drop-down list. For Spheroid, select GRS80 from the drop-down list. Type
–90
for the Central Meridian. Keep the Reference Latitude as 0 . Type 0.9996
as the Scale Factor. Type 520000 as the False Easting. Type
–4480000
as the False Northing. Select OK . Select OK to close the View Properties dialog box.
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Note that the squished appearance of Wisconsin has been changed to the more familiar shape we see on state maps.
We can now convert this theme to a new shapefile and preserve the map projection. This will allow us to compare the differences between the new shapefile we created and the county boundaries shapefile distributed by DNR.
Select
Convert to Shapefile…
from the Theme pull-down menu . Save the new shapefile in the c:\esritrn\arcview\student directory as cnty_wtm_av.shp
.
When prompted whether you want to save your shapefile in the projected units, select Yes . Select Yes in the next Convert dialog box.
Navigate to the Coordinate System Differences view using the Window pulldown menu . Use the Add Theme button to add the new theme
(cnty_wtm_av.shp) from the c:\esritrn\arcview\student directory. Check the box to make the theme visible. Move the Counties (WTM83/91) theme to the top of the Table of Contents by clicking on the theme, holding down the left mouse button and dragging it above the cnty_wtm_av.shp theme.
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Note that you can see some areas where the black outline of the new shapefile shows through the red outline of the Counties (WTM83/91) theme.
Use the Zoom In tool to tightly zoom in on one of these areas. Zoom in again until you notice a difference between the two county boundary lines.
Use the Measure tool to measure the difference between the two lines. The distance units in View properties are set to feet.
The difference between the two projections varies across the state, but is often 20-50 feet in the east-west direction and 5 to 15 feet in the north-south direction. Given the technical issues in map projections and the limitations of using ArcView for precise map projections, it is wise to consult the Geographic Services Unit of DNR for assistance in projecting local government and other data sets into the Wisconsin Transverse Mercator
(North American Datum of 1983, 1991 adjustment) coordinate system.
Click on the Project window to make it active (or choose wiscexer5.apr from the
Window menu). From the File menu, choose Close Project .
Click No when ArcView asks if you want to save your project.
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